THE PODZOL AND PODZOLIC SOILS
Alex. Muir Soil Survey of England and Wales, Rothamrted Experimental Station, Harpenden, Herts., England
I. Introduction ...................................... 11. The Recognition of the Podzol as a Soil Formation . . . . . . . . . . . . . . . . 111. Ortstein and Bleisand ........................................ IV. The Recognition of an Illuvial Horizon . . . . . . . . . . . . . . . . . . . . . . . . . . V. Degree of Podzolization ................................... VI. The Sod-Forming Process of Soil Formation . . . . . . . . . . . . . . . . . . VII. Geographical Variants ........................................ VIII. The Western Contribu .............. IX. What Is Podzolization? ....................................... X. Micromorphology . . , . ............................... XI. The Characteristics of t Podzol and Podzolic Soils . . . . . . . . XII. Summary and Conclusions ................................ References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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When foreign words are adopted into a language there is quite commonly a change or an extension of meaning which is often signified in dictionaries by little danger signals as a guard against mistranslation. It is a pity that the earlier (and also some more recent) writers on soils did not use a similar system to warn their readers that the descriptions being used as illustrations did not necessarily conform to the original definitions. This is perhaps as true for podzols and podzolic soils as it is for any other soil group. Each successive generation of writers has tended to build on the results of immediate predecessors without considering the possibility of some subtle and even serious change in the meaning of the terms. When one is dependent on translations through a second foreign language the potential difficulties multiply considerably. Although the general history of the development of ideas on podzolic soils has been reviewed in the past, little attention has been given to the earliest work, particularly the Russian; and it is just this that provides some clues to the divergent opinions on this group that still exist. In fact it is not inappropriate to quote some remarks by Dokuchaiev from his “Cartography of Russian Soils”: “As the podzol is a formation so far I
comparatively little studied, and therefore controversial, it is undoubtedly true that the term has different meanings in different parts of Russia, so that in discussing the problem of this soil it is probably best to consider it historically and, to begin with, to take account of exclusively factual data.” It is certainly not true now to say that the podzol has been little studied, but it is still a controversial topic. Glinka (1923) listed some 160 publications in which soils of the Russian podzolic zone were discussed, and this figure could no doubt be trebled or quadrupled by subsequent work. This review deals primarily with the Russian evidence as far as it is accessible to me, but touches on the Western European and American literature to bring out the divergences in usage and point to their origin. The translation of Russian words derived from podzol offers minor difficulties in the presentation of the review. I have consistently rendered podzolistii as “podzolic” and opodzolennii as “podzolized.” As I understand the latter word it means that the soils in question have undergone the process of podzolization but may not qualify for the term podzolic in the Russian sense. I have also used it for soils that would commonly be called podzols in Western Europe or North America. It. The Recognition of the Podzol as a Soil Formation
Since Dokuchaiev is commonly given priority for naming the podzol, it is perhaps as well to give in abbreviated form some extracts from his principal writings on the subject. It is convenient to begin with his ”Cartography of Russian Soils” ( 1879). “It is first of all evident that in various parts of Russia the name podzol is given to by no means identical or even similar formations.. , , [A certain number] are peaty deposits with more or less mineral matter; all have been more or less burned; almost all contain fresh-water shells; all are near rivers and some are covered with river deposits. Such deposits are found everywhere on the alluvial banks of our rivers . . . . Their place among other rocks and their period of formation has been elucidated and it is therefore superthous to retain for them the special name of
poazois. . . .
“The remaining examples of podzols are more or less uniform and correspond in general to the description given by Mr. Solov’ev.” “These are their characteristics. “They lie indifferently in lowlands, at the bottom of slopes, on slopes, or on the tops of hills. * Said by Dokuchaiev to have given the “first sufficiently detailed description of
the external features of the podzol” in his “Agricultural Statistics of the Smolensk Government.”
THE PODZOL AND PODZOLIC SOILS
“They generally occur in forests, on the sites of former forests or near bogs into which they sometimes gradually merge. “The podzol occurs sporadically, here and there as small islands, and only rarely occupies large areas. “The podzol lies either on the very surface , , . when it is often used as arable land, or it is covered by a thin layer (several inches) of vegetable earth, generally of forest or bog origin when it plays the part of subsoil. “In either case its depth is not great: 6-12 inches, deepening only in places as it approaches bogs. “Podzols are underlain by clays, loams or sands: in all these cases the podzol appears to pass gradually into the underlying rock. “As regards the composition of podzols, all those investigated so far are of a sandy type and contain 8344% SiOz.. . . “Microscopic analysis shows that the examples examined so far consist of phytoliths, diatoms, soft vegetable residues, quartz grains, silty particles, etc. “External characteristics of the podzol are: an ashy colour, generally with yellowish, bluish or greenish tones; it consists of very small particles so that when dry it breaks down into dust, in the wet state it is slimy like clay and because of this the podzol readily holds water and is therefore sometimes the cause of the appearance of bogs. “Such are the general and most important characteristics of the podzol . . . .” [Dokuchaiev explains the origin of the term podzol as arising from the custom of “slash and burn,” a form of shifting cultivation, still practiced in his time. According to him the name was applied to the infertile layer of soil, gravel, sand or heavy clay, which underlay the ashes resulting from burning the forest cuttings. Vosmer (Russisches Etymologisches Worterbuch) states that the element pod is cognate with Ger. Boden, Gr. pedon, Lat. peda; thus podzol simply means ashy soil.] “There are 4 possible modes of formation of the podzol. “1. . , . that it is due to fires . , . . That cannot be agreed to . . . . “2. . . . due to the burning of p e a t , , , does not apply to podzols. “3. From microscopic evidence it might be considered a simple accumulation of vegetable residues, e.g., phytoliths, but this view . . . cannot be upheld. It does not explain ( a ) the exclusive occurrence of the podzol on the surface or under a thin layer of plant remains; nor ( b ) the striking constancy in thickness (6-8 inches); ( c ) it contradicts the gradual transition of podzols to parent material and finally ( d ) phytolith formations may arise by chemical means alone. “4. These considerations (excluding the last) and at the same time
its position in depressions and on the slopes of hillocks, (e) its small thickness, ( f ) the strange discontinuity of the podzol in one and the same peasant’s field, and finally, ( g ) its content of organic, sometimes structured, material-all these taken together will not allow us to accept the conclusion of Krylov that our podzol can be regarded as a kind of chemical precipitate, as a kind of rock, related by its composition (but the clayey podzol?) and micro-structure to foreign siliceous plant bioliths . , . . “It seems to us that all the characteristics (from a to g) of our podzol compel the conclusion that we are here dealing not with rock . . but with soils . . . “It is evident that these sandy, and rarely loamy, podzols are soils of a mixed, vegetable-bog character; all their differences, in mode of formation, from the common northern dry-Zand vegetable loamy and sandy soils, consist in the fact that the latter, in general, are formed in drier places with free access of air and light and the predominance of a dryland herbaceous vegetation, while podzols are formed mainly in forests with a significant participation of bog and forest vegetation. Here, evidently, there was more moisture, less light and probably less access of air to the soil. That is why podzols occur predominantly in forests and near bogs; why they not infrequently pass gradually into these latter formations; why they are coloured with bluish, greenish (vivianite and ferrous iron) and yellowish tints; why one occasionally finds in them ore concretions and still undecomposed plant remains. “It stands to reason that in the formation of different varieties of podzol special conditions operate . . . . “As regards loamy podzols, to my mind, they are none other than the ordinary northern loamy soils, only changed not by dry-land grass vegetation but predominantly by forests and small bogs . . . .” These extracts show clearly enough that the word podzol was restricted to the ashy gray, often superficial mineral layer and had little reference to what was below. This restricted use of the word podzol continued for many years in Russia and is well exemplified by the following quotation from an account by Sibirtzev (1884) of the soils of the Arzamas district (Nizhni Novgorod) in which he describes soils, with the appearance of podzols, occurring in hollows and depressions of some 60-100 meters diameter, in the chernozem plateau. “A Greyish white, slight yellow tinge; when moist fairly compact; on drying becomes an extremely fine grained, loose, light mass, 1-1sft. or more in thickness. B Gradual transition to brownish subsoil, 3 in. C Yellowish brown or yellowish grey clay.”
THE PODZOL AND PODZOLIC SOILS
Locally the A horizon may appear on the surface, but it is often covered by “black e a r t h to several inches or even by peat, when it contains diatoms and phytoliths. “Therefore we feel that it is simplest to regard podzols, in particular the ‘white land’ of Arzamas district, as vegetable-bog soil, in the formation of which both the subsoil brown clay and its moist peaty, boggy cover took part. The brown clay underwent changes in the moist medium, without access of air, in the presence of carbonic acid and acid bog humus: it is not surprising, therefore, that it lost, by leaching, a considerable part of the soluble material, lost its uniform brown colour, became enriched in silica; the presence in the light grey mass, so formed, of diatom shells and phytoliths is in this way understandable; the yellowish or orange spots (in the A horizon) indicate the original parent material of the podzol . . .” (Sibirtzev, 1884). In all this early work the podzol was very carefully distinguished from the so-called “northern loams and sands,” which were considered as “dry-land vegetable soils” and formed the fifth group of the classification used for the Nizhni Novgorod survey. However, it was becoming recognized that there were soils in this “northern loam” category that had some characters of the podzol. A distinction was therefore introduced between the normal and the “podzolic” varieties, and Dokuchaiev quotes several descriptions in his final report on the Nizhni Novgorod survey ( 1886). “From the data of Sibirtzev and Ferkhmin the structure of the normal (northern) sandy loams in their natural state may be represented by the following schematic section: Horizon A: either ordinary sod or forest mat, s-1-2 in., consisting of leaves, roots, twigs, etc., and under it a grey sandy mass, cemented by clay particles; thickness . . . about 3 in. and not more than 5 in. Horizon B: greyish white, sometimes with yellowish brown and reddish veins and spots, fine sandy loamy mass, without nutty structure or only a slight indication of it, in wet weather it is fairly sticky, in dry it is powdery (thickness 2-3 and up to 7 in.). Horizon C: greyish brown, sticky when wet, consisting of sand cemented by reddish brown particles of clay; on rubbing, numerous colourless, reddish and yellowish quartz grains are exposed; in a word this is a sandy clay or sandy loam; imprints of roots fairly frequent.” The podxolic sandy loams have the following structure according to Amalitzky, one of Dokuchaiev’s colleagues (in Dokuchaiev, 1886) :
“A. Soil horizon of a greyish bluish colour, sometimes with a yellowish
tint; the basic mass is very fine, floury to the feel, dust-like, contains sharp, easily visible quartz grains; in the wet state it is sticky and difficult to work. When dried, immediately following rain, it is hard and cracks, but after prolonged droughts it is powdery and dusty, average thickness about 5-7 in. B. Transition horizon: it is still lighter and unevenly coloured; contains residues of roots and clayey granules of a dark grey colour, occasionally porous; thickness 3-5 in. C . Distinctive light ashy podzolic sandy loam, uncoloured by humus, becoming yellower towards the bottom and then effervesces with acid. There are occasional hard granules of reddish brown colour that are only rubbed down with difficulty, also pores in which there are occasional root residues; towards the bottom it gradually passes into the underlying rock; thickness 0 to 2 ft. C1. Most frequently red-brown sandy loam, sometimes stony; in its upper horizons it has the nutty structure so characteristic of the forest loams; below it is the ordinary boulder clay , . . .” The reference to effervescence with acid is unusual and no explanation is given. On the basis of his description and a chemical analysis of the bleached horizon, Amalitzky concluded that the podzolic sandy loam soil just described is very close to the podzols of Smolensk, referred to by Dokuchaiev. During the Nizhni Novgorod survey a close connection was noted between the thickness (the A horizon of today’s nomenclature) of such podzolic sandy loams and steepness of slope provided there was no change in parent material; the depth of soil increased downslope and movement of material was suggested. ”. . . the material from which this soil [a sandy loam] was formed, because of its occurrence on present or former slopes, has undergone alteration by eluvial processes, mainly resulting in the loss of clay and lime and enrichment in sand . . .” (Dokuchaiev, 1886). “The material (podzol) sorted in this way, consisting of quartz and various silicates, poor in clay, will come under the influence of the different atmospheric agents and vegetation-( in this case forest) and undergo new and further weathering. Quartz will remain unaltered, the silicates will decompose forming amorphous silica which will be left; the clay would be carried by flowing water (slope)” (Amalitsky); the result would be an enrichment in silica. It was considered that amorphous silica formed the cement that made the podzol hard when it first dried out and also conferred its coldness in wet weather. That erosion occurred on these soils is mentioned by Dokuchaiev, who says it was sometimes so severe that it became necessary for peasants
THE PODZOL AND PODZOLIC SOILS
to abandon their fields to forest. Elsewhere he remarks that erosion on slopes has given rise to a “particular fine, floury, podzolic rock,” which increases in thickness from 4 to 6 inches at the top of the slope to 3 to 4 feet at the base. Georgievsky (1888), who has been cited by Glinka (1914, 1931), provides further interesting information on the podzol as it occurs in the former St. Petersburg and Novgorod governments. On the watershed between the rivers Syas and Tikhvinka (Novgorod government) the following types of soil occur: northern loam, sandy loam, sand, and podzol. “Referring in particular to podzols, we see that they nowhere occupy extensive areas, but are everywhere encountered only as bare spots, broken strips, etc.; in addition the following very characteristic feature of their distribution is observed. “Lying on level, relatively high situations or on slopes the formation we are considering nowhere occurs as a layer of constant, definite, thickness but always shows the well known discontinuity, even over the smaller areas: frequently over a distance of 1-2 arshins ( 2 8 5 6 in.) the podzol wedges out several times; sometimes over a whole peasants’ field it appears only at a few points, a search for it being necessary, the more so in that frequently it cannot be determined if there is podzol present or not at any given point either by the colour of the upper soil horizon ( A ) or by any noticeable change in relief. “It is characteristic that in those cases in which the podzol areas are bounded by adjacent swampy ground, the discontinuity of the podzol decreases in the direction of the swamp and its thickness increases. Thus the surroundings of bogs and various kinds of low places and depressions, even though barely noticeable, yet forming more or less wide swamps, are the most usual sites of the podzol in this area” (Georgievsky, 1888, PP. 8-91. Georgievsky divides his podzolic soils into three types according to their position in the relief. In the first type (Fig. 1, D) on the higher elements of relief, the “upper soil horizon A” of about 6 inches comes immediately under the sod (Russian dern) and passes gradually into the “podzolic horizon B” which may be anything up to 10 to 12 inches thick, but more commonly 3 to 4 inches. Frequently the latter horizon tongues and wedges into the subsoil “(horizon C)”. The second type occurs in depressions and moist spots. In this case the upper soil horizon ( A ) , 3 to 4 inches, takes on a whitish tone and in its properties approximates to the material of the true podzol horizon “B”; the latter is much finer grained and floury when very dry. Thirdly a profile from a bog is described, in which, under a layer of 12 inches or so of black bog earth “A,” the podzolic horizon “B” is of a bluish tint with reddish ochre veins
and spots; it gradually passes into the subsoil “C.” The thickness of the podzolic horizon in this case may be 28 inches. In the examples given above the soils have generally been derived from what were called sandy loams (Russian a p e s ’ ) ,and it is surprising that no comment was made about any illuvial horizon. As far as can be seen from these early writings such a horizon had not been clearly recognized. Amalitzky (cf. p. 6 ) comes near it with his reference to a nutty structure in the “C1”horizon; and Georgievsky, after describing some sandy podzols with ortstein, says that “if the subsoil of the podzol is loamy . . . it passes into the country rock very gradually, separated from it by a yellowish loam; the latter occasionally appears as an independent horizon, replacing as it were the podzol.” With regard to the examples from near St. Petersburg referred to by Glinka (1914, pp. 68-69, 1931, pp. 341-342) and quoted by others (e.g., Robinson, 1949, p. 306), the descriptions are given rather differently by Georgievsky ( 1888, pp. 14-16). Near the river Luga, Georgievsky found on a sandy hillock white, fine, quartz sands underlain by loose, mobile reddish yellow or yellowish “diluvial” sand. “They [the white sands] always appear almost at the surface of the soil and are covered with only a thin layer of dark grey earthy sand, they are discontinuous and occasionally, in the form of long root-like salients, wedge into the subsoil; their thickness is 4 to 7 in., reaching 22 in. in the centre of depressions. In this case two features deserve particular attention: 1 ) on passing up-slope the white sands thin out and finally disappear completely; 2 ) under the white sand lies ortstein, occurring in all stages of development, but wedging out at the top of the slope . . . . The structure [of the ortstein] in a dry depression has the following appearance: “Above lies a dark grey sandy (A) soil (thickness 3 to 4 in.); below comes a white, not always uniformly coloured, sand in which white patches constantly alternate with darker; still deeper the dark colour intensifies and the sand is gradually replaced by ortstein; the upper horizon of the latter, in the form of a thin crust (in sections it appears as a black band which closely follows every undulation of the white sand), separates the overlying sand from the lower compact ortstein rock the colour of which is generally pale yellow (but nevertheless contains 0.65% humus), but there are also dark coloured patches of similarly compacted material. In addition it should be mentioned that the degree of cementation of the ortstein as it passes into the loose yellow subsoil sand always gradually decreases. “As to the depth of the ortstein, it is variable and apparently is not closely related to relief, being about 8 to 28 in., and reaching (in the
THE PODZOL AND PODZOLIC SOILS
centre of depressions) 34 in., but in the latter case the ortstein is only weakly cemented. It should also be noted that the surface of the ortstein horizon is extremely uneven . . . .’’ Another example is described from the edge of a bog where the podzol overlies, indifferently, either red boulder clay or sands. ‘When it overlies sand the following interesting section can often be seen.
“A1 Mossy cover-1-2 in. “A Dark-whitish soil, contains 1.7% organic m a t t e r 4 in. “B Sandy mass of a white colour, consisting of sandy fine earth with
an admixture of a moderate number of coarse quartz grains; nevertheless hardens on drying; humus content-O.12%; thickness-12 in. “B1 Sandy loamy mass, stained by humus (up to 2.5% ) to a chocolate colour and gradually passing into the subsoil; on ignition first of all lightens in colour and then reddens; contains some concretions strongly cemented by iron oxide which confers the hardness on this horizon. Thickness 7-9 in. “C Yellowish sand with boulders.
“On the clayey subsoil, immediately at its boundary with the podzol, there was also observed a quite peculiar layer of red, hard ferruginous clay, concretions of which occurred in the lower parts of the podzolic horizon. The podzol in this case appeared as a compact mass which, on prolonged drying turned into absolute flour.” Georgievsky’s conclusions on the occurrence of podzols are similar to Dokuchaiev’s: “The influence of climate is shown by the geographical distribution of the podzol. Judging by the available literature this white earth . . . . in contrast to the chernozem, which is characteristic of the open steppe, is encountered (at least in its typical form), exclusively in the north and north west of Russia, i.e. in places with relatively high rainfall, and with an abundance of both forests and bogs. Such a geographical distribution is not fortuitous. It should not be forgotten that just such an abundance of moisture is one of the essential conditions for podzol formation as this soil is most frequently encountered in bogs, on the edges of bogs, in fields adjoining them-in general everywhere that water can stand for long” ( Georgievsky, 1888, pp. 35-36). It is quite obvious from these various descriptions that the idea of the podzol and its variants was confined exclusively to the bleached or A2 horizon of later nomenclature. This is clearly stated by Dokuchaiev
in a “Short Programme for the Investigation of Soils” (1887-1891) when discussing soil structure: “podzolic, floury structure; the whole mass consists of the finest, dusty, fine earth, almost exclusively quartz, whitish, light yellowish or ashy grey colour; the given structure is especially clearly expressed in the B horizon of typical podzols.” This general attitude to podzolic soils continued up to the turn of the nineteenth century and is summarized by Sibirtzev (1900) in his book “Pochvovedenie.” After contrasting podzolic soils and chernozems and listing the factors influencing podzolization (climate, parent material, etc.) he goes on: “Thus the soils of this type fall naturally into subtypes :
Soddy soils, weakly (sometimes not visibly) affected by podzolforming processes. I1 Podzolic soils proper, with a podzolic horizon clearly separated and sharply distinct from the upper horizon. I11 Podxols or soils strongly podzolised (frequently to the surface).”
The use of the term “sod-podzolic” is considered to be not altogether appropriate “as the majority of the soils are formed under mixed forest , . . ; but we retain the conventional term ‘soddy’ or ‘sod-podzolic type’ in order to indicate that the degree of podzolization is quite variable (Sibirtzev, 1896). The morphology of the ordinary podzolic soil is summarized thus:
. . .”
“A Upper soddy horizon, from 3 to 5 in. thick, usually structureless, coloured light grey, light cinnamon, or grey. “B Under this lies the podzolic horizon proper, whitish, sometimes almost white, sometimes yellowish and bluish; thickness variable, from 2 in. to a foot or more. “C The podzolic horizon gives way to the subsoil material, e.g. brown clay, brownish bouldery sandy loam, yellowish clayey sand, orange coloured loess-like rock and so on. Frequently the podzol penetrates into the subsoil in ‘pockets and spots,’ i.e. the change to the subsoil is gradual.”
“. . , The common new formation of podzolic soils-ortstein-occurs as segregations of varying size, grains and veins . . .” It is surprising that in none of the writings on soil morphology of the “fathers of pedology” do there appear any clear suggestions that the C horizon of their nomenclature might be the illuvial horizon and the recipient of at least some of the material lost from the bleached layer. Although there is frequent reference to ortstein in the early works, it is
THE PODZOL AND PODZOLIC SOILS
only in connection with the formation of this layer in sandy soils that the possible fate of the leached substances is considered at all fully. 111. Ortstein and Bleisand
Soils were often described by competent observers in the seventeenth and eighteenth centuries, but none seems to have noticed or recorded the hard humous or ferruginous layer that is so common in heath and moorland soils particularly. The earliest reference given by Glinka is to a paper by Kindler (1836), who observed the bleaching effect of decomposing roots on ferruginous sands. Following this there appears to have been quite a crop of observations in various countries. Thus, Daubrhe ( 1845 ) confirmed Kindler’s observations and accepted Berzelius’ suggestion of crenic acid as the active agent. Sprengel’s description of Bleisand corresponds roughly to that of the early Russian workers of their podzol, i.e., he is concerned only with the bleached layer: “When, under the fine quartz sand, there is so much carbonaceous, hardened, or very resinous, humus that it has a blue-grey colour, it is known in certain localities, e.g. Liineburg area, as Bleisand. This sand is so very infertile that in the Luneburg area it is reckoned the worst of all soils. It is formed both in heaths and in pine forests in which the carbonaceous, resinous humus is formed (Sprengel, 1837; 1844, p. 154). Here we have reference to the bleached layer only. Later in his discussion of ferruginous loams Sprengel refers to “an earthy layer that is very rich in ferric and ferrous oxides; the iron coming partly from the cultivation implements and partly collected from the soil” (Sprengel, 1844, p. 175). Although some analyses of soils and subsoils are given to illustrate their chemical composition, no attempt is made to link them with the soil descriptions. Following Sprengel, a British writer (Gray, 1839) gave a description of “moorband pan” but had no comment to make on a bleached layer. It is not until we come to Senft (1862) that we get a reasonable description of a German podzolic soil. Senft was primarily interested in the influence of humic matter on the formation of limonite deposits, and ortstein therefore figures prominently in his account. His description of a heath podzol is as follows (Senft, 1862, p. 181). “This Ortstein or Ortsand generally occurs in heath soil, especially on the Luneburg Heath. The plant cover consists either of Calluna vulgaris and other Ericaceae or of Nardus stricta, etc. and Vacciniaceae. Directly under the plant cover there generally occurs a 6 to 8-inch-thick layer of sand of leaden-grey to smoky-brown colour due to the heath humus, or a sand-rich layer of brown loam. This layer is followed by one of variable
thickness, of white colour grading into yellowish or greyish white sand, frequently mixed with rounded boulders and small erratic blocks, often cemented with a little loamy material. Below this whitish sand at 2 to 5 ft. depth is a 10-inch to 4$-foot-thick layer of Ortstein distributed in nests or islets or elsewhere in a continuous formation. It is notable that this Ortstein, according to a written communication from Mr. Schultze, never occurs in fully forested areas of the investigated region, but appears some time after careless deforestation is followed by a period of noncultivation. Furthermore, Ortstein never occurs on the ridges of higher hills, but it does occur on slopes even as steep as 35”.” Elsewhere Senft mentions that many ortsteins consist of sand coated with a skin of ferruginous clay or clayey ochre. Of all the early writers on podzolic soils the greatest is undoubtedly P. E. Miiller, whose name frequently appears in reference lists but who appears to have been little read by soil scientists. His work on natural humus forms is a model of scientific observation, and he provides the most detailed account of a podzolic soil given by any nineteenth century writer. In the following paragraphs the word Torf is translated as raw humus, which seems appropriate to Muller’s definition of his use of the term. “The soil in a rather dense beech forest with a raw-humus layer is only scantily covered with the vegetation mentioned above [i.e., Aira flexuosa, Trientalis europea, and mosses]; it carries a poor vegetation; the surface is covered with small branches, twigs and some fallen leaves between the moss and the few and insignificant phanerogamic plants. The loose leaf-layer always covering the mull is conspicuous by its absence from the raw humus soil. The soil is compact and yields underfoot no more than a thick felt layer over a solid basis. The surface is so compact that rainwater will sometimes collect in puddles on the loose sandy soil where the soil is covered with a raw humus layer; if this layer becomes wetted by continued rainfall, it becomes saturated like a sponge, while the underlying soil remains dry. “When the soil is dug, there first appears a tough blackish brown humus layer, the raw humus. Underneath more or less sharply differentiated from the raw humus is a generally loose sand lacking the ochreous colour common in transported formations. Its colour varies between greyish white and grey or dark grey and usually becomes lighter with depth. Beneath is a dark layer of reddish brown or brown colour, which is underlain by sandy loam, sand or an intermediate form of these two. “These layers [Fig. 1, A and B] are of very different thickness and also show other differences. The greyish sand layer, designated Bleisand, under a thin layer of raw humus may occur either as a very fine band
THE PODZOL AND PODZOLIC SOILS
FIG. 1. Schematic representations of podzolic soils. A-C after Miiller (1887); D after Georgievsky (1888); E after Afanasiev (1930).
hardly one inch thick with indistinct boundaries, or as a sharply delimited, four-inch thick sandy layer of almost pure white colour, such as is often encountered in forest land adjacent to the North Sea; layers of six inches thickness are less frequent, though in forests on the transported sands in Jutland may attain a thickness of a couple of feet. On a flat terrain, the thickness of the peat and Bleisand layers often show some correlation in so far as the thicker layers of raw humus occur over thicker layers of sand. On more varied relief there are many exceptions to this trend where, in depressions, very thick layers of Bleisand may occur under thinner raw humus layers. The sand is generally very loose, but at the boundary with the underlying layer it sometimes becomes more compact and even assumes the appearance of sandstone in some places [Fig. 1, B]. “The brownish layer underlying the Bleisand, designated as red earth or Ortstein, varies both in thickness and consistency. Ortstein and Bleisand are generally the same thickness when the latter occurs as thin bands hardly one-inch thick, but under thicker layers of the sand the Ortstein may be four to six inches or even eighteen inches thick. The total thickness of the three layers can therefore vary between about four inches and two and a half feet. Thin layers of Ortstein are always loose and earthy, and even deposits of four to six inches often have a similar consistency. But thicker layers of Ortstein are generally compact and develop either into a sandstone-like formation or into the true Ortstein, well known on heathland, such as in the Silkeborg Forests in Jutland.. . . “The subsoil, like that under mull, is thus often compact where the Bleisand and red earth layers are thicker; otherwise the subsoil is very variable and may consist of transitional forms between poor sands low in clay, occurring in bouldery formations, and plastic and micaceous loam characteristic of lignite formations. When the subsoil is sandy or gravelly, the two overlying layers differ little except in thickness; on the other hand, where the subsoil is loamy, three different forms of these layers may be distinguished. “The first form is exemplified by the profile at Store Hareskov (Seeland) [Fig. 1, C]. In a depression and under a not particularly firm raw humus layer 21/, in. thick there was a loam layer 5 in. thick having a uniform very light colour below which was a similar thickness of a uniformly reddish brown loam layer resting on a subsoil of irregularlycoloured ochreous yellow sandy loam. “The second form is shown in a profile occurring at the Teglstrupper Gehege ( Seeland). The profile is in a recently dug ditch in loam on an elevation, falling on three sides by 6-8 feet at distances of about 100-200 feet, Under a 4-inch thick extremely tough layer of raw humus there is a
THE PODZOL AND PODZOLIC SOILS
6 or 7 inch thick Bleisand layer of whitish grey colour, underlain by a 16-inch-thick sandstone-like layer of light greyish brown colour with dark brown veins and streaks; a vein of the overlying Bleisand ran downwards through this layer. The subsoil consists, as at the previously described place, of ordinary sandy loam with gravel, sand and glacial boulders. “A third form is shown in the profile of the soil of a high slightly inclined plateau in Laven-Skow, near Silkeborg (Jutland). The very tough raw humus layer of more than 4-inch is underlain by 3 to 4-inches of fine, whitish, micaceous sand, the lowest part being a darker colour and compacted into a very hard but thinner layer. Immediately below this layer is a slightly plastic micaceous clay without any glacial boulders, which undoubtedly belongs to a lignite formation. Though the overlying Bleisand covers the clay in the form of a dense mass without any cracks or fissures, the uppermost part of the clay is full of cracks which divide the soil at many points into larger or smaller clods. The cracks are filled with a blackish brown powdered material which also covers the clods, so that a clean section of the soil appears to be marked with brown veins. If the brown powder covering the clay is scraped off, the surface of the latter is seen to have a greyish colour and a Bleisand appearance and contains less clay; however, at a greater distance from the brown surface (up to 1 inch) the mass is plastic and ochreous-yellow coloured.” After an account of the microscopic appearance of the beech raw humus, Miiller continues: “Mechanical analyses show that the three layers below the raw humus, like those under mull, decrease in stone content with increasing depth, but the clay content may either increase or decrease with depth. In this respect it is particularly noteworthy, that the clay in all profiles in elevated positions increased very markedly with depth, while in the only profile in a very low position the highest amount of clay occurred immediately below the raw humus. It will be seen later that this is due to the effect of water in translocating clay from higher to lower layers of the soil or to lower-lying parts of the relief. In order to ascertain that the process does not take place in the reverse direction and that the sand is not moved by water, as some practical men believe whenever they see a whitish band of sand near the soil surface, a profile about 300 yards long was examined at Store Hareskov. Here the Bleisand was of variable thickness throughout the entire line, but careful levelling showed that it behaved similarly on hilltops, small plateaux and in depressions, showing no removal whatsoever of sand by water. “Further examination of the peculiar structure of Bleisand and Ortstein shows that the former is composed mainly of pure mineral frag-
ments, especially quartz, intermixed with the other components of granite. The humus-like particles, which confer a dark colour on the layer of Bleisand directly underlying the raw humus, as well as containing fine root fibres, consists mainly of small black humus-particles embedded between the grains of sand. Ortstein possesses a quite different constitution: each grain, each solid particle, is surrounded by an apparently structureless substance; each is encrusted with the brown material which confers its colour on the layer, making the entire mass so similar to the topsoil below mull that the uniform colour prevented any differentiation between the structure of Ortstein and of this layer (i.e,, topsoil) when examined under the microscope. A similar incrustation, though less complete and of another composition, occurs in the loamy subsoil.” In a general discussion of forest soils with mull and raw humus, Miiller points out (1887, p. 72) that the whitish gray layer that can occur under mull is not to be regarded as Bleisand for it by no means lacks the encrusting substances characteristic of mull. In other words, we appear to have the first reference to a soil akin to the Parabraunerde or sol lesSiOh.
The heath soil with its ortstein or moorband pan, which provided problems both for the forester and the agriculturalist, gave rise to an extensive literature in Germany and Scandinavia during the later nineteenth century and the early part of the twentieth century. Much of the work was largely repetitive and few adequate soil descriptions were given. Of the various writers, Emeis (1876) and Muller were the most productive and both gave good descriptions with diagrams of heath podzols to which modern horizon terminology can be readily applied. The morphology of the “typical heath soil” is summarized by Muller (1887, pp. 139-140) as follows: “The soil is covered with a layer of compact and tough raw humus, consisting mainly of organic components and residues of heathland plants, densely interwoven with heather roots, fungal mycelia, occasional moss thallus, and roots of other plants. The lowest parts of the heath layer usually contain mineral components, mainly sand, from the subsoil, in amounts increasing with depth, and it assumes the character of a sandy soil containing plant roots strongly intermixed with plant residues, rather than resembling the true raw humus formation on the original soil. Drying, however, also hardens this part of the surface, often making it rock-hard and not sharply distinguishable from the raw humus on the top. The heath layer is underlain by Bleisand in a white layer mixed with humus particles and showing tints of snow white to a greyish black mull-like colour according to the amount of admixture. The boundary between the Bleisand and the heath
THE PODZOL AND PODZOLIC SOILS
layer is sharper in proportion as the above-mentioned lower layer of the raw humus is less developed; sometimes this boundary is quite obliterated, but often it is very marked. The layer below the Bleisand is coloured by humic acid and its compounds and is called Ortstein or ‘red earth.’ The mineral constituents of the soil in this layer are covered with material of a humic nature. The layer is usually very thick, permitting only a very slow penetration of water. The Ortstein layer is never sharply delineated at its lower edge; transition to the ochreous sand of the subsoil is indistinct and usually very irregular, so that the dark to blackish brown layer penetrates the subsoil in tongues. The rich variety in the forms of this transition is mainly due to differences in the movement of water. Emeis published very fine drawings of heath profiles, showing all these characteristics. The boundary between the Ortstein and Bleisand is often very sharp, but also frequently indistinct due to accumulation of humic matter in the lowest layers of the sand.” In view of the importance of ortstein Muller gives a detailed account of its nature and origin which is the fullest to be found in the nineteenth century literature. His recapitulation of its nature and occurrence is very complete and does not appear to have been superseded (Muller, 1887, __ pp. 222-224) : “Nature A. Ortstein formed by deposition. 1. Clay Ortstein. A more or less porous, solid and hard mixture of sand and clay of a rather uniform greyish colour. Does not change its consistency on treatment with alkali or dilute mineral acids. 2. Peat-like (Torfige) Ortstein. Dense, earthy to hard, blackish brown to black or bluish black accumulation of carbonized humus containing humic acids and their salts, and more or less mixed with white sand-grains lacking ochreous films. Acid because of the humic-acid content. On treatment with sodium hydroxide easily decomposes into white sand and amorphous humose mud in a black solution. Disintegrates when exposed to air. (Many fine heather roots often occur. ) B. Ortstein formed by absorption. 3. Humus Ortstein. Dense, earthy to hard, yellowish brown to blackish
On mull-covered sandy loam soils it forms continuous layers 8-200 cm. from the surface; it remains even after the soil surface has become covered with raw humus and has consequently undergone changes. Below ,the leached surface of coarse sandy soils on heathland and in forests, best developed and forming continuous layers in fresh to moist or wet soil: immediately overlies the true humus Ortstein from which it is not clearly differen,tiated.
Always occurs in continuous layers, more or less following the relief of the surface,
Nature brown sandstone, consisting of the skeletal components of the soil (especially quartz sand) which are covered with, and cemented together by, humic acids and their salts. Easily decomposes on treatment with sodium hydroxide into sand and fine soil which contains some amorphous humose mud in a black solution. Is only slightly affected by dilute mineral acids, and disintegrates when exposed to air. a. Ferruginous humus Ortstein containing larger amounts of iron than the subsoil below the layer. b. Iron-deficient humus Ortstein (red earth) containing smaller amounts of iron than the subsoil below the layer. C. Ortstein formed by concretion. 4. Iron-sandstone. Dense, hard sand-
stone of yellowish brown colour, consisting of the skeletal components of soil (especially quartz sand) coated and cemented with iron hydroxide (according to Senft intermixed with some lower iron oxide and other substances). Does not disintegrate when exposed to air, or treated with alkali solution, but decomposes on treatment or boiling with dilute hydrochloric acid.
5. Bog iron (Raseneisenstein) . Porous, slag-like ore of a blackish colour and containing 80-95% iron hydroxide (Senft). Behaves like 4 towards air, alkali and acid.
150 cm. below the uppermost leached layer of the top soil (Bleisand) in forests and heathland with raw humus.
In layers, in the uppermost part of the subsoil in sandy soils very poor in clay. In layers, in the topsoil, at variable depths from the surface, of sands richer in clay or of loamy sands and sandy loams. Occurs in the uppermost layer of raw humus covered sandy soil in the form of aggregates of different sizes and shapes. Probably f;orms larger continuous masses where humus Ortstein occurs in moist heath soils.
Slag-like masses occurring in moors and bogs as well as in clay Ortstein occurring near the humose surface and overlying an impermeable subsoil; also in many other partidly or temporarily anaerobic places in the uppermcost layers of soil affected by humose material especially in sands.”
A little earlier than Miiller, Burkhardt (1870) and Wessely (1873) put forward interesting points about ortstein. Burkhardt stated that it was formed only in sandy soils and did not occur even in sandy loams. Wessely asserted that ortstein was always associated with a heathy vegetation, so that heath humus was a condition of its formation and the thicker the ortstein the harder it was cemented. This idea was taken
THE PODZOL AND PODZOLIC SOILS
up by Pavlinov (1887) in Russia, who suggested that the phenomenon might be termed “callunism” and the ferruginous deposit (ortsand and ortstein) “callunite.” This paper of Pavlinov’s is also interesting in that, apart from a footnote reference, the word podzol nowhere appears and no attempt was made to connect the soil studied with the podzolic type described by Dokuchaiev and others. Following the German writers, he argues for a movement of clay from the surface to the subsoil, but the “clay fraction” (<0.01 mm.) in the ortstein layer is less than the total sesquioxides extracted by strong acids and his argument is not convincing. Many papers continued to appear dealing with heath and other podzolic soils, but the interest in the Western countries was so concentrated on ortstein that no real progress was made in characterizing the group. This is well seen in Sibirtzev’s treatment already referred to: even when considering the fate of the substances lost from the surface horizons of podzolic soils no mention is made of the possibility that they might be deposited in the “C horizon.” In fact, in referring to ortstein as the common accompaniment of podzolized soils, he states that it “lies in the podzolic horizon, usually in its lower part or directly under it. Small pellets of ortstein sometimes appear in the upper horizon of podzolicloamy or light loamy soils.” IV. The Recognition of an llluvial Horizon
The idea of downward translocation of substances from the upper layers of Bleisandboden to the lower was clearly enough brought out by the earliest West European writers on ortstein and was recognized to be highly probable for this formation by the Russian workers. In general, the transference was thought of as occurring in solution. It has already been mentioned that Pavlinov (1887) suggested the movement of clay particles, and some of Muller’s data would support this argument, but as field estimates of texture were probably somewhat rough and mechanical analyses were rarely extended below 20 p, any textural differences were not recognized. The more general concept of illuviation in the pedological sense was put forward by Vysotzky (1899), who defined illuvium as the material washed out of the surface soil and deposited below to form a definite horizon of accumulation (i.e., the illuvial horizon). He used eluvium, however, in the sense of lateral removal of dissolved substances. It was some years before a clear concept of eluviation and illuviation was established. The problem, from the point of view of morphology, was most acutely felt in dealing with podzolic soils that showed bleaching in patches
(either an incipient bleached layer or the beginnings of degradation in the illuvial horizon). According to Zakharov (1906) it was common to find writers who would relegate to the subsoil the patchy bleached layer of a weakly or soddy podzolic soil. He strongly advocated its recognition as part of the podzolic horizon and proposed that the following letters should symbolize the various horizons: A, upper humous horizon; B, transitional or podzolic-eluvial horizon; C, illuvial (ortstein ); D, parent material. This particular usage of symbols continued among many Russian workers until about 1930, when the form earlier proposed by GlinkaA, eluvial; B, illuvial; C, parent material-with the addition of numeral subscripts for subdivisions became the standard. Once this solution of the division of the podzolic soil profile was accepted, profile descriptions became rationalized and at the same time very much improved. It was recognized that ortstein as defined by Senft and later writers was mainly characteristic of sandy soils whereas in podzolic soils of finer texture rounded concretions in the Az and B horizons were the commoner forms of iron segregation and deposition. Tumin (1912) pointed out that ortstein is confined to the A-B boundary, while concretions were disseminated in all horizons. For a podzol from the Smolensk district he gave the following figures for concretion content: Al, 1.15%; Az, 0.75%; B, 0.29%; C, 0.10%. He found that there tended to be an increase in the number of concretions near depressions, the maximum occurring in the upper 10-15 cm. of the B horizon, but in the soils of depressions no concretions occurred, owing to strong reducing processes. The translocation of clay from the surface to the subsoil had been suggested but inadequately established by early writers. The active agent in podzolization was commonly thought to be crenic acid. However, Glinka (1924, 1931) put forward a strong case for his belief that “in general the podzolic process is basically none other than the leaching from the upper horizons of mobile humus sols of low calcium saturation and, under their protective influence, of fine mineral suspensions, and the deposition of these suspensions in the B horizon together with a small amount of humus.” He rejected the theory that crenic acid was the active decomposing agent in podzolization, but in doing so went still further and assumed that no decomposition of the clay fraction took place in soils of the podzolic type (Glinka, 1931, p. 343), because chemical analyses had shown that there was little “colloidal silica” in the bleached horizon (e.g., Georgievsky, 1888; Gedroiz, 1926) and little free alumina in the B horizon (Gemmerling, 1922). In his discussion he incidentally mentions that the “silica powdering” of degraded loams is
THE PODZOL AND PODZOLIC SOILS
nothing more than tiny quartz grains freed from their coatings of soil colloids. This was later confirmed by Rod6 and Feofarova (1955), who also showed that feldspars and small clay aggregates were important constituents of the powder. Good descriptions of the clay coatings on the peds of the B horizon were given by Rod6 (1930) and Gemmerling (1930), who provided the first analyses of the ”crusts,” as he termed them (see below). Rod6’s description of a strongly podzolized soil on varve clay from Lisino Forest, near Leningrad, under a pine-spruce stand (typical Pinetum oxalidostcm) is as follows: “A0
Bedding of forest floor, loose, little decomposed. Immediately below lies the principal mass of roots.
Grey, rather pale, medium loam. Above with irregular clods 5-7 mm. in diameter; below the clods increase in size, become angular and acquire a certain schistosity in their disposition. When dry, clods are scarcely perceptible; fragments of soil do not exhibit any apparent tenacity. Rapidly merges into subjacent horizon, but the boundary is indistinct, being diffuse and broken on account of wedges of humus.
Whitish with a greyish yellow tint. Medium loam, smears much. In dry condition falls into small and unstable pieces, manifesting an evident tendency toward lamination, well exhibited when moist. Contains an appreciable number of dark brown small ore grains (about 1 mm. in section), on the walls of the section usually appearing in the form of commas (smudged by the spade). Rapidly passes into the subjacent horizon, without, however, showing any distinct boundary line, and in penetrating into that horizon to the depth of 8-10 cm. forms tortuous tongue-like projections and streaks.
Pale pinkish yellow heavy loam. When dry, compact and tenacious, of indistinctly expressed usual polyhedral structure with aggregates gradually increasing in size with depth. The aggregates are separated from one another by very fine layers of clay particles of a somewhat paler tint than the aggregates themselves, showing as a kind of “polish” on the surfaces of the latter. In the upper parts contains in some quantity
small ore grains. Gradually passes into the following horizon. ”B2 35-63cm.
Yellowish brown (when fresh darkens to crimsonbrown). In other respects similar to the preceeding. Aggregates are larger. Gradually passes into the subjacent layer.
Varve clay consisting of summer bands, loose, silty, much staining, of a light yellowish grey colour, 6 7 cm. thick, and of winter bands 1-1.5 cm. thick, dark, crimson-brown with distinct fine laminar structure, clayey, very tenacious, apparently consisting of very fine particles.
“The soil is a strongly podzolized soil on varve clay with all the typical podzol features. The humus horizon Al is not very thick, light coloured, although containing above 4 p.c. of humus. The podzol horizon A2 is fairly thick and has a characteristically friable structure. The presence of ore grains should be noted. In horizon B we meet with the polyhedral structure . . , consisting of angular aggregates. It may be mentioned that the faces of the aggregates are paler, but-a very exceptional case within the forest-they do not exhibit any traces of reduction processes; this is evidently due to the fairly satisfactory aeration of the soil” ( RodB, 1930). Gemmerling’s description of a loamy podzol is from near Moscow under young deciduous forest. “A/O-turf. Depth 6 cm. “A/l-no sub-horizon-if a very narrow greyish stripe, merging with the turf is not considered. “A/2,-thick, whitish, mealy structure. In the upper part a lamellar unstable structure is preserved. Iron concretions occasionally are found (very few), the small ones readily disintegrate, the larger grains are very solid. Penetrates pocket-like into horizon B. Depth 6-23 cm. Pockets up to 33 cm. “B/1-light brown, nutty structure, the nuts throughout are podzolized, dense, the edges of the nuts are covered with a brown crust. Depth 33-45 cm. “B/Sbrown colour, breaks into prismatic units, the edges of which are covered with a reddish crust, dense. Depth 45-90 cm. ”B/&yellowish brown colour, breaks into large prismatic units, the edges of which are covered with a reddish blue-grey crust. In the
THE PODZOL AND PODZOLIC SOILS
upper part of the sub-horizon are rather many black spots of manganese, very dense. Depth 90-169 cm. “B/&sandy lentil of yellowish colour. Its depth is over 40 cm.” The characteristic features of these soils are the tendency to a platy structure; the frequent concretions in the A1-AP horizons; the tonguing of the A2 into the B and the nutty or blocky structure and clay skins of the B horizon. The tonguing of the A2 is illustrated in Fig. I, E, which has been drawn after a picture in Afanasiev (1930). The concretions in the A2 are also indicated. The mechanical analyses given by these two workers show strong apparent clay shift, even allowing for small variations in the texture of the drift parent materials. Data given by Gemmerling for the composition of the “crusts” are recorded in Table I. TABLE I Composition of Clay Skins and Peds in Loamy Podzola B3
Clay skin Component SiO, Fe203
Si02/A1203(approx. ) a
(%) 55.58 7.17 24.27
73.45 5.30 13.46 1.70
56.28 6.88 22.29 2.04
75.48 5.18 13.03 1.33
Data from Gemmerling (1930).
Gemmerling states that the thickness of the ”crusts” or clay skins is 0.5 to 2 mm. and “they are much better developed along the vertical than the horizontal sides of the structural units. These crusts are formed from the products, which are washed out from horizon A. In view of the greater density of horizon B, the soil solutions penetrate into this horizon chiefly along the fissures between the structural units; the fine suspensions . . . are gradually deposited on the sides of the units.” While the data do not prove that the material came from above, its composition is obviously close to that of soil clay. Such analyses of more carefully separated clay from clay skins was later reported by Yarilova and Parfenova (1957). Their results were very close to those of Gemmerling, the Si02:A1203 values for podzolic soils being between 3 and 3.5. V. Degree of Podzolization
As mentioned above, Sibirtzev (1900) recognized three main subtypes within the sod-podzolic type: soddy soils, podzolic soils, and pod201s (or strongly podzolic soils). The first attempt to define more closely
the various degrees of podzolization was made by Zakharov (1910,1911). Starting with the relation of soils to relief in which the sod-soils occur on upper parts of slopes and the strongly podzolic toward the foot, Zakharov transposed the concept to a time scale. The position is then: (1)in soddy soils (i.e., the youngest) there is a slight development of an accumulation horizon shown as a graying and browning of the upper layer of the parent material. ( 2 ) As organic matter accumulates the conditions become suitable for the appearance of podzolization, which shows as whitish spots and patches that merge to form a layer-the eluvial horizon; at the same time the brownish ortstein horizon forms. (3) As the podzolic horizon becomes sufficiently developed and thickens, there appears a subdivision of the ortstein horizon into an upper layer, darker red brown when wet, and a lower layer lighter in color. The upper darker part Zakharov considered to be due to humus leached down after the loss of iron from the eluvial horizon. This subdivision for fine-textured materials may be tabulated (Zakharov, 1931) thus: Subtypes Soddy (or cryptopodzolic) Weakly podzolic Podzolic Strongly podzolic Podmls
Degree of development of podzolization Not noticeable As spots Whole horizon (i-e., A, horizon) Whole horizon occupies half the total A horizon Occuuies almost all the A horizon
Such definitions were highly desirable, for the word podzol was loosely used from the start and even long after Zakharov’s proposal. For instance, Glinka (1923, p. 34) describes a “clay podzol” in which the Al horizon was 13-15 cm. thick as against 8-13 cm. for the Aa horizon. The above subdivision, which is applicable to all classes of podzolic soils, is still in general use with minor modifications. Thus: slightly podzolic, Al > 2A2; medium podzolic, Al rr A,; strongly podzolic, A, < % A,. VI. The Sod-Forming Process of Soil Formation
As has been mentioned above, a group of so-called soddy soils was recognized in which little or no podzolization had occurred, Sibirtzev (1896) remarked that “the term sod-podxolic is not altogether appropriate, as the majority of such soils are formed under mixed forest,” but he used the term sod (Russian dernina) and its adjective in the general sense of turf. “In the northern loamy [i.e., podzolic] soils, where moisture is near, the roots and rhizomes of grasses form a shallow, but dense turf horizon” which may contain some 3% organic matter as against about
THE PODZOL AND PODZOLIC SOILS
0.5% in the succeeding whitish horizon. However, the influence of the ground vegetation on the amount of organic matter was considered only in relation to forest versus steppe, and variation within the zone of podzolic soils does not seem to have received much attention. The first worker really to emphasize the importance of woody versus grass-herb vegetation seems to have been V. R. Williams. His full treatment appeared in his textbook on pedology, and his ideas were well disseminated by his colleagues at the Timiriazev Institute, Moscow. From the pedological viewpoint, Williams’ theory is so overlain by irrelevant matter and supported by such fragmentary data that, not surprisingly, it found little favor. The botanical aspects received a slashing attack from V. N. Sukachev (1916), a distinguished ecologist, who concluded that the theory lacked any factual basis. Neustruev (1927) remarked that the description of the processes “is dogmatically stated in such a way as to impede critical judgement” although he admits that some instances of the “soddy” process may be observed. In fact, the Dokuchaiev school more or less rejected the theory and little more than formal mention of it is to be found in textbooks of the 1930’s. Reduced to its simplest form the theory states that on opening up a closed-canopy forest with podzolic soils, a grass-herb ground cover may, in course of time, become so dominant that the woody vegetation cannot regenerate. The grass phase will also occur in cutover woodland used for hay or for grazing. As the herbaceous litter is much richer in bases and undergoes (partly for physical reasons) a different type of decomposition, there is an increase in humus content and base status of the soil. Similar views, without reference to Williams, were expressed by Vysotzky (1927), who also recommended practical measures for the reclamation of forest lands to agriculture. Williams (1940, p. 118) gives the following generalized description of a soddy-podzolic soil, this stage being best developed when “loosetufted grasses” ( e.g., Dactylis glomernta, Poa nemoralis, P . sterilis), together with legumes, reach their heyday in the succession. Four horizons are clearly differentiated: “1. Soddy horizon, coloured brownish black from the top by the presence of humin. With depth the colour changes to brown as ulmin predominates below. In its upper part the horizon consists of loosely held stable clods, all interstices between which are filled with living and dead roots and rhizomes, binding the clods into a sod. “With increasing depth the amount of organic residues and amorphous humus gradually decreases; similar changes take place in the degree of clodiness, stability of the clods and colour of the soil, until the upper horizon merges into the second, podzolic horizon.
“2. Podzolic horizon of grey colour, with a much smaller content of organic residues. Cloddiness in the podzolic horizon gradually changes to an aggregate state. Thanks to this the plasticity of the podzol is strongly decreased and the whole horizon assumes a loose loess-like character. “The podzolic horizon gradually passes into the ortstein horizon. “3. The ortstein horizon may have a great variety of tints of yellow, red, brown, black and grey colours. This horizon may or may not effervesce on moistening with acid. Its structure may likewise be most diverse. In the ortstein horizon there is frequently an abundance of roots, alive and dead. “4. Below this, again with a gradual transition comes the gley horizon of grey colour . . . [in it] there are living and dead roots of legumes only and dead roots of trees. “In its lower part the gley horizon begins to break up into angular nutty fragments which are frequently gleyed on the surface but internally retain the colour of the parent material. The surface of the aggregates is covered with a silica dust and coloured with a brown film of ulmin. [With increasing depth the gleying decreases and the transition to the parent material takes place.]” Although Williams had a low regard for soil morphology (“a children’s disease” he called it), his continued emphasis on the sod-forming process had resulted in the recognition by Russian morphologists of a group of soils that had previously been regarded as ordinary podzolic soils, i.e., those now known broadly as sod-podzolic soils. The &st attempt to show that a grassy vegetation following forest would produce at least some of the effects postulated by Williams was that of I. V. Tiurin (1935). In a comparison of two profiles from the Lisino Forest, near Leningrad, one under an old pine-spruce stand, the other in an adjoining old meadow (at least 90 years old) with scattered trees, Tiurin showed that there was a marked increase in the thickness and humus content of the Al horizon under grass and of pH and exchangeable bases in both the Al and Az horizons. Earthworms were extremely active in the meadow soil. No data were given on the over-all changes in the total chemical composition of the soil or its clay fraction. There is thus no evidence of the establishment of an independent soil type; we simply have forest and meadow phases of the same podzolic soil. The descriptions, however are worth quoting:
THE PODZOL AND PODZOLIC SOILS
Profile I, under pine-spruce forest (approximately 90 years) with bilberry, cowberry, grasses, and mosses. “Aol Ao”
0 . 5 cm. 0.53 cm.
A1/A2 10-16 cm. A2
Undecomposed litter. Brown, partly decomposed litter with fungal mycelia. Transition layer from the litter to A1; grey with a predominance of mineral matter. Light grey with slight yellowish tint, small cloddy, abundant tree roots, loose; gradual transition to Light grey with more definite yellowish tint, more compact than A1. Fewer roots. Whitish podzolic horizon, very compact, small ore grains. Chocolate-brown, very compact, coarse cloddy structure; gradual transition to More uniform, brown, in the lower part some lighter patches; very compact; clayey. At 80 cm. large boulders.”
Profile 2, under meadow adjoining the forest, 150 yards from Profile 1. Grasses include Deschampsia caespitosa, D . flexuosa, Agrostis sp.; herbs and mosses are present. “A0
A2 B1 B2 C
0-1 cm. 1-6cm.
Litter of dead leaves. Upper (soddy) part of humus horizon, strongly permeated by grass roots. Grey humose horizon, fairly sharply separated from 6-14cm. lower podzolic layer into which it tongues; well developed small cloddy structure. In spite of drought the horizon is moist, more so than under forest; large number of earthworms. 1424cm. Whitish, with small ortstein concretions; brown spots in the lower part; more compact than Al; wormholes. 2435cm. Brown, with whitish spots; weakly fissured; coarse cloddy; merges into 3Ei55cm. Brown, compact, clayey, illuvial horizon. 55-100 cm. Brown loamy till.”
Although till was encountered in the second pit, it was assumed that both soils were developed in varve clay.
The changes in soil morphology described by Tiurin can be seen in a comparison of an old grassland soil (Park Grass) at Rothamsted (Guide, 1959) and a woodland soil of the Chilterns (Batcombe silt loam; Avery, 1958). There has been an increase in organic matter in the A horizon of the grassland soil, 3.8% organic C as against about 1.5% for the Al of the woodland soil; there is no change in base status, and both the grassland and woodland soils have a pH range of 4 . M down the profile. It appears that the only source of lime and other bases, the underlying Chalk, is completely out of reach of the roots of the sward and of the moderate earthworm population (Satchell, 1953). In Tiurin’s example, roots and invading worms were presumably able to reach the till of higher base status and thus bring nutrients to the surface. The socalled sod-forming process, therefore, does not necessarily lead to an increase in fertility insofar as mineral elements are concerned. In his book on pedology written for foresters, Tiurin (1933) began his discussion of podzolic soils with what he called the “pre-podzolic” or cryptopodzolic stage, which is obviously equivalent to the sod soils of Sibirtzev, Zakharov, and others, and equated them with the forest brown earths of Ramann. Tiurin considered that under primary grassland within the forest zone it was possible for podzolization to occur and, as grass vegetation may invade and colonize a deforested area, one could have two series of podzolic soil which he illustrated thus: Grassland
(forest brown earths) Weakly podzolic
hlediuni podzolic Strongly podzolic
Soddy soil Sod-weakly podzolic Sod-medium podzolic
The arrows indicate the possible changes that can occur following changes in the vegetation. The sod-podzolic group as described by Tiurin appear to be essentially what were also called meadow podzolic soils (Glinka, 1923, 1931). Whether, in fact, podzolization can take place under meadow grassland, as distinct from heath grassland, seems not to have been adequately demonstrated.
THE PODZOL AND PODZOLIC SOILS
It is probable that some of the soils described in the older literature as meadow podzolic or peaty podzolic are probably essentially surfacewater gley soils and related to pseudogleys; some appear closer to humic gleys. The following description from Glinka (1931, p. 344) illustrates the type from the Amur region.
Almost black when wet, dark grey towards the bottom; structureless. Contains many undecomposed organic remains. Thickness 25 cm. Olive-grey, unevenly coloured; numerous tongues of humus penetrate from upper horizon. Indistinct lamellar structure; porous; hard ortstein concretions of dark brown colour. Thickness 20 cm. Olive-yellow; lamellar stricture breaking down to flat shiny aggregates with pores. Large number of hard ortstein concretions. Thickness 25-30 cm. Brownish yellow sticky clay.”
The dull colors of this profile suggest infiltration of humus and a certain amount of gleying due to impeded drainage. The soil could be described as a humic gley. In proposals for a new soil classification, Gerasimov et al. (1939) included under gray forest soils, brown earth (burozem) and podzolic soils, a subgroup of “sod soils” which was defined as “soils with a greater than typical accumulation of rather saturated humus mostly under grasslands or forest with a well-developed grassy cover.” In other words the sod process was thought of as secondary. This scheme was attacked in principle by S. P. Yarkov (1942), a student and colleague of V. R. Williams, who remarked that “certain pedologists do not recognize the sod-forming process as an independent one.” Following Williams, he considered that the true podzol was found only under close-canopy forest, allowing for variations due to relief and parent material. He considered that the podzolizing process was most vigorous when swamping of the upper layers occurred, but this gave rise to “false podzols” which should be separated from the normal type. The sod-podzolic soils arose as described by Williams, and various subtypes could be distinguished by the thickness of the A2 horizon: weakly ( < 10 cm.), medium (10-20 cm.), and strongly ( > 20 cm.) podzolic, straw-colored podzolic ( Afanasiev ) , and yellow podzolic. However, Yarkov, like Williams, provided no evidence that would enable a satisfactory division to be made between podzolic and sod-podzolic soils apart from the nature of the ground vegetation.
As mentioned above, Tiurin (1935) was able to show that there was an increase in total organic matter in the meadow compared with the forest soil, particularly in the Al, but also to some extent in the A2, horizon. This increase was largely due to the more vigorous root system of the grasses. Quantitative differences showed in a smaller content of uronic acids and a higher content of the lignohumus fraction in the meadow soil. He regarded the grassland soil as showing a slight similarity to the chernozem. In more recent work by Kononova (1951) it has been shown that there is a distinct difference in the proportions of humic and fulvic acids in the podzolic and sod-podzolic types (Table 11). TABLE I1 Humic and Fulvic Acid Contents of Podzolic Soilsa Soil Strongly podzolic soil (Komi A.S.S.R.) Sod-podzolic soil (Yaroslav region)
Fulvic acid (11)
S 1 2 c m . 2.31 15-20m. 3.49
6.93 1.69 1.00 0.52
1.55 0.25 0.14 0.07
1.96 0.45 0.24 0.14
0.79 0.58 0.57 0.50
B,, A,, 4- 7cm. A,A,, 7-15 cm. A,A,, 15-23cm. A,,, 23-38 cm.
Data from Kononova (1951),
Both soils occurred in spruce forest, that from Komi with an undergrowth of spruce, birch, and cedar and a well-developed raw humus ( 8 cm.); that from Yaroslav with a mixture of birch and ash, and ground cover of herbs and grasses (no mention of an A0 horizon). The differences are quite striking, particularly in the apparent mobility of the fulvic acid in the Komi soil, which is probably texturally similar to the sod-podzolic soil, both being formed on silt-loam drifts. Such differences are considered to be due to the presence or absence of the grassy ground vegetation, but no data are available for a comparison of open- and closedcanopy forest in the same region. The ratio of fulvic and humic acids has been suggested by Duchaufour (1957b) as a criterion to distinguish lessiuS and podzolized soils. VII. Geographical Variants
Perhaps we may take as the first recognition of the fact that there might be geographical variants of “podzolic” soils, the suggestion by Glinka (1911) that the Ramann “Braunerde” was really a slightly podzolized soil that had through cultivation lost its bleached layer. The
THE PODZOL AND PODZOLIC SOILS
latter, according to Glinka, was unlikely to be strongly developed mainly because in western Europe, with its higher temperature, longer growing period, and higher rainfall, the decomposition of organic residues would be more energetic. He considered that the “Braunerde” of western Europe was, “as it were, the last stage of the podzolic (acid) type of weathering, intermediate between the latter and the more southern krasnozems and terra rossa.” Glinka compared these “Braunerden” with the soils of the Novo-Alevandrovsk Experiment Station which were undoubtedly podzolic in character both from morphology and total chemical composition. The first to provide some evidence for Glinka’s theory was Afanasiev (1926a), who, in discussing the soils of Bielorussia (White Russia), pointed out that the strongly podzolic soils on clays could be distinguished from those to the north and east of that country. “The most important and interesting characteristics of these soils . , . are (1) the straw-yellow tint of the podzolic horizon; ( 2 ) obvious and deeply penetrating tongues of siliceous formations and ( 3 ) weak development of the underlying, illuvial B, horizon; in comparable soils to the north-east (outside Bielorussia) the podzolic horizon is usually light grey and whitish and the B horizon sharply defined.” Afanasiev, like other before him (e.g., Zakharov, 1910, 1911; Tumin 1912), felt that it was too much to expect uniformity of morphology over the whole of the enormous area shown as podzolic soils on the soil map: as well as variations by latitude, there must also be longitudinal changes. He suggested that the Moscow region might be taken as the central area, with variants to a still more continental climate toward the east and to a more oceanic type toward the west. The latter set of variants he named yellow podzolized in his account of Russian soil classification ( Afanasiev, 1927), and a visit to Czechoslovakia confirmed him in his view (Afanasiev, 1926b). After the 1st International Congress of Soil Science, when an opportunity was afforded of viewing the soil changes from Canada to the southern United States, he could write: “If we compare the colours of the forest soils of temperate latitudes with examples from Western Europe and the United States, it is evident, as should have been expected, that the forest soils of these latitudes are, in the colour of their A horizons, transitional; the grey tones on passing to the south take on a brownishness, a straw-colour, a yellowing, changing very gradually into zhetozems” ( Afanasiev, 1930). He equated his “straw-coloured podzolic soils with the gray brown podzolic soils. This point was not taken up at the time by his colleagues although he showed on the soil map of Bielorussia (1926a) quite large areas of “straw-coloured podzolic soils in the vicinity of Senno (south of Vitebsk), to the east of Vitebsk, and to
the west of Slutsk. On the recent soil maps of the U.S.S.R. this soil type has been merged in the sod-podzolic group (e.g., the 1:4M soil map of 1954), perhaps with some justification, as Nogina (1952) has shown that the evidence in favor of Afanasiev’s suggestion is contradicted by other facts. In her view these soils are simply variants of the sod-podzolics that occur in other regions. However, the geographical aspect of the group of podzolic soils had been raised by Zavalishin (1944) during consideration of podzolic soils of the Trans-Ural region. He pointed out that two groups were in general recognized: (1) Typical podzolic or peaty podzolic soils with a raw humus or peaty horizon. These had a higher acidity in the upper part of the profile, the maximum being usually in the A0 or AoAl horizon. Leaching had been strongest in the upper horizon (i.e., A1). This soil type is associated with “the root system and leaf-fall of spruce forests and their moss cover.” ( 2 ) The sod-podzolic soils had a more or less humic Al horizon and a great profile thickness due to the development of this horizon, a depression of the level of the podzolic A2 horizon, and an increase in thickness of the B horizon. They were also acid, but the maximum acidity was usually at a depth of 30-40 cm.from the surface. The most impoverished horizon was not the Al but the Az. This type occurred in the southern taiga with mixed and broad-leaved forest together with a grassy ground flora, as well as mosses. Zavalishin pointed out that there is no sharp boundary between the two types and asked: “Can one regard strongly podzolic raw humus soils of north-eastern European Russia or Trans-Ural as having the same degree of podzolization as a sod-strongly podzolic soil at the southern border of the podzolic zone under broad-leaved forest. Or is it more correct to regard them as independent series of varying degree of podzolization increasing in some or other directions (e.g., to the north or to the south)?” The point made by Zavalishin about the differences in the way in which the acidity varied down the profile was also discussed by Rod6 (1944), who concluded that the differences were of degree rather than of kind. However, some previous results obtained by Zavalishin for the pH values of soils in the Kuznetz basin, and quoted by Rod6, show that in the more strongly podzolized soils the minimum pH occurred in the B horizon. In soils from Lisino (near Leningrad) the minimum p H was in the surface layer. The type of variation in acidity referred to by Zavalishin is shown in Table I11 where pH data for various kinds of podzolic soils are given. The gradual decrease in acidity with depth is characteristic of sandy podzols described in western Europe and North America. The first two soils in Table I11 are sandy clay loams in the surface and loams and clay
THE PODZOL AND PODZOLIC SOILS
loam in the subsoil; the sod-podzolic soils have sandy loam surface soils over loam subsoils. From an examination of the field data provided by Ufimtseva (1955) and Vadkovskaya (1955) it appears that the soils with close-canopy forest, whether it be coniferous or broad-leaved, tend to be most acid in the A, horizon; in more open forest, particularly with a grassy sward, the pH is at a minimum in the A2B or B horizon. TABLE I11 Variations in Acidity in Podzolic Soils A, soil (pH) Podzolic soil (Leningrad) 4 4.0 3.4 Podzolic soil (Archange1)b S o d - p d d i c soil ( IVZUIOV)~ 5.6 5.4 Sod-podzolic soil (Vo1ogda)b Sod-podzolic soil (Moscow)~ 4.3 Gray brown podzolic soil6 6.3 Gray-wooded soile 6.5 0 Rod15 (1937). b Uht se v a (1955). 0 Vadkovskaya ( 1955 ) . d Brown and Thorp (1942). e Newton et al. (1959).
(pH) 5.0 4.5 5.3 5.4 4.8 5.9 5.8
(pH) 5.1 4.5 4.9 6.3 4.9
(pH) 5.4 4.8 4.9 6.9 5.2 5.2 4.8
(pH) 6.9 7.2
7.3 7.4 7.6 7.3
Zavalishin (1954; Zavalishin and Nadezhdin, 1957) returned to this question of distinguishing the main subtypes of soils in the forest zone of European U.S.S.R. by a study of the Baltic region. He regarded the sod process as the “transformation” of soils under the intluence of a “meadow grassy meso-hydrophyl vegetation” with a consequent concentration in the accumulation horizon (A,) of humified organic matter, more or less saturated with bases and sesquioxides. However, he concluded that the sod process in the western Russian plain was stimulated by calcareous conditions in the ground-water and parent material. Some of the soils described were extremely acid throughout even under oak-hornbeam forest; cultivated variants showed both apparent clay shift and a minimum pH in the B or BC horizon. However, no adequate data were given to enable a separation of podzolic and sod-podzolic soils to be made. VIII. The Western Contribution
In the nineteenth century, theories regarding the problems of the podzol stemming from investigations in Russia and elsewhere developed concurrently in many parts of the world, although with differences in nomenclature. The Russians quite early recognized the similarity of their
podzols to the Bleisand of Senft and Muller, but the reverse process seems to have been a much slower one. When the Russian ideas and the name first spread further is not clear, but in the main it took place through Germany. This is not entirely surprising in view of the proximity of the two countries and the attraction of German laboratories for Russian workers. The first of the Dokuchaiev school to study in Germany appears to have been V. R. Williams, who carried the Russian ideas to the Wollny laboratories at Munich. It is to Ramann ( 1911), however, that we are most indebted for adopting the Russian term podzol and equating it with Bleisandboden, which he thought was a misleading name. Bleicherde he suggested should be used as a group term to cover podzol soils, Grauerden, and others that were depleted in iron and had become bleached. The Russian ideas were adopted surprisingly slowly although information about them had been widely disseminated. V. R. Williams had organized the Russian agricultural exhibit at the Chicago Fair in 1894 for which Dokuchaiev wrote an account in English of Russian soils. Later, Sibirtzev presented a paper in French on the soils of Russia to the 7th Geological Congress and Dokuchaiev’s soil map of the world was shown at the Paris exhibition in 1901. About the same time an excellent English translation of one of Sibirtzev’s accounts of Russian soils appeared in the Experiment Station Record ( Fireman, 1901) . Although in Germany the Bleisand with its associated ortstein was taken as equivalent to the podzol, little consideration was given to the possibility of the “Molkenboden” being similar to the more commonly found heavier-textured podzols of the Russians. Sibirtzev ( 1898) had pointed out that the Russian term podzol nearly corresponds to the German expression Bleisand, but is also applied to loamy and clayey soils if they are clearly affected by the chemical leaching processes characteristic of Bleisand. However, German workers devoted most of their attention to the ortstein layer as it was undoubtedly a major hindrance to soil utilization, particularly when cementation had taken place. The use of the term podzol was thus limited to what w e n nssentially tbsandier varieties. The influence of the German emphasis on ortstein and orterde was reflected in a lecture by C. F. Marbut (1928) when he distinguished the Lakewood, Cecil, and other soils as being podzolic soils developed under the influence of a relatively weak operation of the podzolic process. Although they had light-textured A horizons and heavier-textured B horizons, the latter differed from the ortstein of the “true podzol” in having a percentage of organic matter no higher than was present in the A horizon and in not being indurated as a rule, A similar statement could equally well have been made of many of the true podzols of the Russians.
THE PODZOL AND PODZOLIC SOILS
A selection of data for humus and nitrogen contents of Russian podzolic soils is given in Table IV. Only the peaty sandy podzol shows B humus-illuvial horizon, and the figures for the strongly podzolic soil under pine forest are similar to those for the sod-podzolic soil. TABLE IV Organic Matter and Nitrogen in Podzolic Soils
Horizon Soil Strongly podzolic soil on till under pinea Strongly podzolic soil on till under meadowa Peaty sandy podzol, humus illuvial; forestb Clayey podzol on till0 Sandy straw-colored podzolic soild a
Constituent Humus ( % ) N (%I C:N Humus ( % ) N (%I C:N Humus ( % )
C:N Humus ( ”/o ) N (%I C:N Humus ( % )
3.81 0.14 15.7 4.84 0.18 15.6 37.55 0.99 22 7.92 0.32 14.3 2.0
A2 0.81 0.04 11.7 1.22 0.09 8.0 0.36 0.02 10 0.61 0.031 11.4 1.4
Bl 0.41 0.05 4.8 0.41 0.05 4.8 0.92 0.04 9.6 0.42 0.035 7.0 0.7
17.4 0.43 0.048 5.2 0.3
Tiurin ( 1935). Rod6 ( 1937). Kossovich and Krasiuk (cited from Rod6, 1937) : first layer A,A,; last layer C,. Nogina (1952): layers 3 and 4, A,B and B,, respectively.
The advent of Glinka’s “Die Typen der Bodenbildung (1914) stimulated interest in the main groups of soils, but its description of podzolic soils and podzols on clay and loess did little to alter the prevalent idea that such soils were confined to sandy parent materials: the later work of Frosterus (1914) and Tamm (1920) rather confirmed it, although Tamm (1930) described a “mull podzol” which, from its morphology, could equally well have been called a sod-podzolic soil. During the 1930’~~ both in the United States and Britain the analysis of the clay fraction of soils became a standard procedure in soil studies and at once provided a fairly certain distinction between podzols and brown earths (including gray-brown podzolic soils). This aspect received strong emphasis by Robinson (1930, 1949), who showed that in the Welsh podzols there was a marked change in the composition of the clay in passing from the A to the B horizon in comparison with the relative constancy in the whole profile of the brown earths. This received confirmation in the work of Anderson and Byers (1931), who examined
the clay fraction from American podzols, gray-brown podzolic, and redyellow podzolic soils. There were even more marked differences in the clays from their podzols than in Robinson’s, whereas in the other soils the composition was almost constant. The use of the clay fraction in defining soil groups was extensively used in Britain (e.g., Kay, 1934, 1939; Muir, 1934, 1935; Muir and Fraser, 1940; Robinson, 1935). Muir showed that the separate ratios of alumina to iron oxide and of silica to alumina were of more value than the ratio of silica to sesquioxides, as the marked differentiation of iron and aluminium in podzols was a characteristic feature (Table V ) , The method for distinguishing between podzols and brown earths using acid-oxalate extraction introduced by Tamm (1920) was applied by him and by Aaltonen (1935) in Finland to elucidate the rate of podzol development. Tamm’s method was used by Lundblad (1934) to distinguish brown earths and podzols and, together with clay analyses, was used by Muir (1935) to separate podzols and a variety of brown earth common in Britain. The method is not satisfactory for heavier textured soils (cf. Tamm, 1934), but this difficulty has been overcome by the use of dithionate (Deb, 1950; cf. Duchaufour, 1956). These variations in clay fraction ratios, however, give no indication of the mineral constituents of the various horizons. MacEwan (1948) showed for some Scottish podzols that there generally appeared to be an increase in the illite content at the expense of biotite, hydrobiotite, and chlorite. Tedrow (1954) showed an increase in the quartz content of the clay from Az horizons of podzols; the dominant minerals in the soils were interstratified 2:l layer silicates and illite. Recent studies by Jackson and his co-workers (e.g., Brown and Jackson, 1958) have shown that in some sandy podzols montmorillonite may form in the A2 horizon at the expense of a vermiculite-chlorite that is present in the B and C horizons. None of these workers produced evidence for mineral synthesis in the B horizon. In the case of the gray-brown podzolic soils, McCaleb (1954) found that illite was the dominant mineral in all horizons; and in the more base-saturated profiles, some vermiculite and montmorillonite. The more acid soils had some kaolin. In soils of a similar character Avery et d. (1959) showed a tendency to increase in vermiculite content-possibly at the expense of clay mica. On the morphological side it was generally accepted that raw humus was characteristic of the podzols while mull was one of the principal features of brown earths in the widest sense (Robinson, 1949). The latter soils lacked any marked deposition of iron oxides in the subsoil, the profile having a much more uniform appearance, although in the more strongly leached varieties there might be a clear textural and structural
TABLE V Variation in Clay Fraction Ratios of Podzols and Podzolic Soil Soil Brown eartho Soil (England) Podzol ( Scotland)b
2.94 2.90 3.03 5.85 3.22 5.72 3.15 3.98
M2O3/Fe2O3 Si02/A1203 A1203/Fe203
Cray-brownd Podzolic (USA.) a Kay (1939). Muir (1934). 0 Anderson and Byers ( 1931). d Brown and Thorp (1942).
3.04 3.00 2.41 1.58 1.85 0.84 3.05 3.66
3.10 3.05 1.80 2.58 1.83 2.40 3.70 3.12
3.38 3.02 1.38 5.90 2.12 3.71 3.21 3.10
1.03 4.07 -
F 5 8
change with depth. Such varieties were represented by the gray-brown podzolic soils and what were later to be called sols lessiue’s by French workers (cf. Duchaufour, 1956, 1960) and the Purubraunerde of the German workers (cf. Altemuller, 1956; Kubiena, 1956). The microscope had often been used in soil mineralogy, but its applications by Kubiena (1938,1953) to the examination of soil fabrics was a revolutionary stage in the study of soil morphology. Kubiena’s techniques, particularly that of thin sections, have now been widely used and, as applied to the groups of podzols and podzolic soils (lessive’, etc.), have provided more definite morphological criteria for their separation. However, although Kubiena gives Dokuchaiev credit for priority in the use of the term podzol, it is certain that many of the soils described by Dokuchaiev and his colleagues would not qualify for inclusion in Kubiena’s group of podzols. IX. What Is Podzolization?
The discussion of the geographical variants of podzolic soils has shown that, while there was a general realization of differences in both morphology and chemistry between podzolic and sod-podzolic soils, the kind of data that it had been customary to collect did not satisfactorily distinguish between the two types if they were to be regarded as more than variants. This is not entirely surprising, for from the beginning the terms podzol and podzolic had been loosely used. Both Dokuchaiev and Sibirtzev criticized this tendency, but with little effect. Thus, in an excellent account of the aspen groves of the Voronezh steppe, Popov (1914) repeatedly refers to podzolized solonetz. Such usage, at least in the earlier period, was not unnatural as the folk term referred solely to the bleached layer and neither Dokuchaiev nor Sibirtzev had ever clearly distinguished an illuvial horizon except in the sandy varieties. However, following Vysotzky’s definition of the illuvial horizon the terms podzol and podzolic were normally restricted to the soils found in the taiga zone. From the time of Senft and Dokuchaiev it was agreed that the basic process involved was the decomposition of the primary minerals with subsequent removal of sesquioxides together with more or less humus from the surface soil. Residual quartz and secondary silica formed the gray or white surface layer. The analyses quoted by Dokuchaiev and his co-workers were almost all confined to the bleached layer, and they invariably showed an extremely high silica content. Early Russian data for the other horizons are scarce, but analyses given by Sibirtzev (1898) are shown in Table VI.
THE PODZOL AND PODZOLIC SOILS
TABLE VII Podzol from Tikhvin: Total Chemical Analysis5 Sample Soil covering to pOdZ0l Podzol Subsoil of the
Loss on ignition
Data from Georgievsky (1888).
THE PODZOL AND PODZOLIC SOILS
Later Russian workers carried out more complete chemical analyses, and many hundreds, if not thousands, must be in print. They were usually supplemented by determinations of exchangeable bases and organic matter and sometimes by analyses of the HC1, H2S04,KOH, and water extracts. Mechanical analyses when made were only rarely extended below 20 p. Bulk chemical analyses, of course, provided clear evidence of apparent movement of material from the surface layer to the subsoil, and the distribution of exchangeable bases showed the leaching of the more mobile ions. The KOH extraction seems first to have been used by Georgievsky (1888) as an improvement on the soda extraction introduced by Il’enkov (1869) in an attempt to study the state of the silica in the “podzol.” Il’enkov and later Amalitsky had considered “amorphous” silica the cause of the adverse agricultural properties of the soil. A selection of Georgievsky’s data for a sandy loam podzolic soil is given in Tables VII and VIII. Georgievsky concluded from his results TABLE VIII Analysis of KOH Extracta Sample SiO, Fe,O, Podzol 0.112 1.45 Subsoil of podzd 0.99 0.06 a Data from Georgievsky (1888).
A1,0, 0.56 1.52
CaO 0.05 0.02
MgO Trace Trace
that, although some amorphous silica might be present, the removal of alumina, etc., suggested that aluminosilicate materials had been attacked in the treatment. He thought that the small amounts of silica extracted could not appreciably affect the soil properties, as had been suggested. The 5% KOH extraction was later used by Gedroiz (1926) to distinguish between podzolic and solodized soils. The latter invariably showed a large excess of extractable silica over the amount required to form kaolin with the alumina simultaneously extracted; podzolic soils showed little or no excess. No development in the laboratory study of podzolic soils was made until in the early 1930s A. A. Rod6 began to publish the results of his researches on the soils of the Lisino Forest near Leningrad. These results, together with others, formed the basis of his monograph on the podzol-forming process (Rode, 1937), which was partly an extension and partly an amplification of 0. Tamm’s work on Swedish soils, with a wider survey of the available literature. In addition to detailed total chemical analyses of podzolic soils, Rod6 also gave a range of clay analyses, which if they had been more extensive might have provided
the basis for a distinction between the ‘‘true” and the “soddy” podzolic soils. Rod6 (1944) concluded that “the nature of the podzol-forming process, as regards the mineral part of the soil, consists in the complete breakdown of all minerab of the parent material, except quartz, and the removal of all products of this decomposition from the upper layers of the parent material.” He considered the most characteristic feature of the process to be the instability of the secondary clay minerals. In a later account, Rod6 (1955) again stressed these aspects and in regard to the sod-forming process remarked that indications of it can be found in all three zonal subdivisions of podzolic soils, even in the absence of a grassy vegetation. He therefore, concluded that there is no strict division between podzolic and sod-podzolic soils, a view that would not find general acceptance with Russian pedologists. Rod6s definition of podzolization implies differentiation of the clay fraction, but this is not so in many sod-podzolic soils which thus cannot be regarded as “true” podzolic soils (see p. 44 and Table XI). Yarilova and Parfenova (1957) suggested that clay is resynthesised in the B horizon of podzolic soils, a view to which Rod6 also subscribes. This, however, is not substantiated, although not excluded, by their data, which could equally well be given by clay that had been moved from the A horizon. Other Russian pedologists (Minashina, 1958; Fridland, 1958; Karpachevsky, 1960) accept clay movement. TABLE IX Clay Fraction Data for Lisino Podzolic SoiP Fraction
Fraction 2.5-0.25 p
< 0.25 p
A1 A2 B C
26.5 29.2 33.0 39.5
4.32 4.12 3.79 3.89
5.63 5.26 3.89 4.46
1.7 1.3 23.2 17.7
2.26 2.04 2.49 2.49
3.74 4.70 3.01 2.94
Data from Rod6 (1937).
Clay fraction data obtained by Rod6 are given in Table IX. No standard clay fraction has ever been adopted, so that results by different authors are not always strictly comparable. Rode’s coarser clay probably contained quartz, which makes the Si02/A1203 value for it somewhat high, particularly in the upper layers. The relative uniformity of the si1ica:alumina ratios does not suggest alteration of the clay to the extent indicated by the data for the podzolized soils given in Table V. The differentiation of iron oxide and alumina is, however, clear in both coarse
THE PODZOL AND PODZOLIC SOILS
and fine fractions. The ratios for the fine clay shows that it is the more aluminous and ferruginous and is the main fraction that is migrating. However, on the clay fraction criteria mentioned above, this soil cannot be regarded as strictly comparable with the sandy podzols of the United States and Great Britain. Clay mineral analyses have been made on Russian podzolic soils with results that are probably similar to those obtained elsewhere for this general group. The results obtained by Rod6 and Sedletzky (1939) for sandy podzols indicate an interstratified mineral or vermiculite as the dominant constituent of all horizons, with little variation with depth. Yarilova and Parfenova (1957, 1959) have published similar data on B horizon samples from Russian podzolic soils. Skrynnikova ( 1958 ), from a consideration of pedogenic processes in the forest zone, concluded that podzolization is no longer an active process, except perhaps in some northern podzolic soils under conifers. She believes that podzolization was initiated at the end of the Ice Age under a colder and moister climate than the present. The advent of broad-leaved forest in the south of the forest zone has caused a reversal of the earlier processes, leading to “regradation.” She does not clearly define podzolization, but implies decomposition and leaching of mineral matter so that the Al is the most impoverished horizon. Under broadleaved forest and meadows there is not only an improvement in base status in the A horizon, but an increase in clay content in the A1 horizon following decomposition of mineral-rich litter. This higher clay content in the Al horizon is very common in what have been called sod-podzolic soils, and in many it is a very striking feature. A few examples are given TABLE X Clay Content of some Podzolic Soils ( ”/o of Air-Dry Soil)@
I I1 I11 IV V VI A, 4.47 3.39 14 12 9 15.9 5.13 0.04 9 6 8 15.5 A2 13.65 15 8 6 16.4 -42B 9.80 16.39 20 13 22.7 Bl B2 21.87 14.45 28 13 36.9 C 19.34 11.97 21 14 14 21.3 5 I, Strongly podzolic soil, Leningrad ( Rod&, 1937 ) : < 2 c~ clay. 11, Loamy podzol, Moscow (Gemmerling, 1930) : < 1 ~1 clay. 111, Podzolic soil, Archangel ( Ufimtseva, 1955) : < 1 P clay; last layer CD. IV, Sod-strongly podzolic soil, Vologda ( Ufimtseva, 1955) : < 1 p clay. V, Straw colored sod-podzolic Bieloxussia (Nogina, 1952) : < 1 clay: last layer CD. VI, Gray-brown podzolic (Brown and Thorp, 1942) : < 2 p clay; third layer is Horizon
in Table X with “true” podzolic and gray-brown podzolic soils for comparison. Some western European soils of the lessive’ type also show a somewhat higher clay in the A1 than in the A? horizon (cf. Duchaufour, 1957a; Avery, 1958) although none of these is as strongly bleached as the Russian soils. Results more comparable with those for sod-podzolic are given by Muckenhausen (1957) for a pseudogley. The same feature is seen in the gray-wooded soils of Canada (Newton et al., 1959), but it does not seem to occur in the gray-brown podzolic type. The distribution of podzolic soils with lower clay in the A2 horizon is not clear, but it is possible that they represent cases of extreme degradation of former “chernozemic” soils as described by Glinka (1924). From data given by Zavalishin and Firsova (1960), it appears that in the zone north of the 60th parallel podzolic soils should not show this feature, but exceptions occur as can be seen from the data for the Leningrad and Archangel soils in Table X. These presumably reflect purely local conditions. There is no very clear connection between the occurrence of this very depleted A2 horizon and vegetation, and it may occur in the presence or absence of raw humus. Of the soils listed in Table X, I and I11 occur under a raw humus layer; Zavalishin and Firsova (1960) describe a “strongly podzolic raw humus soil” from Komi which shows a decrease in clay in the lower A?. Russian ideas on podzolization have undergone a change since Fridland (1958) introduced the term and concept of “illimarization,” which is the exact equivalent of the “lessivage” of Duchaufour. The process of illimarization he considers to operate in brown forest soils, yellow podzolic soils, etc., and partly in the sod-podzolic group, whereas podzolization is the effective process in “typical podzolic” soils, gley podzolic, and others. The distinguishing feature is the presence or absence of differentiation in composition of the clay fraction down the profile. Fridland also recognizes that there are intermediate phases in which podzolization may accompany illimarization. The question of what are “typical podzolic” soils is left unanswered. The issue was somewhat confused by Gerasimov (1959) when he stated that “soils of a gley-pseudopodzolic type have been called sols lessiue’s and pseudogleys in Western Europe. Thus they have been rightly distinguished from the true podzolic soils characteristic of Eastern Europe, but have been wrongly regarded as genetic formations contrasting with and of equal status to that group.” Gerasimov does not define his “true podzolic soil” but he accepts Rodd‘s definition of podzolization which, as we have seen, implies a marked differentiation in the clay complex. A selection of Russian clay fraction data is given in Table XI which shows that there is no great differentiation in
THE PODZOL AND PODZOLIC SOILS
these sod-podzolic soils. The Al horizon tends to be somewhat more siliceous than the A2 horizon, which may indicate a certain breakdown of the clay minerals, but this is also shown by some soils that would be called Zessiue' by Duchaufour's definition. The difference could equally well be, and is more probably, due to the removal of the finest clay, as indicated by Rode's data. The figures in Table XI given by Karpachevsky TABLE XI Variation in Clay Fraction Ratios for Podmlic Soils Horizon
soil Sod-podzolic Moscow regiona Moscow regiona Bielorussiab Bielorussiab Moscow regionc Moscow regiond Podzolic Kama Rivers a b
SiO,/Al,O, A1,O3/Fe,O3 Si0,/A1,03 A1203/Fe203 Si02/A1,03 Al2O3/Fe2O3 SiO,/AI,O, A120,/Fe,03 SiO,/AI,O, A1203/Fe203 SiO,/AI,O, A1,03/Fe203
4.98 4.32 4.26 3.70 3.72 2.58 4.56 2.96 3.26 3.26 4.7 4.5
4.13 3.90 3.09 4.43 3.59 2.61 4.12 2.64 3.63 3.44 4.0 3.5
4.08 3.68 3.00 4.23 3.20 2.80 3.90 2.34 3.12 3.05 3.5 3.5
3.33 2.67 4.4 2.5
Morozov (1940). Nogina (1952) Vadkovskaya (1955). Karpachevsky (1960). Rod6 and Sedletzky ( 1939 )
(1960) for a soddy medium podzolic soil of loamy texture under oak show a more marked differentiation of iron oxide and alumina between the A1 and A2 horizons than most of the other results. He nevertheless believes that Rode's argument for the breakdown of secondary minerals is wrong and that translocation of unaltered clay minerals can occur in both light- and heavy-textured podzolic soils. A more extreme example is that of the Kama River soil which contains only about 1.5% clay in the B horizon and is thus comparable with the American podzols referred to in Table v. Thus, on the available evidence of clay composition no clear distinction can be made between the two main Russian classes of podzolic soil. They both show a marked apparent transfer of clay to the subsoil, but
a E X . MUIR
without any strong differentiation of its constituents. This does not preclude transformation from one clay mineral to another (e.g., possible increase in vermiculite or illite in the A horizon), but it does not support RodGs contention unless one assumes equal rates of removal of the main weathering products and that the new minerals formed in the subsoil have the same chemical composition. The very high losses, particularly of silica, from podzolic soils shown by Rod6 (1937) could well be accounted for by the breakdown of primary minerals. There would inevitably be some loss of the weathering products before resynthesis of clay was complete. In any case the change from, say, feldspar (SiO2/ A1203 = 6 ) to a clay mineral (SiOz/A1203 = 2-4) involves a significant loss of silica. It would seem that, on the basis of the evidence adduced so far, the Russian podzolic soils on heavier textured materials have much in common with the sols lessive‘s (and Parabraunerde) of western Europe as well as the gray-brown podzolic and gray-wooded soils of North America. As many of the first group were formerly considered to be brown earths, Glinka’s suggestion that they formed the western end of a sequence through from the podzolic soils of eastern Europe was nearer the truth than at one time seemed to be the case. So too with Afanasiev’s equation of gray-brown podzolic soils and his straw-colored podzolics. Thus, it appears that the concept of podzolization as it applies to the great majority of Russian soils described as podzolic is simply an extreme expression of “lessivage.” X. Micromorphology
The use of thin sections to investigate micromorphology has only recently begun in Russia, and little information on soil fabrics is yet available. Karpachevsky ( 1960), Minashina ( 1958), Kundler ( 1959), and P. Bullock ( 1960, unpublished) have described the micromorphology of some podzolic soils in sufficient detail for comparisons to be made with Kubiena’s and other criteria. Yarilova and Parfenova (1957) printed excellent photographs suggesting clay deposition in B horizons, but gave no indication of the soils from which the specimens came. Two sandy soils, one from Leningrad (Bullock) and one from Moscow (Karpachevsky), showed all the features of Kubiena’s humus or humusiron podzols. P. Bullocks description is as follows:
Largely clear quartz grains, no evidence of iron oxide or humus; little clay. Largely quartz sand; little clay; a few iron oxide concretions. Quartz grains coated with humus which forms bridges be-
THE PODZOL AND PODZOLIC SOILS
tween the grains and shrinks to form a network of fissures; local accumulations of humus pellets. Mainly quartz grains with little iron oxide; some humus coatings locally.
Sod-podzolic soils were described by all the authors mentioned; the description by Kundler is as follows: Al
Porous crumbly fabric; finely dispersed organic matter mixed with minerals, many fine plant remains; frequent small dark brown concretions; the clay is unoriented. Consists of a series of roughly parallel layers (0.5-3 mm. thick) of a light yellow tint; the fine material is “unoriented; conspicuous dark brown concretions. Soil mass in section is yellowish brown and interspersed with a network of strongly birefringent platy minerals and mineral aggregates; the skins on the peds and linings to pores show layering and strong birefringence.
Bullock’s descriptions of a loamy podzol and a loamy strongly podzolized soil from the Moscow region correspond quite closely to that of Kundler, and they both show much strongly birefringent clay in the channels and pores of the B horizon. The schistosity in the A2 mentioned by Kundler was not seen in Bullock‘s specimen. It is unfortunate that the heavier-textured podzolic soils so far examined in thin section are those occurring in the sod-podzolic zone. However, Karpachevsky considers his sandy soil to be a sod-podzolic. It thus appears that sandy soils in the sod-podzolic zone can have the micromorphology of the western and American podzols. The heaviertextured podzolic soils have all the attributes of strongly developed sols lessiv6s or bleached Parabraunerde. Whether an examination of heaviertextured soils from areas shown on the soil map as unqualified podzolic soils will prove that they are likewise of the lessive‘ type remains for the future. XI. The Characteristics of the Russian Podzol and Podzolic Soils
Gorshenin ( 1958) recommended that the podzolic and sod-podzolic soils should be recognized as independent soil groups at the highest category of classi6cation. Tiurin (1960) has written that soils called ilovka and gley soils by Sibirtzev “can now without difficulty be recognized as pseudogleys (or surface gley podzols) .” He also mentions that Sibirtzev had commented on “the yellowish colour of the podzolic horizon
in sod-podzolic soils on the loess-like loams of Bielorussia and the plains of Central Europe. Such soils are now known as sod-straw-yellow podzolic (or sols lessivds).” However, this statement does not go far enough, for Nogina’s data showed that there was really little difference between the ordinary sod-podzolic and the straw-yellow variety. It has been shown above that in the absence of an adequate definition of a podzolic soil the present course seems to be to make a subdivision as already done in the West: i.e., the sandy soils of the north (and perhaps in the southern taiga also) correspond to the humus and iron-humus podzols of Kubiena whereas the podzolic soils on medium and heavytextured materials belong to the lessive‘ or Parabraunerde group. On the available data it is possible to distinguish two types within this broad group which correspond to Russian division into podzolic” and “sodpodzolic,” but it is doubtful if they are independent groups as Gorshenin suggested. They are, rather, expressions of degree of development ( RodB, 1944) or alteration due to human interference, as indicated by Tiurin (1935), or to difference in age. The sod-podzolic perhaps should have a subdivision for those soils that show a relic humus horizon in the lower Az. Kubiena (1953) has suggested that his Molkenpodzol is equivalent to Georgievsky’s clayey podzol although the description of the latter could well apply to soils that are probably of the lessive‘ type but are influenced by surface waterlogging. This effect of surface-waterlogging can be recognized in many of the heavier-textured Russian podzolic soils and podzols and has been commented on by many Russian workers. Some, like Yarkov (1954, 1956), have put considerable emphasis on anaerobic reduction processes in the formation of podzolic soils. There is no doubt that these must be responsible in part for concretion formation in the Az horizon, but there is no evidence that they are essential for the translocation of ferruginous clay. Once the textural and structural B horizon is well formed, temporary waterlogging in cracks and fissures can readily occur and give rise to the bluish gray colors that are sometimes seen on clay skins. It may be that waterlogging in the surface contributes to the breakdown of the structure aggregates at the top of the lower B horizon (cf. Grossman et al., 1959, p. 75) which results in the powdering that is such a common feature of the well-developed Russian podzolic soils (cf. p. 20). Soils showing the more extreme effects of surface waterlogging are usually separated as a subtype which approximates to the pseudogley (cf. Tiurin, 1960, quoted above ) In the description of the characters of the various Russian groups of podzolic soils attention has been concentrated on the better-drained varieties. Russian workers recognize some 20 subtypes and varieties of
THE PODZOL AND PODZOLIC SOILS
podzolic soils, some of which have a large literature devoted to them. The range of chemical analyses made is very wide, and only a selection of results that appear to be useful for characterization is mentioned in the following paragraphs.
A. Sandy podzolic soils, The podzol is probably the commonest variety. Usually associated with pine woods or heaths. 1. Field morphology. Raw humus, or moder, forms the A. horizon; the thickness is variable, 5-10 cm. The Al horizon is not well developed and if present is normally only slightly humic; its structure is weak. The A2 horizon is variable, 10-20 cm. thick; structure weak or absent. Usually sharp change to B horizon which may be more humic in its upper part ( Bl ); usually well cemented. 2. Micromorphology . Depending on the degree of development of the soil the sand grains in the A horizon may be coated to some extent with humic matter or be quite clean. The B horizon is characterized by the pronounced coating of the grains with humus or humus-iron complexes. These coatings tend to crack and flake off. Segregations of iron oxide may be present. 3. Chemistry. Sandy podzols are commonly strongly acid, this acidity decreasing gradually with depth; the minimum pH is usually in the A, or A2. Clay translocation may be shown by mechanical analysis, but in very sandy varieties this can be accounted for by “free” sesquioxides. Clay fraction shows marked differentiation of silica and alumina as well as alumina and iron. Organic matter content in B horizon normally higher than in A2 and has a high C:N ratio ( > 10). B. Loamy podzolic soils. This group appears to consist mainly of strongly podzolized soils and podzols, i.e., the A, horizon is poorly developed or absent. They occur under coniferous forest often with an ericaceous and mossy ground flora. 1. Field morphology. Raw humus or moder forms the A. horizon: the thickness varies and some varieties have been termed peaty. The bulk of roots occur in the raw humus and the Al, if present. Most of the humus in the Al is infiltrated and not due to decomposing roots. The structure of the A1 is weak, but sometimes shows a platiness; when dry breaks down to floury consistency. The A2 horizon is well developed and may be up to 15-20 cm. thick and shows a platy structure which is unstable when dry, Concretions of iron oxide are common. Humus content normally low but may reach 2 3 % . Frequently this horizon tongues into the illuvial layer, residues of which may form an AZB transition, which in part consists of a “powdering” on the B horizon peds. The B horizon is, however, clearly defined. It shows a strong increase
in clay content and has a well-developed blocky structure, the size of the aggregates increasing with depth. Clay skins on the peds are common. Concretions are usually present in the A2B, but may be absent below. Toward the bottom the structure becomes massive and passes into the parent material (varve clay, till, loesslike loam). Soil faunal activity is low. 2. Micromorphology. No data. 3. Chemistry. The soils are usually strongly acid with the minimum pH in the Ao, or Al horizon when present: the pH rises steadily with increasing depth. Maximum humus in Al horizon, decreasing rapidly below, but a few soils have been described in which there is a second accumulation at the top of the B horizon ( 2 Molkenpodxol). When this second humus horizon is present it has a high fulvic acid content. The clay shows a fairly uniform Si02/A1203 ratio, but the A1203/Fe203values suggest differentiation of the sesquioxides. The finer clay fraction is the more sesquioxidic. No data for “free” iron oxide. C. Sod-podxolic soils. This group shows the whole range of development from slightly podzolic to strongly podzolic. Podzols have also been described in which the Al horizon is vestigial, but the soils have the other properties of the group. 1. Field morphology. There may or may not be a raw humus layer, depending on the density of the forest canopy. When present it may be up to 2 inches thick; under more open conditions leaf fall is rapidly mineralized. The Al horizon usually contains abundant roots forming the “sod” which gives its name to the group. It is medium to dark gray in color, has a well-defined crumbly structure, and shows evidence of earthworm activity. Concretions are scarce. The transition to the A2 horizon is often gradual, allowing an AlA2 horizon to be separated. The A2 horizon i s commonly a slightly yellowish gray color when wet but dries very light gray. It is usually clearly lighter textured than the A1 and contains numerous concretions. The structure is usually platy. Tongues of Az penetrate in the B, the upper part of which shows a “powdering” on the peds. The powdering may extend down fissures into the B horizon. The B horizon is brown to strong brown or reddish brown in color and shows a marked increase in clay content. The subangular blocky structure is well developed, the peds increasing in size with depth until the structure is massive. Clay skins on the peds are well developed and sometimes thick. Concretions are uncommon, but segregations of manganese oxide may be present. In some soils the peds break horizontally to form coarse platy aggregates. Soils in the southern taiga (broad-leaved and mixed forest) often
THE PODZOL AND PODZOLIC SOILS
show a patchy second, “relic,” humus horizon in the middle or lower part of the A2. 2. Micromorphology. In the less strongly developed soils there may be some humus staining of the sand in the Al horizon and some unoriented clay. Concretions or segregations of iron oxide are present in both A1 and A2. The latter may show schistosity. The B horizon in thin section is yellowish brown with much birefringent material and oriented clay films in fissures and pores which decrease in frequency with depth. No information is available on the fabric of the second humus horizon. 3. Chemistry. The soils are moderately to strongly acid, the pH of the A1 horizon commonly being higher than that of the A2. The minimum pH is in the A2B or B in the more typical examples. The humus content is highest in the Al and usually falls sharply in the A2, decreasing still further with depth. The content of fulvic and humic acids is proportionate to the humus content. The fulvic acid is less mobile than in the podzolic soils. The clay has fairly uniform silica:alumina ratio throughout the profile and there is little differentiation of iron oxide and alumina. Determination of so-called “mobile” iron by the Kirsanov (1937) method (extraction in cold with 0.05 N HC1) gives lower amounts in the A2 than in the Al, the values of the latter being similar to those for the B horizon. The humus content of the second humic horizon is about 1% (cf. Ufimtseva, 1955) and is said to have a humic acid content suggesting its origin from earlier grassland conditions. XII. Summary and Conclusions
It has been shown that in the early Russian work the term podzol was applied solely to the bleached horizon that occurred at or near the surface of soils in the taiga zone. The word was by no means restricted to soils that were well drained. The appreciation of “illuviation” as an important feature of soils with bleached surface horizons led to the application of the word podzol and its derivatives to a wide variety of soils. The distinctions between many of these podzolic soils have never been clearly made, but two main groups of the better-drained varieties have been recognized: podzolic and sod-podzolic. The podzolic soils can be subdivided into those with and those without a humus B horizon. The former are commonest in sandy soils but apparently may occur in heaviertextured materials. From the available descriptions, the sod-podzolic soils may or may not carry a raw humus or moder cover. On the basis of laboratory data the sandy podzolic soils are similar to what have been called podzols or humus-iron podzols, etc. in western Europe and North America. The podzolic soils of heavier texture and
the sod-podzolic soils show a relatively uniform clay composition which, together with thin section evidence, suggests that the textural B horizon results from clay movement. In the sod-podzolic soils of the sprucemixed forest zone, however, there is usually a distinctly lower clay content in the A2 than in the Al horizon. The latter feature might serve to distinguish soils that are close to pseudogleys from those that are strongly bleached sols lessivds or Parabraunerde. The apparent disintegration of the B horizon is clear in both cases. The following tentative correlations can be suggested, although further work is essential before they can be considered as established. Russia
Sandy podzolic soils Sandy podzolic soils with illuvial humic horizon Podzolic soils Podzolic soils with illuvial humic horizon Sod-podzolic soils
W. Europe and N . America Iron or iron-humus podzols Iron-humus or humus podzols Sols lessivds or sols podzoliques (bleached Parabraunerde)
Molkenpodzol Bleached Parabraunerde, pseudogleys, or gray wooded soils
I wish to thank Dr. I. Sladits for making the translations from the German, Mr. D. V. Jones for making the diagrams, P. Bullock for the thin sections, and Dr. D. A. Osmond for much helpful discussion.
In their textbook on Soil Science, published at the end of 1960, Gerasimov and Glazovskaya (1960) accept the idea of lessiuage, i.e., translocation of clay with little or no decomposition, but consider that gleying is an important accompaniment and suggest that the complex of processes giving rise to sols lessiv6s and related soils should be called “pseudopodzolization” (cf. Gerasimov, 1959). They contrast with this podzolization-“the podzolic process”-which, they say, “is accompanied by intense decomposition of the mineral part of the soil under the influence, principally, of humic materials, and the removal from the upper layers of the soil mass of the most diflicultly mobile products of weathering and soil formation-the hydrated sesquioxides.” Following leaching of the readily mobilizable bases as humates and fulvates, there takes place “the dissolution of substances entering into the nucleus of the colloidal particles. In particular, colloidal hydrates of iron, aluminium and manganese oxides are dissolved by reaction with fulvic acids” which also attack minerals coarser than clay. “Gradually from the upper horizons,
THE PODZOL AND PODZOLIC SOILS
lying directly under the forest litter, all the mobile products of the exchange between the mineral part of the soil and organic acids are leached out; the layer becomes impoverished in colloids and sesquioxides. Quartz, being most resistant to decomposition, accumulates as the residual product.” The more mobile constituents may be removed from the soil completely, “the less mobile fulvates (or crenates) of iron and aluminium (and also colloidal solutions of hydrated silica) precipitate from solution somewhat below the podzolic horizon and form a distinct ore-like or illuvial horizon.” It is considered that the various hydrated oxides may crystallize or combine to form secondary alumo- and ferrisilicates in the illuvial horizon. In discussing the podzolic soils of the taiga forests (predominantly coniferous), in the middle zone of which the typical podzol is developed, Gerasimov and Glazovskaya repeat the above arguments, but unfortunately use data for a sod-strongly podzolic soil to illustrate the typical podzolic profile. “The sod-podzolic soils are formed by the superimposition on the podzolic soil forming process of a more or less developed process of humus accumulation.” The explanation of this process is similar to that given by Williams and others, but one is left with the impression that sod-podzolic soils are in the main simply a phase or variant. REFERENCES Aaltonen, N. T. 1935. Conimuns. Inst. forest. Fenn. 20, No. 6. Afanasiev, J. N. 1926a. Zapiski Belaruskai DzarzhaunaK Akad. ScZ’skae i Lyasnoe Gaspadurki No. 1, 92-126. Afanasiev, J. N. 1926b. Pochvovedenie No. 2, 84-89. Afanasiev, J. N. 1927. Rzcss. Pedol. Invest. No. 6. Afanasiev, J. N. 1930. “Basic Outlines of the Earth‘s Soil Cover.” (Bielarusskaya Akademiya Navuk, Minsk). AltemuUer, H.-J. 1956. 2. [email protected]
?hr. Diing. Bodenk. 72, 152-157. Anderson, M. S., and Byers, H. G. 1931. U. S. Dept. Agr. Tech. Bull. 228. Avery, B. W. 1958. J. Soil Sci. 9, 210-224. Avery, B. W., Stephen, I., Brown, G., and Yaalon, D. H. 1959. J. Soil Sci. 10, 177195. Brown, B. E., and Jackson, M. L. 1958. Clays and Clay Minerals, Proc. 5th Natl. Conf. 1956, Washington, pp. 213-226. Brown, I. C., and Thorp, J. 1942. U. S. Dept. Agr. Tech. Bull. 834. Burkhardt, H. 1870. “Saen und Pflanzen.” (4 Aufl. Hannover). Quoted from Pavlinov ( 1887). DaubrBe, A. 1845. C m p t . rend. mad. sci. 20, 1775-1780. Deb, B. C. 1950. J . Soil Sci. 1, 212-220. Dokuchaiev, V. V. 1879. Cartography of Russian Soils. In “Collected Works,” Vol. 2, p. 226 et seq. ( Acad. Sci. U.S.S.R., Moscow.) Dokuchaiev, V. V. 1886. Materials for the evaluation of the lands of Nizhny Novgorod government. In “Collected Works,” Vol. 5, p. 510 et seq. (Acad. Sci. U.S.S.R.,Moscow.)
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