Phosphate fractions in some tanzania soils

Geoderma, 10 (1973) 181-192 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

PHOSPHATE FRACTIONS IN SOME TANZANIA SOILS*

A.P. URIYO and A. KESSEBA Department o f Agricultural Chemistry and Soil Science, Faculty of Agriculture, University of Dar es Salaam, Dares Salaam (Tanzania) (Accepted for publication June 13, 1973)

ABSTRACT Uriyo, A.P. and Kesseba, A., 1973. Phosphate fractions in some Tanzania soils. Geoderma, 10: 181-192. Inorganic-P fractions were determined in 17 soil profiles from eight soil orders widely occurring in Tanzania. Most of the inorganic-P fractions decreased with depth. Where soils were young, or calcareous, or the parent material rich in phosphorus-bearing minerals, Ca-P was the dominant inorganic-phosphorus fraction. Where the soils were highly weathered, A1-Pand Fe-P were the dominant fractions. INTRODUCTION The distribution of the various forms of inorganic phosphorus in soils is of prime interest to those working with fertilizer advisory services. Chang and Jackson (1957) described a procedure based on the selective solubility of soil phosphate in various extractants, in which soil phosphorus was fractionated into five discrete chemical forms: aluminium, iron, calcium, reductant-soluble (iron oxide coated) and occluded iron-altiminium phosphates. Norrish (1968) showed that the mineral phosphate has a complex chemistry with two structural positions being occupied by P and AI. The other positions contained appreciable amounts of Ca, Sr, Ba and rare earths which would indicate that the chemistry of soil phosphorus is so complex that identification of the various forms on the basis of indirect methods should be treated with great caution. In their later work, Chang and Jackson (1958) applied their fractionation procedure in an investigation of several widely different soils. Forms of phosphorus were observed to be related to soil pH, cation activities, the solubility products of various phosphorus compounds, the d~gree of chemical weathering which had occurred and fertilizer practices. Chang and Juo (1963) have also shown that the relative amounts of the inorganic-phosphorus fractions reflect genetic differences among soils. As weathering intensity increases, iron and the reductant-soluble phosphorus tend to increase at the expense of calcium and aluminium *Part of a thesis submitted by the Senior author for a Ph.D. degree in the University of Dares Salaam, Faculty of Agriculture.

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phosphates. The latter two forms apparently are less resistant to weathering and are thought to be the principal forms of phosphorus prior to much chemical weathering. The explanation given for this is that calcium and aluminium ions have higher activities than the iron ion, the activity of which is controlled by the silicates and iron oxides. In the literature, it is not clear which phosphorus fraction is most active in the replenishment of plant-available phosphate in the soil solution. Laverty and McLean (1961) found A1-P the most-available fraction, whereas Pratt and Garber (1964) state that in the acid soils studied by them the NH4F extractable P (presumably A1-P) is the most important P fraction, other than the water-soluble P. Suzuki et al. (1963) found that in Michigan soils Ca-P and A1-P contributed most of the P removed by plants. On the other hand A1-Abbas and Barber (1964) showed in their experiments that Fe-P was most highly correlated with plant growth. Westin and Buntley (1966) found that Fe-P, AI-P and NHaC1-P all correlated well with available P determined by the methods of both Bray and Olsen in the soils studied. This is to be expected because the availability of phosphorus in a given form may not be the same in soils of different kinds. In addition to the ionic and the inorganic forms of P, this element also occurs in soil organic matter. The amount of organic phosphorus present and the ratio of carbon to phosphorus in the organic matter was used by Hawkins and Kunze (1965) to explain the response of certain Vertisols to fertilizer phosphorus. In this study the aim was to examine the distribution of the various inorganic-phosphorus fractions in some Tanzanian soils. METHODS AND MATERIALS 17 soil profiles in Tanzania were sampled and then characterized into 8 soil orders according to the 7th Approximation (U.S.D.A., 1960, 1967), namely Vertisols, Inceptisols, Aridisols, Motlisols, Spodosols, Alfisols, Ultisols and Oxisols. Their classification down to the Great Group level is shown below.

Profile No.1 Order Suborder Great Group

Vertisol Ustert Pellustert

Profile No. 4 Order Suborder Great Group

Profi&No.2 Order Suborder Great Group

lnceptisol Aquept Haplaquept

Profile No.5 Order Aridisol Suborder Argid Great Group: : Paleargid

ProfileNo.3 Order Suborder Great Group

Inceptisol Aquept Tropaquept

Profile No. 6 Order Suborder Great Group

lnceptisol Aquept Tropaquept

: Mollisol : Ustoll : Haplustoll

PHOSPHATE FRACTIONS IN SOME TANZANIA SOILS

Profile No. 13

Profile No. 7 Order Suborder Great Group

: Mollisol : Ustoll : Paleustoll

: Mollisol : Udoll : Hapludoll

: Mollisol : Udoll " Hapludoll

: Spodosol : Ferrod : Ferrod

: Ultisol : Humult : Palehumult

Order Suborder Great Group

: Oxisol : Orthox : Acrorthox

Order Suborder Great Group

: Oxisol : Orthox : Acrorthox

Profile No. 17

Profile No. 11 Order Suborder Great Group

Order Suborder Great Group

Profile No.16

Profile No.l O Order Suborder Great Group

: Ultisol : Aquult : Umbraquult

Profile No. 15

Profile No.9 Order Suborder Great Group

Order Suborder Great Group

Profile No.14

Profile No.8 Order Suborder Great Group

183

: Alfisol : Udalf : Natrudalf

Order Suborder Great Group

: Oxisol : Orthox : Acrorthox

Profile No. 12 Order Suborder Great Group

: Alfisol : Udalf : Hapludalf

METHODS OF ANALYSIS The inorganic-phosphorus fractions were determined by the method of Chang and Jackson (1957) with two modifications whereby (1) the NH4F reagent was buffered at pH 8.0 (Uriyo and Kesseba, 1972) and (2) the dithionite citrate extract was oxidized with perchloric acid instead of hydrogen peroxide (Uriyo and Kesseba, 1973). Organic phosphorus was determined according to the method of Saunder and Williams (1965). Organic carbon was determined by the wet-combustion method of Walkley and Black (1965).

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RESULTS Data for the profiles are shown in Tables I - V I I I . For all P fractions, concentration decreased with depth except in a few profiles. The Vertisols, Aridisols and Oxisols generally contained low amounts of inorganic-P fractions. In Vertisols, Fe-P was the most abundant fraction, while A1-P was next, followed by occluded aluminium iron phosphate and Ca-P. The decrease in amounts of phosphorus with depth was not consistent for all fractions and this is thought to be due to mixing which occurred in the profile because of the vertic movement so characteristic of Vertisols. In the Aridisols as shown in Table III, A1-P and Ca-P were higher than the Fe-P fractions. Ca-P increases with depth, reductant-soluble iron phosphate is present only in the top horizon, and the occluded iron phosphate is very low. In the Oxisols, as shown on Table VIII, most of the inorganic P was present in the form of Fe-P. The Oxisols, though low in A1-P, contained considerable occluded aluminium-iron phosphate and no reductant-soluble iron phosphate. The distribution of inorganic-P fractions in four Inceptisol profiles is shown in Table II. In profile 2 the A1-P fraction was very small, Fe-P and Ca-P were in equal proportions, the reductant-soluble iron-phosphate fraction was higher than the other fractions with an apparent accumulation at the bottom which could not be explained. In both profiles 3 and 4, Fe-P formed the largest portion of the inorganic phosphorus, followed by A1-P. The Ca-P was small and appeared to have shifted to the iron and aluminium forms. The reductant soluble iron phosphate and the occluded aluminium-iron phosphate comprised a substantial part of the inorganic-P fractions in these two profiles. The distribution of inorganic-P fractions in Mollisols is shown on Table IV: in both profiles 6 and 7, Ca-P was the dominant fraction, whereas iron and aluminium P were much lower. Reductant-soluble iron phosphate was higher in both profiles than that of any other inorganic P fraction and its distribution in both profiles showed no consistent trend. Both profiles 8 and 9 showed a trend of inorganic-P distribution similar to profiles 6 and 7 with the exception of Fe-P which increased with depth. Table V shows the distribution of the inorganic-P fractions in Spodosols. Most of the inorganic P was in the form of aluminium and iron-bound phosphates. DISCUSSION The distribution of inorganic-P fractions in soil profiles has been used widely in soil fertility and genesis studies. The Oxisols, which cover large areas of Tanzania, are represented in this study by profiles 15, 16 and 17. These Oxisols had low levels of inorganic-P fractions. These soils are believed to be highly weathered (Saggerson, 1962). Most of the inorganic phosphorus was found in the iron-bound and occluded forms. The Aridisols, which occur in the dry central plateau of Tanzania, are derived mainly from rocks of the basement complex (Saggerson, 1962). These soils were even lower in

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A.P. URIYO AND A. KESSEBA

organic-P content than the Oxisols. They were also particularly low in organic phosphorus and organic matter. This is due to the fact that the vegetation is very scanty because of restricted and unreliable rainfall of an annual average of less than 30 cm. The Ultisol, Alfisol and" Vertisol profiles showed fairly moderate inorganic-P fractions distribution. In the Ultisols most inorganic P is found in the iron-bound and the reductantsoluble iron phosphate forms and the A1-P was found only on the top layers. The reasons for this pattern of inorganic-P distribution are not yet clear. There is a marked contrast in the distribution of the inorganic-P fractions for the two Alfisol profiles (No.11 and 12). In profile 11 the distribution pattern is very similar to those of the Oxisol and the Ultisol profiles. Profile 11 and all the other Oxisol and Ultisol profiles were excavated from one geographical area and the similarities in the distribution pattern of inorganic-P fractions suggest that the soils have been subject to similar weathering conditions. For profile 12, which is situated in the Serengeti National Park, where there is little rainfall and there has been limited activities by man, the percentage distribution of A1-P and Fe-P is considerably higher in the surface horizons than in the subsoil, although Ca-P is the predominant form throughout the profile. According to Pagel (1972), the high content of the Ca-P, of over 60% of total inorganic P throughout the profile, suggests that the soil has not been intensely weathered. The contrast between these two profiles does point out the fact that although two soils may belong to the same order in the classification system, it does not necessarily follow that the dominant inorganic-phosphorus fractions will be the same. The parent material and the intensity of weathering play a leading role in determining the dynamics of the inorganic-phosphorus fractions in the soil. Similar observations were made by Westin and De Brito (1969) who showed that as weathering intensified, total P, active P and organic P all declined along with pH. The Vertisols are derived from ultra-basic rocks and other intrusives high in calcium and magnesium (Saggerson, 1962). They receive medium rainfall and the distribution of the inorganic-P fractions showed that Fe-P is the dominant fraction, with A1-P and Ca-P present in fairly small proportions. The distribution of inorganic-P fractions in Inceptisols showed a similar pattern for profiles 2 and 4. The magnitude of the inorganic-P fractions in profile 3 is higher because the parent rock is basic (Saggerson, 1962). In the Spodosol profile most of the inorganic-P fractions were in the iron- and aluminiumbound forms. Although the parent rock is basic volcanic ash, the heavy rainfall of the area and the continued leaching under cold conditions seems to have transformed most of the Ca-P into aluminium and iron-bound phosphorus. In the Mollisols the nature of the parent material seems to play a key role in the distribution of the inorganic-P fractions. In profiles 6 and 7, which were obtained from the Mpwapwa area in central Tanzania where rainfall is limiting, most of the inorganic@ fractions were calcium-bound and reductant-soluble iron phosphates. In profiles 8 and 9, which are situated in the Kilimanjaro area where soils are formed from basic volcanic ash, the quantity of inorganic P in all fractions is quite high and it is mostly in the calcium-

PHOSPHATE FRACTIONS IN SOME TANZANIA SOILS

191

bound form despite the heavy rainfall the area receives during the greater part of the year. In general, most of the soil profiles contained much less reductant-soluble and occluded phosphates in their A horizons than the B horizons. The high proportion of the total P in the organic form is probably a contributing factor to the lower reductant-soluble phosphorus content of the A horizons. Similar observations were made by Bauwin and Tyner (1957). The removal of a large part of the soluble phosphorus from all soil sources through root and microbial activity and its return to, and accumulation in the soil as relatively stable organic phosphorus, including the tendency of organic matter to complex soluble iron, represent mechanisms which would tend to keep large proportions of the phosphorus in the A horizon in a relatively non-reactive state and simultaneously contribute to the transport of considerable iron from A to the B horizons during soil development. The organic carbon-organic P ratios (C:P) reported by Pearson and Simonson (1940) for the A horizons of seven Iowa soils ranged from 53 to 230 while Nye and Betheux (1957) reported ratios up to 500 for gneiss and granitic soils of Ghana. In contrast Hawkins and Kunze (1965) working on the A horizons of Texas soils obtained C:P ratios for the A horizons ranging from 27 to 296. In this study the C:P ratios for the A horizons ranged from 25 for a virgin Alfisol in the Serengeti National Park to 493 for a Vertisol. The low C:P ratios observed on most of the profiles would suggest that plants growing on them might obtain much of their P from mineralized organic phosphates (Tisdale et al., 1970). On the other hand, the generally much greater content of organic P in most of the profiles and the accompanying low C:P ratio may partly account for the lack of response to P applications on many soils in this country as reported by Hasewell (1970) and Kesseba and Uriyo (1971). CONCLUSION The findings of this investigation on the distribution of inorganic P in soil profiles have a practical significance concerning the use of chemicals for the determination of available phosphorus for fertilizer recommendations. In the first place, if one method is used on a group of soils with the same phosphate distribution pattern, the correlation between available phosphorus and crop responses to fertilizer will be better than on a group of soils with different phosphate-distribution patterns. Whereas the parent material, climate and the stage of development of the particular soil profile appeared to control the distribution of phosphorus in the profiles studied, no general conclusion can be made that a particular soil order will contain say predominantly A1-P and very little Fe-P and Ca-P since wide differences were observed between profiles in the same soil order. ACKNOWLEDGEMENT The authors wish to express their appreciation to the University of Dar es Salaam for the financial assistance received during the period of this study.

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REFERENCES A1-Abbas, A.H. and Barber, S.A., 1964. Soil tests for phosphorus based upon fractionation of soil phosphorus. Correlation of soil phosphorus fractions with plant available phosphorus. Soil Sci. Soc. Am. Proc., 28: 218-221. Bauwin, G.R. and Tyner, E.H., 1957. The nature of reductant-soluble phosphorus in soils and soil concretions. Soil Sci, Soc. Am. Proc., 21: 250-257. Chang, S.C. and Jackson, M.L., 1957. Fractionation of soil phosphorus. Soil Sci., 84: 133-144. Chang, S.C. and Jackson, M.L., 1958. Soil phosphorus fractions of some representative soils. J. Soil Sci., 9: 109-119. Chang, S.C. and Juo, S.R., 1963. Available phosphorus in relation to forms of phosphate in soils. Soil Sci., 95: 9 1 - 9 6 . Hasewell, D.R., 1970. Response of Maize to NPK Fertilizers. Annual Research Conference. Ilonga Research Station, Tanzania, 32 pp. Hawkins, R.H. and Kunze, G.W., 1965. Phosphorus fractions in some Texas Grumusols and their relation to soil weathering and available phosphorus. Soil Sci. Soc. Am. Proc., 29: 650-656. Kesseba, A. and Uriyo, A.P., 1971. Response of maize to NPK fertilizer application in the Morogoro area. Proceedings of the second landuse conference in Tanzania, 1971, 92 pp. Laverty, J.C. and Mclean, E.D., 1961. Factors affecting yields and uptake of phosphorus by different crops, III. Kinds of phosphate native and applied. Soil Sci., 91: 166-171. Norrish, K., 1968. Some phosphate minerals of soils. Trans. lnt. Congr. Soil Sci., 9th, Adelaide, 2: 713-723. Nye, P.H. and Betheux, M.H., 1957. The distribution of phosphorus in forest and savana soils of Ghana and its agricultural significance. J. Agric. Sci., 4 9 : 1 4 1 - 1 5 9 . Pagel, H., 1972. Distribution of inorganic forms of phosphorus in important soils of the arid and humid tropics. Beitr. Trop. Subtrop. Landwirtsch., Karl-Marx Univ., Leipzig, 10: 53-70. Pearson, R.W. and Simonson, R.W., 1940. Organic phosphorus in seven Iowa soil profiles. Distribution and amounts as compared to organic carbon and nitrogen. Soil Sci. Soc. Am. Proc., (1939) 4: 162-167. Pratt, P.E. and Garber, M.J., 1964. Correlation of phosphorus availability by chemical tests with organic phosphorus fractions. Soil ScL Soc. Am. Proc., 28: 443-449. Saggerson, E.P., 1962. The geology of East Africa. In: E.W. Russell (Editor), Natural Resources of East Africa. D.A. Hawkins and E. Aft. Literature Bureau, Nairobi, pp.52-56. Saunder, W.H.M. and Williams, E.G., 1965. Observations on the determination of total and organic phosphorus. J. Soil Sci., 6: 254-267. Soil Survey Staff, 1960. Soil Classification. A Comprehensive System, 7th Approximation. U.S.D.A. Washington, D.C., 265 pp. Soil Survey Staff, 1967. Supplement to Soil Classification System, 7th Approximation. U.S.D.A. Washington, D.C., 207 pp. Suzuki, A.K., Lawton, K. and Doll, E.C., 1963. Phosphorus uptake and soil tests as related to forms of phosphorus in some Michigan soils. Soil ScL Soc. Am. Proc., 27: 401-403. Tisdale, S.L. and Nelson, W.L., 1970. Soil Fertility and Fertilizers. MacMillan, New York, N.Y., 201 pp. Uriyo, A.P. and Kesseba, A., 1972. An evaluation of NH4F as a selective extractant for AI-P in two soils of the tropics. Geoderma, 8: 207-220. Uriyo, A.P. and Kesseba, A., 1973. The use of HELO 4 in the determination of reductant soluble iron phosphate. Plant Soil, in press. Walkley, A. and Black, C.A., 1965. Determination of organic carbon. In: C.A. Black, D.D. Evans, J.H. White, L.E. Ensminger and F.E. Clark (Editors), Methods o f Analysis. Am. Soc. Agron., Madison, Wisc., pp. 1372-1375. Westin, C.F. and De Brito, J.G., 1969. Phosphorus fractions of some Venezuelan soils as related to their stage of weathering. Soil Sci., 107: 194-202. Westin, C.F. and Buntley, G.J., 1966. Soil phosphorus in South Dakota, II. Comparison of two availability tests with inorganic phosphorus fractions among soil series. Soil ScL Soc. Am. Proc., 30: 245-253.