Pedogenic Changes and Mineral Weathering in Three Soils from the Indus River Plain of Pakistan

161

PEDOGENIC CHANGES AND MINERAL WEATHERING IN THREE SOILS FROM THE INDUS RIVER PLAIN O F PAKISTAN

M . S . AKHTAR, J.B.

DIXON AND L.P. WILDING Texas A&M U n i V . , C O l l . Stat., TX 77840. USA

Dep. S o i l and C r o p S C i . ,

ABSTRACT Three soil series were investigated in East and Northwest Pakistan to determine mineral weathering and degree of profile development. Micromorphic evidence suggests that these soils are generally weakly differentiated and weathered in spite of their previous field classification as Udic Haplustalfs (Peshawar and Gujranwala) and Ustalfic Haplargids (Lyallpur). Thin-section observations failed to support clay translocation in Peshawar and Lyallpur series but the Bt2 and Bt3 horizons of the Gujranwala did exhibit illuvial void ferriargillans. In all three soils the s-matrix was porphyroskelic; the plasmic fabric ranged from silasepic to argillasepic in carbonate-poor zones to crystic in carbonate-rich horizons. Translocation of plasma was primarily by redistribution of carbonates and Fe-Mn oxyhydroxides as evidenced by sesquioxide nodules and concretions, ferrans, calcans and carbonate nodules and concretions. Most nodules contain mica and other skeletal grains indicative of in situ formation. The Bt2, Bt3, and Bt4 horizons of the Gujranwala soil had larger and more abundant sesquioxide nodules suggesting more strongly reducing conditions in the horizons either due to irrigation or more pluvial paleoclimates. All soils have mixed mineralogy. Sand and silt fractions contain significant amounts of mica, quartz, and feldspars with smaller amounts of kaolinite and chlorite. The mica grains are smaller and less numerous in the Peshawar soil than in the Gujranwala and Lyallpur soils. Most grains, except some biotite, showed little or no weathering. In the light of the microfabric observations and other laboratory data new classification of these three soils has been suggested.

INTRODUCTION

Pakistan lies in a subtropical region characterized by predominantly warm and dry climate and low moisture effectiveness. Most of the arable soils in the country are derived from alluvium and are classified according to the Soil Taxonomy (USDA, 1985) as Haplargids and Haplustalfs (Soil Survey of Pakistan 1 Soil Management Support Services USA,1986). However, any conclusion based on field evidences without thorough micromorphological data is hazardous, especially in alluvial soils since many features are sedimentary in origin (Sanders, et al.. 1986). Hence, evaluation of pedogenetic processes using an array of physical, chemical, micromorphic and elemental techniques was undertaken to assess the degree of profile development. Mineral weathering, especially layer silicates, has been studied under different soil environmental conditions (Bisdom et al., 1982; Pal and Deshpande, 1987; Tomar. 1987). Formation of Fe and Mn nodules associated with seasonally wet soils and their genetic implications were reported by Chakravarty et al. (1982), Rahmatcllah et al. (1988), Ransom et al. (1987). Formation of argillic horizons has been studied by Manchanda et al. (1983).

The objective of this study was to examine differences in the weathering intensity among three

162 pedons as manifested in micromorphological. chemical, and mineralogical properties of selected calcareous alluvium-derived soils of the lndus River plain of Pakistan.

MATERIALS AND METHODS

Study area and Pedon morphology: The study area (Fig.1) consists of (a) a subhumid old river terrace (Gujranwala series), (b) semi-arid peidmont plain (Peshawar series), and (c) semi-arid interfluve between the Ravi and Chenab Rivers (Lyallpur series). The soil parent materials of (a) and (c) regions include mixed, calcareous alluvium derived from the Himalayas and the adjoining upland. The material has been transported by the lndus River system and deposited during late Pleistocene and Holocene periods. Parent material for the piedmont plain (b) is quaternary loess redeposited and mixed with piedmont calcareous red shale. Most of the soils in the lndus plain are cultivated and therefore subject to more or less severe anthropogenetic-induced sedimentation from irrigation water.

,7 6”

72O

Fig.

sltes in East and Northwest Pakistan: OGujranwala, m Peshawar, and A Lyallpur.

1 Locatlon of sampllng

The three pedons representative of these areas in the order of decreasing profile development are Gujranwala, Peshawar, and Lyallpur series. They were classified in the field as follows: (1) The Gujranwala pedon was classified as fine-loamy, mixed, hyperthermic Udic Haplustalf. Segregations of

163 Fe-Mn oxides occur from 66-180cm. In the field continuous, thick (Btl) to patchy and thin (Bt3) clay coatings were observed on vertical and horizontal ped surfaces. (2) The Peshawar pedon was classified as fine-silty, mixed, hyperthermic Udic Haplustalf and is a very deep, well drained moderately alkaline soil. Field observations suggested discontinuous thin to continuous and thick clay coatings on vertical ped faces and root channels throughout the profile depth except Ap. (3) The Lyallpur pedon was classified as fine-silty, mixed, hyperthermic Ustalfic Haplargid and is a deep, well drained, mediumtextured, calcareous soil; continuous distinct-thin clay films on peds and in pores were reported in field description of the profile.

Field and Laboratory Methods:

Bulk and undisturbed core samples were $ken from each genetic soil horizon. Fine-earth subsamples, passed through a 2mm screen, were treated to remove carbonate, organic matter, and iron oxide components (Jackson, 1974) prior to fractionation into sand (2-0.05mm), silt (50-2 urn ), and coarse (2-0.2 urn ), and fine ( c0.2 urn ) clay separates. Mineralogy was determined using X-ray diffraction on randomly oriented powder mounts of sand and silt, and preferentially oriented mounts of Mg- and K-saturatedclays. Following initial XRD analysis, Mg-saturated clays were glycerol solvated, while K-saturatedclays were heat-treatedto 350" C and 550" C in two steps and X-rayed again. Calcium exchange capacity (CaEC) and potassium exchange capacity (WEC) were determined and served as the basis to calculate smectite and vermiculite percentages; potassium was determined on HF-digested samples and used to calculate mica percentages in both clay fractions (Jackson, 1974). Elemental Ti and Zr in the silt fraction were determined using X-ray spectrographic techniques (Smeck and Wilding, 1980).

Two thin-sections of 7.5~5cm size with known orientation were made from undisturbed cores from each horizon (Innes and Pluth, 1970) and were examined. Part of the thin section was treated to remove carbonate (Wilding and Drees, 1988). Transmission electron microscope examination was made of selected clay samples drop-mounted on carbon-coated gold grids.

RESULTS AND DISCUSSION

Mineralogy: All three soils have mixed mineralogy The sand and silt fractions were composed of quartz, mica, and feldspar with small amounts of kaolinite and chlorite. Similar mineralogy for these other and soils in the region, developed from the same type of parent material under similar, has been reported (Manchanda et al 1983, Pal and Deshpande, 1987 and Tomar, 1987). Sand- and silt-size mica particles were distinguished as biotite and muscovite. Mica particles in Peshawar soil appeared to be fewer and smaller as compared to Gulranwala and Lyallpur soils. Most grains, with the exception of a few biotite

164 grains seen in the thin-sections showed little or no weathering. The coarse clays contain mica, kaolinite, smectite, and chlorite with traces of vermiculite and quartz. The fine clays were comprised malnly of smectite, mica, and vermiculite with traces of kaolinite

(XRDdata not reported).

Mica percentages (Fig.

2) in the Lyallpur soil range from 39-46 and 27-38% in coarse and fine clays, respectively. A smooth decrease in mica content with profile depth was noted in Lyallpur pedon. Amount of mica in the Gujranwala pedon varies from 37 to 49% in coarse clays and 19 to 25% in fine clays. The increased mica contents in clay fractions toward profile surface are attributed to mica additions in the upper sediments through canal irrigation and to the disintegration of coarse mica into fine particles. Electron microscope observations of the clay particles revealed predominantly clean smooth-edged platy particles with little evidence of weathering. Selected electron diffraction (SED) indicated both diand trioctahedral nature of the mica particles. Presence of easily weatherable minerals, i.e. feldspar, biotite, and their morphology suggested an early stage of mineral weathering in the pedons studied.

Guiranwala

0

10 20 30 4 0 50 6 0 Fraction (%)

Peshawr

0

10

20 30 40 50 Fraction (%)

60

Lv a IID u r

9

0

Mica 2-0.2itm

10 20 30 40 50 6 0 Fractlon (%)

Fig. 2 Distribution of clay minerals and total clay with soil depth.

165 Sandisilt and Ti12 ratios were used to judge parent material uniformity. The TiiZr data suggest lithologic breaks at the Bt2-Bt3 and 2BC-2BCk horizon boundaries in the Gujranwala pedon; the later, 2BC-2BCk, discontinuity in the profile was also supported by a change in sandisilt ratio. In the Peshawar pedon both sand/silt and TiiZr ratios indicate a break in the parent material uniformity at the 2Bwbl-2Bkbl horizon boundary (66 cm depth). Field observations indicate stratification at the 2BC-2BCk boundary (156 cm) in the Gujranwala and at the Bw-2Bwbl boundary (66 cm) in the Peshawar profile. Laboratory data suggest that the 2Bbl of Peshawar soil should be considered as a part of the same parent material of the upper fluvial unit.

Micromorphology: Micromorphological features (Table 1) suggest that these soils are generally weakly differentiated and slightly weathered. All three soils have a porphyroskelic s-matrix and a silasepic to argillasepic plasmic fabric in carbonate-poor to crystic fabric in carbonate-rich horizons. The Gujranwala Btl and Bt2 horizons have an insepic plasmic fabric in some places. The Ap and BA horizon of the Lyallpur soil has argillaceous glaebules with lamellar fabric (Fig. 3a). The Ap horizon of all three soils has spongy microstructure with many interconnected meta vughs (Fig. 3b). Evidence of a plowpan can be seen in the upper B horizons, especially in the Gujranwala pedon. The BA horizon has a dense s-matrix with few skew planes and channels (Fig. 3c). Lack of macrovoids restricts the penetration of air, water, and roots.

TABLE 1 Summary of micromorphological features of selected horizons Horizon

Depth(cm)

Plasmic fabric

% Btl

Bt 1

0-10 10-24 24-58 58-66

Silasepiciskelasepic Argillaspic Argillasepic Argillasepic, insepic

AP AB Btl 2Bbl

0-1 1 11-43 43-66 66-98

Argillasepic Argillasepic Argillasepic Crysticiargillasepic

0-10 10-25

Argillasepic Argillasepic

114-162

Crystic!silasepic

BCk

Other features

Gujranwala

Spongy microstructure; Fe-Mn, calcite nodule; Dense matrix in the lower half; skew planes Dense matrix; Mod. oriented channel argillan; Few patchy mod. oriented argillan; Fe Mn nodules

Peshawar

Spongy microstructure; calcite lithorelicts; ferran Neo and quasi channel sesquan; micritic calcite Chamber neo sesquan; thin vugh ferriargillan Quasisesquan; chamber calcan; calcite lithorelicts

Lyallpur

Lamellar clay papules; spongy microstructure Clay papules with lamellar fabric; calcite nodules fecal cast Complex argillan/calcan; calcite nodules

Thin-section observations suggest that translocation of clay is occurring only in the Btl and Bt2 of the Gujranwala soil where few, moderately oriented ferriargillans were noted in vughs and channels (Fig. 3d&f). Enrichment of clay in Bt horizon of the Gujranwala is supported by an increase in total clay in the Bt of 33% (Fig. 2). However, part of the clay may have been formed in situ. Relatively prominent evidence of clay accumulation in the Gujranwala soil may represent a higher frequency of the wetting-

166 drying cycle, lessivage, weathering in situ, and reorganization Of illuviated constituents (Tomar, 1987). In the other two soils (Peshawar and Lyallpur) argillans were absent often in the horizons field

designated as Bt. In the Lyallpur pedon argillans were found in BC and BCk (Fig. 39).

Thin-section observations failed to provide any evidence of clay lessivage in the Peshawar soil, except in the Bw. In this horizon moderately-oriented continuous vugh ferriargillans were present in .c 1% of the thin-section area. However, this horizon does not qualify as an argillic horizon due

to lack of

clay gain. Occasional evidences of complex compound argillanicalcans banded in BC and BCk vughs in the Lyallpur soils were observed (Fig.3g). Complete genetic interpretation of such clay coating remains hazardous. However, it may suggest a multistage development of these features by carbonate leaching, precipitation of secondary calcite, and subsequent illuviation of argillans (Smith and Wilding, 1972).

Segregation of Fe-Mn oxyhydroxides was an important morphological feature from 66 to 150cm of the Gujranwala profile (Fig. 3h). These accretionary Fe-Mn nodules have concentric fabric. Some nodules contain mica and other skeletal grains in the same proportion as the s-matrix indicative of in

situ nodule formation. The nodule embeded-grains, including biotite, appear to be least weathered among the other grains of the same size (Fig. 3h). However, nodules are sparse in the Peshawar and Lyallpur soils. The genesis of the Fe and Mn nodules reflects reduction and translocation of free Fe-Mnoxides under anaerobic conditions in upper horizons either due to irrigation or strong pluvial paleoclimates.

The minimal availability of water in the Peshawar and Lyallpur soils limits dissolution-reorganization and translocation of plasmic constituents when compared with Gujranwala soil. Peshawar soil contains numerous micritic and sparry calcite nodules and less numerous calcite lithorelicts, calcans (except Ap) and neo and quasi ferromangans through out the profile. The Peshawar Ap and AB horizons have numerous evidences of lithorelicts and quasi ferromangans. The Lyallpur soil has a zone of calcite nodule accumulation below 60cm, increasing toward the lower depths. In all three soils some calcite nodules have been observed containing skeletal grains and others to be surrounded by neo ferromangans which penetrate due to the mobility of the oxides under reduced conditions (Chakravarty et al. 1982). Biological homogenization of the parent material appears to be the dominant soil forming process active in the B horizons of the Peshawar and Lyallpur profiles as indicated by numerous biopedofeatures i.e. fecal pellets (FIG. 3e), biotubules etc.

CONCLUSIONS

The three pedons are generally weakly differentaited and are slightly weathered. The Gujranwala soil is the most developed among the three. The clays are composed mainly of layer aluminosilicates:

167

Fig. 3 Photomicrographs of selected micromorphic features from the three alluvial soils: (a) lamellar clay papule (Lyallpur Ap); (b) spongy microstructure (Peshawar Ap); (c) dense s-matrix with crossed skew plane (Gujranwala BA); (d) Vugh argillan (Gujranwala Btl); (e) fecal pellet (Lyallpur Bw); (f) argillan (Gujranwala Bt2); (9) compound argillanicalcan (Lyallpur BCk); (h) part of Fe-Mn nodule with embeded skeletal grain and surrounded by calcan (Gujranwala Bt2). Bar = 0.5mm. Plane light in (a), and crossed polarizer in all others.

168 Mica constitutes > 1:3 of the clays. Minimal leaching in the Peshawar and Lyallpur soils limited dissolution, reorganization, and translocation of plasmic constituents. The field and micromorphological observations appear to be contradicting about the occurrence of argillans in the Lyallpur and Peshawar pedons and is a source of confusion. This contradiction can only be explained considering the parallel oriention of micaceous platelets along void and ped surfaces and occurrence of neo and quasi ferromangan. These features were probably mistaken as illuviated clay due to a limited resolution available in the field. Further more, the laboratory data on particle size analysis do not support any significant accumulation of argillaceous clay in the horizons designated as Bt in the field. Hence, it was necessary to reconsider the field classification. In view of the all vailable evidences, following classification has been propsed: (1) Gujranwala, Fine-loamy, mixed, hyperthermic Udic Haplustalf (same as the original), Peshawar, Fine-silty, mixed, hyperthermic Udic Ustocrept, and Lyallpur, Fine-silty, mixed, hyperthermic Typic Camborthid.

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