Effect of Time on Soil Formation in Selected Alluvial Soil Series of Pakistan

Introduction

The nature and the intensity of changes occurring in the soil with the passage of time have been studied by many investigators (Brewer and walker, 1969; Campbell, 1971; Alexander, 1974). Such studies involve a comparison of certain characteristics of soils having different ages, but formed from a similar type of parent material under the similar environment. The studies related to chronosequence revealed that, with time, organic matter content increased, soluble salts, bases and carbonates were leached whereas pH was decreased (Wright et al., 1959; Foss and Rust, 1962; Mehdi et al., 2000). Parsons et al. (1962) and Campbell (1971) found that after initial accumulation of organic matter there was a distinct trend of decreasing values with further passage of time. Campbell (1971) showed a decrease in cation exchange capacity with increase in age, while Brewer and Walker (1969) reported that in an age sequence, with degree of weathering illuviated clay profile was progressively developed. As weathering proceeds, the clay content increases as a result of physical and chemical alteration of primary minerals. With further chemical changes, the transformations may takes place within the clay fraction of soil. The type of clay minerals formed depends on the climatic conditions and the chemical environment within the soil. Brady (1984) and Jackson (1968) described progressive modifications of the clay minerals with increasing weathering under various environments given as under:

The chlorite and illite apparently represents the younger, the mont- morillonite and vermiculite, the intermediate and the oxides, the oldest state of weathering. Franzmeier and Whiteside (1963) observed illite and chlorite during the initial stages of development and these two intergrade to vermiculite and montmorillonite with soil development. Bajwa (1989) Akhtar and Jenkins (1999) and Mehdi et al. (2000) also reported similar results. Keeping all this in view present study was undertaken to see the effect of age on soil formation in three soil series of Pakistan.

Materials and Methods

Profiles of three soil series i.e., the Shahdara (recent, 100 year old), the Sultanpur (Subrecent, 600-1000 years of age) and the Lyallpur (the pleistocene, 10⁴ to 2 x 10⁴ years old) were exposed at the selected locations and were described with the technical help of the soil survey staff, Lahore. Their classification is given in Table 1. Bulk soil samples were collected from Ap, B and C-horizons for physical, chemical and mineralogical analysis. These samples were air dried, ground and passed through a 2 mm sieve and were analyzed for texture, EC_e (dSm^-1) pH_s, 0.M (%), CEC (me 100 g^-1), NH₄OAc extractable bases, soluble ions and mineralogical composition.

All the analyses were done according to the methods given in Hand Book No.60 (U. S. Salinity Lab. Staff, 1954) except texture by Moodie et al. (1959). In case of mineralogical composition determination all steps were performed for clay samples preparation for X-ray diffraction by Jackson (1979) including:

Destruction of carbonates and removal of divalent cations, Decomposition of organic matter and dissolution of MnO₂, Removal of free iron oxide, Dispersion and separation of less than two-micron fraction, Pretreatment and sample preparation for X-ray analyses.

Clay samples were X-rayed by using a Norelco X-ray diffractometer, equipped with a flow counter and Bristol Dynamaster recorder. Copper K or iron K radiation were employed with A, 1.5418 and 1.9373 A^O, respectively. Each of the sample was scanned through the range of 0-30⁰ at a speed of 20/2θ/minute (Jackson, 1979).

Table 1:	Classification of the soils series

Results and Discussion

The data for the physical, chemical, morphological and mineralogical analyses of three soil series i.e., the Shahdara, the Sultanpur and the Lyallpur were considered to determine the effect of time on soil formation. These soil series varied in age forming a chronosequence.

Physical analysis: The distribution of soil particles i.e. sand, silt and clay (Table 2) was haphazard in the various horizons of the Shahdara soil series. Profile indicating a kind of stratification of the alluvial material. In the sub recent Sultanpur soil series, the clay content increased from 21 to 24 percent from surface to subsoil (up to 66 cm). In case of the Lyallpur soil series, the older one, there was a continuous increase in the clay content from surface to subsoil up to the depth of 90 cm and then it decreased. The particle size distribution showed sharp changes in the various size fractions in the relatively un-altered part of the profile i.e. the C-horizon. The relative particle-size differentiation increased with soil age and an illuviated clay profile was progressively developed. Brewer and Walker (1969), Ruhe (1956) and Campbell (1971) reported an increase in the clay content of the B-horizon with increasing soil age. Wright et al. (1959) found clay illuviation only in the relatively older members of some soil chronosequences. Ahmad et al. (1977) also has reported similar findings.

Chemical properties: The organic carbon content was low (0.50%) in Ap of the Shahdara series than that in the Sultanpur (1.0%) and the Lyallpur (0.62%) soil (Table 3). Depth wise distribution of organic carbon was irregular in the recent, Shahdara, while a consistent decrease in the sub recent, Sultanpur, and the old Lyallpur series.

Table 2:	Physical characteristics of the three soil series forming a Chronosequence

Table 3:	Chemical characteristics of three soil series forming a chronosequence

Table 4:	Main morphological features of the selected soil series.

Table 5:	Clay mineralogy of < 2Fm fraction

Organic mater accumulates in the soil with increasing development (Franzmeier and Whiteside, 1963) and after reaching a stable equilibrium in mature soils; it declines (Campbell, 1971). In the present study there was no consistent increase in organic matter with age but Syers et al. (1970) observed that organic matter continued to increase even after 10,000 years of soil development. However, these findings pertain to the possible reason, for low organic matter in the soils, may be high temperature, which promoted its rapid oxidation.

The CaCO₃ content, which indicates the intensity of leaching of a soil profile is an important index of the degree of soil formation. The distribution of CaCO₃ in the profile of the Shahdara soil series was inconsistent, in the Sultanpur a total amount was slightly less but there was a little increase with depth wile in the Lyallpur series, the percent CaCO₃ was low and it also increased with depth. Under climatic conditions permitting net downward movement of water at some time of the year, the amount of lime in soil decreased with greater soil age (Wright et al., 1959). A definite trend of the cation exchange capacity of the soils of different series was observed. Actually the CEC of a soil is a function of colloidal particles in its mineral and organic fractions.


Plate I:	The shahdara profile (Typic torrifluvent) prominent depositional stratification and lamination.


Plate II:	The Sultanpur profile (Fluventic camborthid) because of relatively younger age, profile development is only to limited depth and horizon not much conspicuous. The substratum comprises unaltered parent material exhibiting depositional stratification.


Plate III:	The Lyallpur profile (Ustalfic haplargid) very distinct color horizon. Zone of secondarylime accumulation occurred below about 90 cm depth.

Soil with low organic matter content, as was in the present case, the CEC is generally controlled by the mineral colloids. There was slightly high increase in the CEC of the surface soil of the Sultanpur series probably due to more organic matter oldest, the Lyallpur soil series. The ammonium acetate extractable base also did not exhibit any relation with soil age.

Morphological features: The main morphological features of the profiles are presented in Table 4. Plates 1,11 and 111 also shows some morphological features of the Shahdara, Sultanpur and Lyallpur soil series, respectively. The topsoil color was uniform throughout the soil sequence but the subsoil color varied. The top soil structure was essentially the same in all the soils, however, the subsoil structure ranged from depositional stratifications in the youngest soil (the Shahdarab series) to well developed structural aggregates in the oldest soil (the Lyallpur series). Both the electrical conductivity and pH of the surface soil had comparatively higher values in the youngest soil than that in the others. Solum depth, horizon thickness, and structural development increased and clay illuviation became pronounced with time. Similar conclusions were drawn by Ruhe (1956), Gile (1966), Brinkman and Rafiq (1971) and Ahmad et al. (1977).

Mineralogical composition of the clay: The results of X-ray diffraction analysis of < 2µ clay fraction from surface and subsoil horizons from all the three soils are presented in Table 5. The illite content was the same throughout the profile in all the series. Smectite was more in the Shahdara and it decreased with soil age. In the case of the Lyallpur series, smectite content increased in the C material. Vermiculite was absent while chlorite and kaolinites were present, though in minor amounts, in all the three series. Minerals in the clay fraction are subjected to weathering and transformation with soil development (Brewer and Walker, 1969) The chlorite and illite apparently represent the younger, the smectite and vermiculite, the intermediate; and the oxides, the oldest state of weathering (Jackson, 1968; Campbell, 1971; Franzmeier and Whiteside, 1963; Brady, 1984). But, in this study, the mineralogical variation of alluvial soils reflected the differences of their original material rather than that of pedogenic processes, so, the loessial materials (alleviated) carry principally hydrated mica (illite) and smectite in association with chlorite and kaolinite. Bajwa (1989); Akhtar and Jenkins (1999) and Mehdi et al. (2000) also reported similar results.