INVITED ARTICLE

Introduction

The Indo-Gangetic alluvial Plains are among the most extensive alluvial plains of the world. They extend over a length of about 1,600 km and a width of 320 km including the arid and semi-arid environments in Rajasthan and Punjab and the humid and perhumid deltaic plains in Bengal. Of great significance to the country s agricultural production, they comprise an area of 95,714 sq km in Punjab, Haryana and Rajasthan, known as the Punjab plains, and 373,606 sq km in Uttar Pradesh (U.P.), Bihar and West Bengal, known as the Ganga plains. The Punjab plains have been built by the river systems of the Ravi, the Beas, the Sutlej, and the Yamuna over long ages with the sediments of Siwaliks and Himalayas in the north and the Aravallis and the Vindhyans brought by the tributaries of the Yamuna and laterites and Gondwana formations in the Damodar basin. The Ganga plains are formed mainly by the Ganga, the Yamuna, the Ghagra, the Gandak and the Gomati. The thickness of the alluvium may have been 2,000 to 3,300 m during the Pleistocene. The nature of the alluvium varies in texture from sandy to clayey, calcareous to non-calcareous, and acidic to alkaline. The total geomorphic variations assume considerable significance even with low gradients and the average elevation ranging from 150 m in Bengal plains to nearly 300 m in Punjab plains (Shankarnarayana, 1982).

Issues

Research in soils of IGAP during the past decade has pointed out some issues that need to be disseminated to other participating scientists working in soils of IGAP.

a) Fertility: Despite the fact that soils developed in the Indo-Gangetic alluvium are known to be fertile, they have low organic carbon, nitrogen and phosphorus in arid and semi-arid parts. However, they are rich in K due to enormous release of interlayer K from biotite. In presence of biotite, muscovite mica appears to be a useless K reserve. Studies on K and P fertilization indicate that crop do not respond to K fertiliser over long periods of time while they respond to P in sodic soils after few years of cropping (Pal and Mondal, 1980; Pal and Durge, 1989; 1983; Bhumbla and Chhabra, 1982).

b) Natural degradation process in soils of IGAP: Soils under arid and semi-arid climate lack in organic carbon due to high rate of decomposition. The adverse climate conditions induces the precipitation of CaCO3 thereby depriving the soils of Ca2+ ions on soil exchange complex with a concomitant development of sodicity in the subsoils. The subsoil sodicity impairs the hydraulic conductivity of soils. This impairment of percolative moisture regime provides an example of a soil where gains exceeds losses. This self terminating process (Yaalon, 1983) will lead to the formation of sodic soils with ESP decreasing with depth. Therefore, the formation of pedogenic (secondary) CaCO3 is a basic process that initiates the development of sodicity and thus needs to be included as basic and natural process of soil degradation. CaCO3 is formed during the semi-arid climate prevailing for the last 4000 year BP. The rate of formation of CaCO3 is thus 0.86 mg per 100 g of soil per year in the first 100 cm of the profile (129 kg ha-1 yr-1 for mean bulk density of 1.5 mg m-3) (Pal et al, 1999).

c) Soil carbon stock and its role in decision support system for land resource management: Estimation of soil organic carbon (SOC) and soil carbonate carbon (SCC) stocks together give a total mass of carbon in the Indo-Gangetic Alluvial Plains which range from 0.74 in the 0-30 cm depth to 6.6 Pg in the 0-150 cm depth. The OC content in soils of humid and subhumid regions has reached a quasi-equilibrium (Shaikh et al, 1998). This suggests that the existing agricultural management practices in such areas needs to be sustained. Unlike pedogenic carbonates in soils of arid climates, the geogenic carbonates in soils of humid and subhumid regions can act as a useful source of calcium in soil solution that will maintain the soil productivity for a longer time.

About 36.7 percent area of IGAP is covered by arid and semi-arid ecoregions. The soils of this region are poor in OC but rich in SCC content. Rehabilitation of these soils will, therefore, help in sequestration of OC and also in dissolution of natural CaCO3 and thus these soils can emerge as a formidable production zone within IGAP.

The role of basic information on both organic and inorganic carbon stock of IGAP as a decision support system can thus be realized in two ways. Firstly it can identify the degraded soils that are potentially resilient. Secondly it can guide the land resource managers to decide location specific organic matter management programmes (Velayutham et al, 1999).

d) Paedogenesis in soils of IGAP: During the Holocene the major paedogenesis of the soils of the north western and central IGAP, has mainly involved clay illuviation and decalcification. Weathering of biotite was substantial. Preferential downward movement of weathered products of biotite (di and trioctahedral expanding minerals) resulting in decreasing trend of clay mica is a sure test of clay illuviation even when clay skins are absent (Pal et al, 1994).

Clay illuviation in soils of IGAP has not always resulted clay skins or, where present, in pure void argillans. Instead "impure clay paedofeatures" is typical in these soils because of impairment of parallel orientation of clay platelets, a specific process different from those described so far for genesis of less oriented void argillans (Pal et al, 1994).

e) Polygenesis in soils of IGAP: Clay mineral assemblages of soil chromoassociation of the IGAP between Ramganga and Rapti rivers in north central India, demonstrate that pedogenic interstratified smecitite-kaolin (Sm/k) can be considered as a potential indicator for paleoclimatic changes during the Holocene from arid to humid climates. During the pedogenesis two major regional climatic cycles are recorded: relatively arid climates between 10000-6500 yr BP and 3800-? yr B.P. were punctuated by a warm and humid climate. Biotite weathered to trioctahedral vermiculite and smectite in the soils during arid conditions, and smectite was unstable and transfommed to Sm/k during the warm and humid climate phase (7400-4150 cal yr BP). When humid climate terminated, vermiculite, smectite, and Sm/k were preserved to the present day. During the development of soils in the Holocene in alluvium of the IGAP; climatic fluctuations appear to be more important than realized hitherto. The soils older than 2500 yr BP are relict paleosols, but they are polygenetic because of their subsequent alterations (Srivastava et al,1998).

f) Impact of climate chanqe on human civilization: All the warm period in which human civilization developed and flourished represents a short epoch which begins 10,000 yr BP. Within the present interglacial period too, thermal conditions have continued to change. It is believed that the monsoons were much stronger in the early part of these interglacial. As a case in point, around 4500-3700 yr BP, the rainfall in the Indus valley was probably much more than double the amount received now as for which the agriculture flourished. The drought conditions that followed could have laid to the end of the great Harappan civilization. Climatic fluctuations have continued in this century also with a warming trend occurred during the 1920's and 1930' followed by a cooling trends till date. Analysis of the fluctuations of monsoon rainfall over India in the last century indicates that the drought in the warm epochs in 1930 - 1960 were considerably less frequent when compared to the previous or succeeding three decades (Prasad and Gadgil 1986).

Polygenesis of soils in IGAP during the Holocene will act as a viable tool in identifying such climatic fluctuations when climate persisted for a longer periods. Climatic change to humid may bring prosperity in agriculture due to higher annual rainfall, while such change to aridity is a bane because this will cause soil degradation in terms of depletion of OC, pedogenic formation of CaCO3 and the concomitant development of sodicity/salinity. Therefore, acquisition of data on the changes of climate in the past may act as a decision support system for a perspective agricultural development.

References

Bhumbla, DR; Chhabra, R (1982): Chemistry of sodic soils. Review of Soil Research in India, Part I, 12th Int. Congr. Soil Sci., New Delhi, pp.169-180.

Pal, DK; Durge, SL (1980): Release and adsorption of potassium soils of India in relation to their mineralogy. Pedologie, 39: 235-248.

Pal, DK; Durge, SL (1993): Potassium release from clay micas. J. Indian Soc. Soil Sci., 41: 67-69.

Pal, DK; Mondal, RC (1980): Crop response to potassium in sodic soils in reaction to potassium release behaviour in salt solutions. J. Indian Soc. Soil Sci., 28: 247-254.

Pal, DK; Dasog, GS; Vadivelu, S; Ahuja, RL; Bhattacharyya, T (1999): Secondary calcium carbonate in soils of arid and semi-arid regions of India. Advances in Soil Science (Ed. B.A. Stewart); in press.

Pal, DK; Kalbande, AR; Deshpande, SB; Sehgal, JL (1994): Evidence of clay illuviation in sodic soils of north-western part of the Indo-Gangetic Plain since the Holocene. Soil Sci., 158: 465-473.

Prasad, CR; Gadgil, S (1986): Climatic change -a global perspective. Science Age, pp 35-40.

Shaikh, H; Varadachari, C; Ghosh, K (1998): Changes in carbon, nitrogen and phosphorus levels due to deforestation and cultivation: A case study in Jimplipal National Park, India. Plant and Soil. 198: 137-145.

Shankarnarayana, H (1982): Morphology, genesis and classification of soils of Indo-Gangetic Plains. Review of soil research in India, Part II, 12 th Int. Congr. Soil Sci., New Delhi, pp. 407-473.

Srivastava, P; Parkash, B; Pal, DK (1998): Clay minerals in soils as evidence of Holocene climatic change, central Indo-Gangetic Plains, north central India. Quat. Res., 50: 230-239.

Velayutham, M; Pal, DK; Bhattacharyya, T (1999): Organic carbon stock in Indian soils. Advances in Soil Science (Ed. B.A. Stewart); in press.