Soils of the Indo-Gangetic Plains, India : Their Historical Perspective and Management.
D.K. Pal1, T. Bhattacharyya1, P. Srivastava2, P. Chandran1, and S.K. Ray1. (1) Division of Soil Resource Studies, National Bureau of Soil Survey and Land Use Planning, Amravati Road, Nagpur, 440 010, India, (2) Dept of Geology, Univ of Delhi, Delhi, 110 007, India
The Indo-Gangetic Alluvial Plains (IGP) are among the most extensive fluvial plains of the world and cover several states in the northern, central and eastern parts of India. The agriculturists of ancient India (2500 BC to 600 AD) were quite conscious of the nature of soils and its relation to the production of specific crops. However, the scientific interest of the Indian soils began when the Geological Survey of India started studying the soils in 1846 and then the soils of the IGP were recognized as one of four major soils of the country. In the past, the IGP are thought to consist of older and younger alluvia of Pleistocene age. Recent soil geomorphic studies suggest the presence of more than two soils/surfaces younger than 13,500 BP. The IGP witnessed climatic fluctuations, especially after the last glaciation. A cold, arid to semi-arid climate prevailed during the early Holocene to about 7,390 BP in the central and western parts of the IGP. Later, a warm and humid climate set in, followed by drier conditions that continued until the present. Neotectonic movements in response to the northern push of the Indian Plate also played a significant role in the evolution of geomophology and soils of the Gangetic Plains. By creating microhigh and microlow sites, it has been ultimately responsible for the formation of more and less sodic soils in the semi-arid part of the IGP. During the early Holocene to 7,390 BP and also during the latter warmer and wetter periods, weathering of minerals, illuviation of clay, decalcification and little addition of organic matter have been the major pedogenic processes. Clay illuviation in soils of the IGP has not always resulted in clay skins or, where present, in pure void argillans. Impure clay pedofeatures appear to be typical in these soils of the Plains. Clay mineral assemblages of the soils indicate the pedogenic development of smectite-kaolin mineral which can be considered as a potential indicator of paleoclimatic change during the Holocene from arid to humid climates. Development of soils in the IGP indicates that climatic fluctuations are more important and soils older than 2,500 BP are relict paleosols but they are polygenetic because they were again altered during the period of subsequent climate change. Polygenesis of soils of the IGP appears to be a common phenomenon and each pedogenic event needs more attention to record the different climates under which they took place. The soils under arid and semi-arid climates are impoverished in organic carbon and phosphorus but rich in potassium. The adverse arid climatic conditions induce the formation of pedogenic CaCO3 and as a result, sodicity develops in the subsoils. At present the formation of CaCO3 is proceeding at a very fast rate. Even during the early to middle part of the 19th Century, much of the land in the IGP was under cultivation with traditional mixed cropping. Over the last three to four decades the production of foodgrains, chiefly of rice and wheat, increased tremendously by introducing high input technologies. However, this has ended with the degradation of many natural resources that has implications on the sustainability of soils for rice-wheat cropping systems. Soil Organic Carbon (SOC) sequestration has been reported to be a boon whereas Soil Inorganic Carbon (SIC) sequestration a bane for farming. High SIC in semi-arid climates, however, does not pose a significant problem to soil productivity. In fact, adoption of appropriate management techniques has increased the SOC stock of the IGP soils cutting across dry and humid tracts. This has been realized after comparing soil survey data over two time periods representing relatively low and high management programmes. Recent C-modeling indicates that perhaps the IGP soils have reached a quasi-equilibrium in terms of SOC over a period of 30-40 yrs of Green Revolution. In view of the extensive area of the IGP, a new initiative in registering pedogenic thresholds during polygenesis and their interaction in a landscape are essential to manage the agricultural productivity with time.