1. Indian Tropical Soils: An Overview

The Indian subcontinent, which collided with the Asian mainland during the Eocene period, is a very old mass and has not been under water since the Carboniferous period. A girdle of high mountains, snow fields, glaciers and thick forests in the north, seas washing lengthy coasts in the Peninsula, a variety of geological formations, diversified climate, topography and relief have given rise to varied physiographic features. Temperature varies from arctic cold to equatorial hot. Such varied natural environments have resulted in a great variety of soils in India compared to any other country of similar size in the world. Many however think of tropical soils as the deep red and highly weathered soils, and are often thought are either agriculturally poor or virtually useless. The major soils of India are Vertisols, Mollisols, Alfisols, Ultisols, Aridisols, Inceptisols and Entisols. Although soils of India occur in 5 bio-climatic systems, but only a few soil orders are spread in more than one bio climate. Vertisols belong to arid hot, semi-arid, sub-humid and humid to per-humid climatic environments. Mollisols belong to sub-humid and also humid to per-humid climates. Alfisols belong to semi-arid, sub-humid and also in humid to per-humid climates, whereas Ultisols belong to only humid to per-humid climates. Both Entisols and Inceptisols belong to all the 5 categories of bio-climatic zones of India, and Aridisols belong mainly to arid climatic environments. This baseline information indicates that except for the Ultisols and Aridisols, the rest 5 soil orders exist in more than one bio-climatic zones of India. The absence of Oxisols and a small area under Ultisols, suggest that soil diversity in the geographic tropics in India, is as large as in the temperate zone. These soils are not confined to a single production system and generally maintain a positive organic carbon balance. Thus they contribute substantially to India’s growing self-sufficiency in food production and food stocks. Therefore, any generalizations about tropical soils are unlikely to have wider applicability in the Indian subcontinent. The genesis of Ultisols alongside acidic Alfisols and Mollisols for the millions of years in both zeolitic and non zeolitic parent materials in Indian humid tropical (HT) climatic environments indicates how the parent material composition influences the formation of Alfisols, Mollisols and Ultisols in weathering environments of HT climate; and also how the relict Alfisols of semi-arid tropical (SAT) environments are polygenetic. The critical evaluation of the nature and distribution of naturally occurring clay minerals, calcium carbonates, gypsum, gibbsite and zeolites can yield valuable and important information to comprehend the complex factors involved in the pedogenesis of soils formed in the present and past climates. Thus, the conventional management protocols to improve and sustain their productivity need to be revised in the light of new knowledge gained in recent years. Global distribution of tropical soils and the recent advances in knowledge by researching on them (Entisols, Inceptisols, Mollisols, Alfisols, Vertisols and Ultisols) in the Indian sub-continent indicates that some of the agricultural management practices developed in this part of the tropical world for enhancing crop productivity and maintaining soil health, might also be adoptable to similar soils elsewhere. In the following chapters from 2 to 9, arguments are presented in terms readily understood by all stake holders of tropical soils and with both scientific and economic rigor so that they are not easily refuted.