Degradation in Indian Tropical Soils: A Commentary

Abstract

Land and soil degradation are often believed to be an effect of only anthropogenic processes although both land and soil degradation are induced by natural and human-induced processes, which are considered as chemical, physical or a biological phenomenon. To record the area of the severity and extent of soil degradation in the country has been a continuous endeavour by many national agencies. Historical account on this issue indicates that the earliest assessment of the area affected by the land degradation was made by the National Commission on Agriculture at 148 mha, followed by 175 m ha by the Ministry of Agriculture. The ICAR-NBSS &LUP projected an area of 187 m ha as degraded lands in 1994, and it was revised in 2004, and the area re-estimated to be 147 m ha in contrast to the estimate of 123 m ha by the National Wasteland Development Board. In view of the different estimates, the National Academy of Agricultural Sciences (NAAS) took a pioneering leadership to bring all the concerned agencies together through a series of meetings and deliberations to achieve a harmonized classification system comprising wasteland classes and soil degradation. A map was prepared, and the total area under degraded and wastelands in the country stood at 114.01 mha. Out of this estimated area, the areas under water erosion, wind erosion, chemical degradation (salinization/alkalinization and acidification), salt-affected soils and acid soils are provided. The present status of degradation is projected to be alarming because India supports 18% of the world’s human population and 15% of the world’s livestock population but has only 2.4% of the world’s land area. Area under soil degradation is 114.01 m ha, whereas arable land covers ~141 m ha. On the other hand, the country aspires to fulfil both the current and future projections of creating food stock and self-sufficiency in food grain production. This strongly suggests that the analytical protocols followed to include areas under different forms of above-mentioned soil degradation are not made on sound science-based data. In the estimate of 114.01 mha under various degradation processes, water erosion is considered to be the most widespread form of soil degradation in India especially in the red ferruginous (RF) soils of both humid tropical (HT) and SAT climatic environments without assigning due weightage to the rate of top soil formation in HT soils of north-eastern hills (NEH) and southern peninsular areas. It would be prudent to delineate an area under degradation only after the establishment of cause-effect relationship that are developed on operative basic pedogenic processes linked to regressive and progressive pedogenesis in SAT and HT climates, respectively. Without ascertaining these operative pedogenic processes that are linked to soil/crop productivity, any attempt to estimate the land areas under water erosion and chemical degradation based on arbitrary assumptions will end up as unrealistic. In view of remarkable anomalies observed between the observed potential of soil productivity and the projection on the extent of soil erosion hazards in soils of both SAT and HT climates, and also designating productive acid soils as degraded soils, a revision is necessary to confirm their degradation status. Similarly, sodic soils show enough resilience even under prolonged natural vegetative cover and also after typical short-term management interventions, suggesting that sodification is not an irreversible pedogenic process to cause a permanent senility in soils. Therefore, it would be prudent to keep these soils out of degraded class so that the present projected area under degradation status could be reduced and highlight their ecosystem services and thus become compatible with the nation’s aspiration of food stock and food grain production. The gain in soil formation in HT climate justifies the existence of pedogenetically matured Ultisols, Alfisols, Mollisols and clay-enriched Inceptisols on a stable landscape, which have no major issue of physical soil degradation in term of water erosion as observed in some states of NEH areas, Goa, Maharashtra, Madhya Pradesh, Karnataka, and Andaman and Nicobar Islands. This is also supported by the major pedogenetic processes operative in such soils, which are enriched in OC concentration of ~ 1–3% in the 0–30 cm of the soil depth effected through the addition of C by litter falls and its accumulation as soil organic matter under adequate vegetation and climate. The major pedogenic processes are the translocation of clay particles and transformation of 1.4 nm minerals to 1.4–0.7 nm mixed layer minerals which are not pure kaolinite but are kaolins with a basal spacing of around 0.7 nm. Therefore, such soils do not represent nutrient impoverished kaolinitic/gibbsitic soils of most advanced weathering stage, and they demonstrate their unique ecosystem services. RF soils occurring under SAT environments in the southern states of Karnataka, Andhra Pradesh and Tamil Nadu are reported to suffer from soil loss due to water erosion at the rate of >10 t ha⁻¹ year⁻¹ as the threshold for soil degradation. But the RF soils of SAT are dominantly matured soils like Alfisols followed by Mollisols. In view of the operative pedogenic processes, such soils may not have any major problem of soil loss due to water erosion. Under successful agricultural production system, RF soils do not suffer from any major soil loss and are also capable of providing ecosystem services. The threshold value of soil loss by water erosion as a sign of degradation needs to be based on pedogenesis and experimental results. Any initiative to assume an arbitrary value of >10 t ha⁻¹ year⁻¹ as the threshold limit for degradation by water erosion would be an imprudent approach and thus to be wisely avoided to prevent the unnecessary national expenditure for the current and future conservation measures.