A Treatise of Indian and Tropical Soils

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.

A synthesis of recent developments in the pedology of Vertisols achieved through the use of high resolution micro-morphology, mineralogy, and age control data along with their geomorphic and climatic history, has contributed to our understanding of how the climate change-related pedogenic processes during the Holocene altered soil properties in the presence or absence of soil modifiers (Ca-zeolites and gypsum), calcium carbonate and palygorskite minerals. The climate change has caused modifications in the soil properties in the presence or absence of Ca-zeolites, gypsum, CaCO₃ and palygorskite minerals. The formation and persistence of Vertisols in the Deccan basalt areas under humid tropical (HT) climatic conditions, provides a unique example of tropical soil formation. Such soil formation remained incomprehensible unless the role of zeolites was highlighted by the Indian soil scientists during the last two decades. Persistence of these soils in HT climate for millions of years has provided a deductive check on the inductive reasoning of the conceptual models on the formation of Vertisols in HT climate. The novel insights will serve as guiding principles to improve and maintain their health and quality while developing suitable management practices to enhance and sustain their productivity. However, much of the success of the management interventions still depends on the proper classification of Vertisols at the subgroup level, identifying the impairment of drainage in Aridic Haplusterts (ESP ≥5, <15), Typic Haplusterts (with palygorskite) and the improvement of drainage in Sodic Haplusterts/Sodic Calciusterts with soil modifiers. The semi-arid tropics (SAT) Vertisols at present are less intensively cultivated because of their inherent limitations. It is hoped that new knowledge on pedology, mineralogy and taxonomy of dry and wet climates will fulfil the need for a handbook on Vertisols to facilitate their better management for optimizing their productivity in the 21st century.

Red ferruginous (RF) soils of Indian tropical environments belong to five taxonomic soil orders (Entisols, Inceptisols, Alfisols, Mollisols and Ultisols), which clearly indicate that tropical RF soils in India have captured wide soil diversity. The spatially associated Ultisols with acidic Alfisols and Mollisols in both zeolitic and non-zeolitic parent materials in humid tropical (HT) climatic environments provides a unique example of tropical soil formation. Such soil formation discounts the exiting conceptual models on tropical soils. For a long time, this fact was not much appreciated, until the role of zeolites and other base rich parent materials was implicated in pedology and edaphology by the Indian soil scientists and soil mineralogists during the last two decades. Indian tropical soils support multiple production systems and generally maintain positive organic carbon balance. The recent developments on the pedology of RF soils, including their physical, chemical, biological, mineralogical and micro-morphological properties are very timely as the new knowledge improves the understanding as to how the parent material composition influences the formation of Alfisols, Mollisols and Ultisols in weathering environments of HT climate. This knowledge also explains how the relict Alfisols of semi-arid tropical (SAT) areas is polygenetic created by climate shift during the Holocene. Pioneering research efforts have improved the basic understanding of why the formation of Oxisols from Ultisols is an improbable genetic pathway in tropical environment of India and elsewhere in the world. There is a strong need to modify the mineralogy class of highly weathered RF soils. This basic information will help to dispel some of the myths on the formation of tropical soils and their low fertility.

Indian earth scientists and soil scientists based on large number of well-presented pedons in the Indo-Gangetic Alluvial Plains (IGP), spread along the west hot arid climate to per-humid climate in the east, have led to new perspectives on the historical development of the IGP and the soils therein. This addresses the hitherto little known subtleties of pedogenesis and polygenesis due to recorded tectonic, climatic and geomorphic episodes and phenomena during the Holocene. Based on degree of development, five geomorphic surfaces, with soil ages 0.5, 0.5–2.5, 2.5–5.0, 5.0–10, >10 ka respectively, are mappable in the IGP and correspond to the post-incisive chronosequences that evolved in response to interplay of fluvial processes, climatic fluctuations, and neotectonics during the Holocene. The polygenetic signatures, illuvial clay pedofeatures, pedogenic carbonates, clay mineralogy, and stable isotope geochemistry, suggest the evolution of the IGP soils witnessed two humid phases (13.5–11.0 and 6.5–4.0 ka) with intervening dry climatic conditions. The IGP soils with varying climate from hot-arid to per humid belong to Entisols, Inceptisols, Alfisols, and Vertisols orders. Addition and depletion of OC, formation pedogenic CaCO₃, illuviation of clay particles and argilli-pedoturbation are the major pedogenic processes in soils of the IGP during the Holocene. The IGP soils are, in general, micaceous, but the soils with vertic characters are smectitic. A better understanding of the pedology of the IGP soils and their linkage to climate change, and landscape stability appear to be potentially useful as guideline for their management. Thus the new knowledge base has potential as a reference for critical assessment of the pedosphere for health and quality in different parts of the world and may facilitate developing a suitable management practices for the food security in the 21st century.

Use of models to explain adequately the formation of tropical soils indicates that although among the most popular models applicable in soil formation, the residua and haplosoil models have relevance to formation and persistence of Indian tropical soils, they cannot explain the existence of million years old Vertisols, Alfisols and Mollisols under humid tropical climate because these models did not consider the stability of base rich primary minerals over time. This novel understanding provides a deductive check on the inductive reasoning so far made on the formation of soils in tropical humid climate and also establishes the validity of Jenny’s state factor equation in the formation of the Indian tropical soils in the intense weathering environments under HT climate.

Most published research on soil degradation in general emphasizes the role of anthropogenic factors. Even among the natural soil degradation processes the regressive pedogenic processes that lead to the formation of CaCO₃ and concomitant development of subsoil sodicity and the adverse effects of palygorskite mineral on the soils of the semi-arid tropics (SAT), have received little global attention as the natural processes of chemical degradation of soils. Pedogenic calcium carbonate, soil sodicity and palygorskite mineral impair the hydraulic properties of the SAT soils, which reduce their crop productivity. This type of unfavourable soil health triggered by the tectonic-climate linked regressive pedogenic processes (formation of pedogenic calcium carbonate and development of sub soil sodicity) needs to be globally considered as the natural soil degradation process. The regressive pedogenic processes that are inherently connected to the development of natural soil degradation, expands the basic knowledge in pedology and thus it may have relevance in soils of other SAT areas of the world. Research efforts made in the Indian subcontinent explains the cause-effect relationship of the degradation and provides enough insights as to how the remedial measures are to be invented including the role of pedogenic CaCO₃ and geogenic Ca-zeolites as soil modifiers along with gypsum, in making naturally degraded soils resilient and healthy.

Identification of paleoclimatic signatures in paleosols is a major challenge to soil scientists. It is realized that the clay minerals of the paleosols are potential promising materials for documenting and resolving a wide spectrum of different genetic environments and reactions. It is often difficult to determine which soil minerals are characteristic of different climatic zones as the environment itself changes over time with consequent further modification of mineral assemblage and this is particularly true for clay minerals. Clay minerals such as kaolinite often remain unaltered through subsequent changes in climate, and therefore, may preserve a paleoclimatic record. Other layered silicates at a less advanced stage of weathering may adjust to subsequent environmental changes and thus may lose their interpretative value for paleoclimatic signatures. However, Indian soil scientists, clay mineralogists and earth scientists indicate that minerals of intermediate weathering stage can act as potential indicators of paleoclimatic changes in parts of central India and Gangetic Plains. They have demonstrated how secondary minerals like di- and trioctahedral smectites (DSm and TSm), smectite-kaolinite interstratified minerals (Sm/K), hydroxy-interlayered smectite (HIS), hydroxy-interlayered vermiculite (HIV), pseudo-chlorite (PCh) of intermediate weathering stage, and CaCO₃ of pedogenic (PC) and non-pedogenic (NPC) origin can be regarded as potential indicators of paleoclimatic changes in major soil types of India and also in paleosols of the alluvial sediments of the Himalayan river systems and Cratonic source from Peninsular India.

Minerals in soils are the result of both inheritance and authigenic formation of both primary and secondary minerals. Comprehensive reviews on the soil clay minerals and other minerals in the past indicate that there are not many attempts to show the influence of minerals in soil genesis and management. A search for links between mineralogy and soil properties of agricultural importance is likely to be difficult because many a time the description of minerals actually present in a soil is inadequate or incomplete. Further, as soil minerals often differ from “type” minerals, it is very much necessary to investigate the properties of these minerals relevant to the properties of the soil in bulk. Despite our general understanding on the role of minerals in soils, it is necessary to investigate the properties of the minerals, especially clay minerals, their mixtures and surface modifications in the form that they occur in the soil. From the few examples under different agro-climatic situations cited it is evident that unless the mineralogical description is accurate enough for the purpose intended, it would not be prudent to look for their significance in soils. With the use of high resolution mineralogy, identification and explanation of many enigmatic situations in soils can be conveniently solved. Therefore, the advanced information developed provides adequate mineralogical database that would explain discretely many unresolved issues of the nutrient management in terms of specific soil minerals in general, and clay minerals in particular and their significance in soil as a sustainable medium for plant growth. It is hoped that this synthesis would help to assess the health and quality of soils while developing suitable management practices to enhance and sustain their productivity in the 21st century.

Many consider that the soils of the tropical soils are acidic, infertile, and that they do not support a reasonable sustained agricultural production. Recent research in agricultural production in tropical Asia and Latin America indicate that universal infertility of tropical soils is a myth. In India, during the green revolution period, a renaissance was initiated by the National Agricultural Research Systems (NARS) in a modest way by managing the tropical soils properly for their restoration and preservation. It is clear now that the substrate quality of Indian tropical soils is good enough to support the agricultural land uses, horticultural, spices and cash crops, in making India self-sufficient in food production. The substrate quality is maintained by progressive pedogenic processes (pedology) in tropical Indian soils, which are inherently linked to many edaphological issues. Recent advances in pedology of the Indian tropical soils have demonstrated their considerable potential and also amply established the basic necessity of pedological research, in better understanding some queer edaphological aspects of Indian tropical soils (Vertisols, RF soils and IGP soils), which are affected by the climate change during the Holocene period. Thus edaphology is inherently based on deep fundamental understandings of soils and thus basic pedological research in tropical soils needs to be encouraged vigorously to link some of their major unresolved edaphological aspects to develop improved management practices as guiding principles to improve and maintain soil health through adequate national recommended practices in other tropical parts of the world.

The treatise of Indian and tropical soils ends with a chapter ‘Summary and Concluding Remarks’, which projects a concise but a precious synthesis of unique research results obtained by the soil and earth scientists on major soil types of tropical Indian environments. In the past, much valuable work has been done throughout the tropics, but it has been always difficult to manage these soils to sustain their productivity and it is more so when comprehensive knowledge on their formation remained incomplete for a long time. Soil care continues to be the main issue in national development and thus needs to be a constant research agenda in the Indian context. This is imperative since soil knowledge base becomes critical in meeting the food demand for ever increasing human population. In this task basic pedological research is required to understand some of the unresolved edaphological aspects of the tropical Indian soils to develop improved management practices. This chapter highlights the major theme areas of soils (Chaps. 2–9) that have been dealt in the perspective of the recent developments in pedology, mineralogy, taxonomy and edaphology with context of tectonics and climate change in the Indian sub-continent. The usefulness of such information in unravelling many interesting pedological, edaphological, mineralogical and taxonomical issues of soils of the country has been well established. The synthesis of research results finally transforms to state-of-art information, which may serve as guiding principles to improve and maintain soil health through adequate national recommended practices in other tropical parts of the world.