Mineralogy of Soils of Semi-Arid Tropical India

Abstract

The Semi-Arid Tropical (SAT) soils occupy the major part of India covering about 50% of the total geographical area in northern and central part of the Indo Gangetic Plains (IGP) and Peninsular region of black soil regions (BSR) including red and black soil association. Indo-Gangetic Plains (IGP) are most extensive fertile fluvial plain, capable of feeding about 50% of the country’s population is over utilized. The black soil regions are developed from the alluvium of weathered Deccan basalt is, on the other hand, underutilized due to its physical, chemical and climatic constraints. Here an attempt has been made to study the mineralogy of these soils and relate it with their pedo-genesis, potential and degradation and propose pathways for probable redressal of problems. Soils of the IGP vary from Entisols, Inceptisols and Alfisols, sandy to clayey, calcareous and alkaline. The majority of the soils indicate micaceous mineralogy with substantial weathering of biotite resulting in decreasing trend of clay mica with depth. Mica is dominant in silt and clay fractions along with vermiculite, mixed layer mineral, smectite, kaolinite and chlorite. The micas are both di- and trioctahedral type and vermiculite and smectite are the weathering product of trioctahedral mica through 1.0 – 1.4 nm mixed layer phase. The 0.7 nm peak of K is not true kaolinite but interstratified with 1.4 nm mineral is an indicative of earlier humid climate. The presence of both NPC and PC in these soils throughout the climatic region was noticed, with the former formed in the humid and later in arid climate. Micro morphological features indicated impure clay pedofeatures because of the impairment of parallel orientation of clay platelets due to their development in sodic environment of soils. Thus illuviation and PC are two concurrent pedogenic events in the semi-arid part of the IGP. The mineral assemblage and micro morphological evidence indicated that IGP has experienced two phases of aridity punctuated with a humid phase during (6500-4000 yr B.P.). The adverse climatic condition induced inorganic-carbonate-formation-facilitated sodicity which impaired hydraulic properties. The problem is further aggrevated by poor quality of irrigation. Black soils of semi-arid tropics of India are developed from alluvium of weathered Deccan basalt occur in a wide range of climatic environment and occupies an area of 76.4 m ha as per the revised estimates. The first weathering product of Deccan basalt high in plagioclase feldspar is dioctahedral smectite which detached and transported downstream and deposited as alluvium in lower topographic position during humid climate. The other minerals observed in clay fraction are little amount of mica, vermiculite and kaolin. Vertisols contain both muscovite and biotite mica in the silt and clay fractions. Dioctahedral smectite (DOS) cannot be formed from muscovite because of its sensitivity of K levels in soil solution. Biotite converts into tri-octahedral vermiculites (TOV) and thus the simultaneous formation of DOS and TOV from mica is unlikely. Thus the DOS is a weathering product of plagioclase in humid climate. The presence of Sm-K is common in Vertisols of India. In arid climates smectite shows no sign of transformation except hydroxy-interlayer in smectite layers. The presence of hydroxy-interlayered dioctahedral smectite in the fine clay fraction as well as HIV and pedogenic chlorite in the silt and coarse clay fraction indicate that hydroxyl-interlayering in smectite and vermiculite occurred in humid climate and their crystallinity is preserved in the non-leaching environment of the present. This was also confirmed by the pedogenic and non-pedogenic carbonate with relict Fe-Mn coated carbonate nodules which are older and developed in wetter climates than the pedogenic carbonate (PC) of dry climate. As explained in the IGP soils, in black soils also development of PC and clay illuviation are two pedogenic processes simultaneously. Presence of Vertisols or Alfisols with vertic properties in subhumid and humid tropical part of India is attributed to the supply of sufficient bases from zeolite. Otherwise the smectite might have transformed into kaolin due to transformation. Development of subsoil sodicity due to precipitation of carbonates in dry climates is a threat to natural resources. The subsoil sodicity caused by the Na-clay smectites in Vertisols of semi-arid and arid climate impairs the hydraulic properties of soils which is considered as a self-terminating processes wherein sodic soils were formed with a decreasing trend of ESP with depth. The hydraulic properties are also impaired by high Mg in the exchange complex of Vertisols due to development of PC and ESP. However, continuous release of Ca+2 ions from minerals like Ca –rich zeolites and gypsum present in Vertisols improves the sHC. However, in some other Vertisols palygorskite, a mineral with high Mg,causes dispersion of clay colloids to form 3D mesh in the soil matrix. Thus the Vertisols with palygorskite mineral should be treated separately as naturally- degraded soils. The SAT area of India has enormous potential and is considered the next food bowl of India. The soils of IGP and BSR have climate-related degradation and management problems. Future management interventions amidst the threat of climate change is to utilize the databank developed so far for developing best management option which will control development of SIC and improvement of SOC.