Pedogenic Processes Derived Plant Available Water Capacity (PAWC) for Rain-Fed Deep-Rooted Crops in Indian Vertisols: A Pragmatic Method

Abstract

Global pedologists and edaphologists have created a huge database on the properties and management of smectitic Vertisols (cracking clay soils, CCS) but they are not adequate to optimize their use and management. This review is a synthesis of research done in the recent past at ICAR-NBSS, Nagpur, to search for real soil moisture status during crop growth. Despite the fact that the smectitic CCS do retain the highest amount of water both at 33 kPa and 1500 kPa, they have limitations to both rainy season and winter crops in the majority of CCS of central India under semi-arid tropical (SAT) environments due to lack of adequate moisture and poor porosity in their subsoils. Due to these predicaments, farmers of central India are unable to grow both rainy and winter crops in a year. Research indicated that the laboratory determined available water content (AWC) is not fully released during the crop growth period. Thus, the inclusion of AW capacity as one of the soil parameters for soil suitability analysis of cotton is inappropriate and inefficacious bio-physical parameter because soil moisture after cessation of rains remains at 100 kPa for non-sodic CCS and for sodic CCS (ESP > 5 but < 15), it remains at 300kPa. Inadequate soil moisture is due to the impairment of soil hydraulic properties caused by the dispersion of smectitic clays in the presence of both Mg and Na ions on the soil exchange complex. Recent research on depth-wise soil moisture characteristics curves indicates that the release of soil water beyond 800 kPa is negligible in Vertisols, indicating the failure in the release of soil water beyond 800 kPa is due to the dispersion of dominant nano-size smectite. The difficulties in releasing soil moisture at tension at or higher than 800 kPa suggest that moisture in micropores is held very tightly by Mg- and Na- smectite clays and thus, the release of soil moisture beyond 800 kPa is not significant throughout the depth of Vertisols. Therefore, the calculated PAWC considering the soil water held between 100-800 kPa for non-sodic CCS, and for sodic CCS, the soil water held between 300-800 kPa, showed a better significant positive correlation with cotton yields, in comparison to the correlation obtained between PAWC at 100-1500 kPa for non-sodic and 300-1500 kPa by the earlier method. The better correlation highlights how fundamentally pedogenetic processes driven by PAWC controls the movement of rainwater and its retention and release in SAT Vertisols. Therefore, PAWC is a unique bio-physical property, which can act as a guiding principle forthe growing of deep-rooted crops in abiotically stressed Vertisols of Indian SAT areas. In addition, PAWC could be applied as a useful parameter in further revision of agro-ecological sub-regions (AESRs) in black soils areas for better crop planning and ecosystem services of SAT Vertisols.