Modern Technologies for Diagnosis and Prognosis of Salt-affected Soils and Poor-quality Waters

Abstract

Soil salinity and brackish groundwater are primary concerns for reduced productivity of 953 million ha in the world. In India, 6.73 M ha of salt-affected soils (SAS) is distributed in 15 states and 13 agroclimatic regions. Salt deposition at low topographic zones, high evaporation in arid zone, salty parent materials and brackish ground use in peninsular plain, and inundation of saline seawater in coastal areas are primary processes controlling genesis and distribution of SAS. Canal irrigation practices have contributed to waterlogging, soil salinization, and losses of soil productivity in poorly drained soils. Remote sensing data with improved spatial and spectral resolutions has facilitated detection and delineation of SAS with limited ground truth and soil studies. Using high-resolution remote sensing data, prognosis of soil salinity was studied to quantify soil salinization processes in canal command areas integrating topography, hydrology, and aquifer characteristics. Spatial variability of sodic and saline soils with salty groundwater was studied at farm scale at experimental farms. Visual and digital analysis of remote sensing data facilitated the identification of strongly salt-affected soils by high spectral reflectance of salt crusts from barren surfaces. High energy absorption in the SWIR band enabled identification of waterlogged soils in canal-irrigated areas with poor natural drainage. Temporal dynamics of salty surfaces, vegetation, and normalized difference vegetation index (NDVI) data during June, March, and October seasons were used for SAS with poor-quality or high RSC groundwater. Mixed spectral signatures for salt crusts, moderate cropping density, and surface wetness in moderately and slightly sodic soils are authenticated by ground truth study. Thermal band was used for salty soils and sand dunes showing close spectral signatures in visible range. Natric horizon formation, clay illuviation, iron and manganese mottles, higher moisture content, and precipitation of subsurface calcareous materials are typical soil-forming processes in sodic soil. Chemical analysis indicated high pH, exchangeable sodium percentage (ESP), and sodium adsorption ratio (SAR) values and the dominance of carbonate and bicarbonates of sodium. High moisture content and salt saturation (commonly saline soils of sodium chloride and sulfate salts) are typical features associated with waterlogged (surface ponding) soils and high water table depth (potential waterlogging). Periodic irrigation with water of high pH, SAR, and RSC (residual sodium carbonate) values favored the formation of salty soil profiles in arid and semiarid regions. High clay contents, smectite clay minerals, critical ESP (5 or more), and salty groundwater are primary constraints of black soil in the peninsular plains. The complex saline-sodic soils of alluvial (A), aeofluvial/arid (B), and others (H) are classed as sodic. Soils of coastal (D), deltaic (C), and mud flats/mangrove swamps (G) were classified as saline. Benchmark salt-affected soils were identified for monitoring in agroclimatic and physiographic regions. The largest areas (67% and 75%) of SAS lie in the arid to semiarid (300–1000 mm rainfall) and strong hyperthermic (25–27.5 °C) temperature zones and in the Pleistocene and Recent (39%) geological formations. Recent IRS data (years 2010–2013) revealed 315,617 ha of SAS lies in 18 districts of Haryana state. Attempts were also made to update similar databases for Uttar Pradesh and Gujarat states.