The need for the spectral characterization of dominant salts and recommended methods of soil sampling and analysis for the proper spectral evaluation of salt affected soils using hyper -spectral remote sensing

Abstract

ABSTRACT Soil salinity and seasonal dynamics are typical soil characteristics along the west coast of India. The low-to-high soil/water salinity are classified based on the nature, content and composition of salts: the salinity (SAR) and sodicity (ESP, RSC) hazards. Soil salinity has been characterized based on typical reflectance in 427, 487, 950, 1414, 1917, 2206, 2380 and 2460 nm bands using hyper-spectral remote sensing (HRS). The inverse relation between soil salinity and spectral reflectance indicated the hygroscopic properties of dominant salt. Recent studies on spectral properties of saline soils (EC 3.2 to 30.46 dS m−1) have revealed an increasing trend from visible to SWIR1 and a dip at SWIR2 bands. Spectral properties of saline–sodic soils in IGP revealed i) a higher spectral response from dry-barren surface of a strongly (pHs >10) sodic soil and ii) a change in spectral response pattern due to vegetative cover and higher moisture content in irrigated soils. The results suggest for further validation of spectral data using natural salts. Studies on technical grade salts have indicated a relatively high reflectance of NaCl and CaCO3 than Na2CO3, NaHCO3, Na2SO4, CaSO4.2H2O (gypsum) and pyrite (FeS) salts; the later show a decreasing trend at higher wavelengths. Hygroscopic salts have shown prominent energy absorption of water molecules at 1400, 1900 and 2250 nm. The reflectance from surface soils varies with colour, texture, structure and surface roughness. Statistical and index-based models have also been successfully used for the diagnosis of soil salinity. The recommended methods for soil analysis, the available forms of nutrients and spatial variability studies are suggested for better salinity appraisal.