Legacy data-based national-scale digital mapping of key soil properties in India

Mapping soil resources at a national scale in large countries such as India is a challenge because of limited soil data available and efforts to collect them. Legacy soil information shows promise; but, there are still challenges that need to be addressed. In this study, we deliver the first digital maps of key soil properties down to 2 m depth across India using legacy data and quantified the relationships between mapped soil properties and bioclimatic and terrain attributes. A legacy database containing analytical data for 1,707 soil profiles with 7,337 soil horizons was collated from reports published by the National Bureau of Soil Survey and Land Use Planning and other Indian organizations. 3D regression kriging based on the random forest model was used to map sand and clay contents, pH and soil organic carbon (SOC) contents at depths as per the GlobalSoilMap specifications. Important covariates included mean monthly temperature and precipitation data from the World Climatic Centre and terrain attributes derived from the NASA's Shuttle Radar Topography Mission (SRTM) digital elevation model. The uncertainty of the model was quantified as the coefficient of variation of the predicted soil properties by repeated random sampling of the profile dataset into calibration and validation samples at a ratio of 75:25. The performance statistics for the surface soil properties were superior to subsurface soils with the highest Lin's concordance coefficient (LCC) recorded for soil pH. Estimated LCC values in validation datasets ranged from 0.81 to 0.84 for pH, 0.30 to 0.59 for SOC, 0.48 to 0.56 for clay content, and 0.34 to 0.44 for sand content. Elevation, topographic wetness index, high rainfall, and temperature were observed to be the major drivers for the variability of selected soil properties. Prepared digital soil maps across different agroecological regions showed that sandy soils dominated Western Plains while clayey soils were dominant in the central Deccan Plateau. Soils of the North-Eastern hills were acidic in nature while the Western Ghats and Coastal Plains showed high SOC accumulation. Although finer soil fractions are considered as major drivers of SOC stabilization, rainfall during June (onset of monsoon) was a major climatic driver for SOC in Indian soils. The national maps of soil properties may be linked to soil productivity and provisioning of ecosystem services to guide policy makers for creating region-specific soil management plans.