2.1. Study Area and Soil Sampling
The study was carried out in the middle Indo-Gangetic plain zone, India situated in the state of Uttar Pradesh covering the administrative districts of Varanasi, Chanduali, Sant Ravidas Nagar and Mirzapur, localized between 82°30′ and 83°30′ East and between 24°30′ and 25°30′ North covering a total area of approximately 9604 km2(Fig 1). A total of 280 geo-referenced composite soil samples were collected from surface (0–15 cm) layers after crop harvest. Stainless steel soil auger was used for collection of soil samples and the coordinates of the sample points were recorded via hand held Global Positioning System (Model Garmin etrex ). Samples were air dried and ground in a wooden pestle and mortar to pass through a 2 mm sieve and electrical conductivity of soil was measured in 1:2.5 soil water suspensions using electrical conductivity meter and expressed as dS m-1 (Ghosh et al., 2012).
The processed soil samples were further homogenized in Retsch Mortar Grinder RM 200 for 3 minutes wherein samples were grounded up to 0.5 mm size. The RM 200 is suitable for the homogenous and reproducible sample preparation for precision of analysis. These ground soil samples were subsequently used for recording spectral signature in alpha- MIR spectrometer and the processed soil samples that passed through 2 mm sieve was used for obtaining spectral signature in NIR spectroradiometer.