3.1. Descriptive statistics of soil electrical conductivity
The data for electrical conductivity was not normally distributed as evident from the Q-Q plot (Fig 2a), where, considerable deviation from the straight diagonal line at both ends can be observed and hence the data was log transformed (to the base 10) to make it normally distributed (Fig 2b). The descriptive statistics revealed considerable variability of soil electrical conductivity (Table 1). The minimum and maximum values of electrical conductivity were 0.01 to 1.71 dS m-1, with a mean value of 0.46 dS m-1 and the values of skewness and kurtosis were 1.0 and 2.34 respectively. Development of soil salinity is sometimes geogenic, being affected by parent material and arid climate, but is largely anthropogenic in the Indo-Gangetic plains, being affected by fertilization, crop production or management history. Soil salinity is generally measured as electrical conductivity in the saturation paste or its liquid extracts, with soils having ECe> 4 dS m-1 being referred to as saline. It may be reiterated that the procedure for measuring EC in the present study is that which is routinely followed in soil testing laboratories where EC is measured in the soil suspension (1: 2.5) used to measure pH, and would be considerably higher when measured in saturation extract, normally used to differentiate saline and non-saline soils. Moreover, soil salinity is an important parameter of estimating soil quality, is highly variable, changes rapidly over small distances spatially, and hence a large number of samples are required to adequately characterize soil salinity across landscapes (Aldabaa et al., 2015). Further any increase in electrical conductivity, has great bearing on efficient soil and water management and sustainable crop production (Bilgili et al., 2011). To alleviate the cost of extensive sampling, followed by sample preparation and detailed laboratory analysis, hyperspectral reflectance spectroscopy is a lucrative alternative for rapid characterization of salt content in soil, and justifies investigation.