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.