Spatially refined salinity hazard analysis in gypsum-affected irrigated soils

The global extent of salt-affected agricultural land, 20% of which is deemed gypsiferous, results in billions of dollars of annual economic loss, a serious problem deserving of attention. However, the analysis of gypsiferous saline soils, such as in the irrigated Lower Arkansas River Valley (LARV) of Colorado, can result in an inflated estimation of soil salinity when using the traditional soil saturated paste extract electrical conductivity (EC_e), leading to inaccurate crop yield loss predictions and misguided decisions for remediation. Sparingly soluble gypsum (CaSO₄ $·$ 2H₂O) in these soils dissolves more readily during laboratory preparation of saturated paste extracts because of excess soil water dilution coupled with sample disturbance. We present a pragmatic linear-regression approach to correct for this phenomenon, calibrated using two adapted methods for correcting EC_e on an individual sample basis. The novel approach used electrical conductivity of pore water samples from saline fields to evaluate the accuracy of the correction methods. The approach was applied on soil samples from two surface-irrigated, saline fields in the LARV, which were mapped using electromagnetic induction data and analysis of covariance linear regression, calibrated for EC_e and EC_e corrected for excess gypsum dissoultion (EC_eg). Average EC_eg values are as much as 26% lower than uncorrected EC_e in gypsum-biased portions of the fields. Estimation of corn salinity hazard in these gypsum-affected areas using EC_eg in lieu of EC_e in a traditional yield response function generated mean relative yield values that are higher by up to 13 percentage points. We discuss lessons learned and suggest enhancements to the techniques.