Root-Zone Salinity

Abstract

Six empirical functions were compared for describing the product yields of agricultural crops grown while subject to increasing levels of root-zone salinity. The four nonlinear functions fit the test data from a spring wheat (Triticum aestivum L. , cv. Biggar) experiment conducted in Canada's Salt Tolerance Testing Facility closer than the two linear functions. Although each of the four nonlinear declining functions could reasonably describe the data, the modified compound-discount equation recorded the lowest root mean square error and the highest R 2 value. Additional response data using the nonlinear discount function obtained from 33 separate trials averaged 11% closer in statistical fit and 45% lower in statistical error than the best linear function. The discount function {Y I = 1/(1 + [(C/C 50 ) exp(sC50) ]} follows a sigmoidal form and relates relative yield (Y I ) to a measure of root-zone salinity (C) such as the solute concentration with an electrical conductivity of an equivalent saturated soil paste extract (EC e ). This function features two parameters, the salinity level producing 50% of the nonsaline crop yield (C 50 ) and the absolute value of the general decline in relative yield with salinity at and near C 50 , the steepness constant (s). These parameters combine to form a single-value, salinity-tolerance index (ST-Index) consisting of the 50% reduction in crop yield (C 50 ) plus the tendency to maintain some product yield as the crop is subjected to increasing salinity levels approaching C 50 , i.e., ST-Index = C 50 + s(C 50 ). The ST-Index for the Biggar wheat registered 6.44. Approximations for C 50 and s can be derived from the threshold salinity (C,) and declining slope (b) parameters of the threshold-slope linear response function [Y, = 1 - b(C - C t )]. Procedures for converting C, to C 50 and b to s offer linkages between these linear and nonlinear response function parameters, and are further explored in this paper's companion. The resulting ST-Index-values equal 6.56, 9.43, and 5.67 for sample field (corn, Zea mays L.), forage (alfalfa, Medicago sativa L. and falcata L.), and vegetable (radish, Raphanus sativus L.) crops, respectively.