Root‐Zone Salinity

This paper provides the tools for distinguishing levels of tolerance to root‐zone salinity in agricultural crops. Such distinction rests on the response of a crop's product yield following the declining, sigmoid‐shaped, modified compound‐discount function { Y r = 1/[1 + ( C / C 50 ) exp( sC 50) ]} for plants grown as crops exposed to increasing root‐zone salinity. This nonlinear function relates relative yield ( Y r ) to root‐zone salinity ( C ) measured in equivalent saturated soil‐paste extract electrical conductivity with two nonlinear parameters, the salinity level producing 50% of the nonsaline crop yield ( C 50 ) and a response curve steepness constant ( s ) equal to the absolute value of the mean d Y r /d C from Y r = 0.3 to 0.7. These discount parameters suggest the existence of a single‐value salinity tolerance index (ST‐Index) equal to the 50% reduction in crop yield from that of the nonsaline yield plus a tendency to maintain some product yield as the crop is subjected to salinity levels approaching C 50 , i.e., ST‐Index = C 50 + s ( C 50 ). The explicit purpose of this study is to determine if the discount function using biophysically relevant parameters can be applied to historical data sets. Approximations for C 50 and s were identified in the threshold salinity ( C t ) and declining slope ( b ) parameters of the well‐known threshold‐slope linear response function. Several procedures for converting C t to C 50 and b to s offer the linkage between these linear and nonlinear response functions. From these procedures, two regression equations, C 50 = 0.988[(0.5/ b ) + C t ] − 0.252 and s = 1.52 b , proved the most appropriate for the eight representative field, forage, and vegetable crops tested. The selected conversion procedures were applied to previously published C t and b values to obtain a list of the relative root‐zone salinity tolerance in agricultural crops. In addition to C 50 and s , values for exp( sC 50 ) and the ST‐Index were computed for each crop. The revised list provides extension personnel and plant growth modelers the parameter values from a nonlinear analog of crop yield response to root‐zone salinity.