Characterization of Soybean Yield As Affected By Tissue Chloride Concentration and Cultivar Chloride Rating.

Chloride (Cl) toxicity is recognized as a yield limiting problem for irrigated-soybean [Glycine max (L.) Merr.] production in Arkansas.  Elevated Cl concentrations are known to limit soybean yield, but criteria for diagnosing Cl toxicity are not available.  Our objective was to define critical trifoliolate leaf-Cl concentrations at which yield loss begins for soybean cultivars categorized as Cl-includers or -excluders. Three Cl-includer and three Cl-excluder cultivars were planted in strips and each strip received a total of 0, 280, 560, or 840 kg Cl ha^-1 in five separate applications between the R1 and R5 stages. Chloride was added to the soil as a solution of MgCl₂ and CaCl₂. The experiment was a randomized complete block design with a split-plot treatment arrangement where Cl rate was the main plot and variety was the subplot. Recently mature, trifoliolate leaves were collected at the R2, R3, R5, and R6 stages from each variety and analyzed for Cl concentration.  The relative yield of each variety among the four Cl rates was calculated using mean actual grain yields. Relative yield was regressed against leaf-Cl concentration at each growth stage using linear and quadratic models. Relative grain yield decreased at a uniform linear rate as leaf-Cl concentration increased but the intercept depended on variety Cl rating.  Leaf-Cl concentration explained 60, 62, 61 and 47% of the variability in relative yield at the R2, R3, R5, and R6 growth stages, respectively.  At the R3 stage, leaf-Cl concentrations less than 1076 and 3735 mg Cl kg^-1 were predicted to produce 95% relative yields for the excluder and includer varieties, respectively. Relative yield decreased by 1% as leaf-Cl increased by 336 mg Cl kg^-1. Leaf-Cl concentration increased substantially from the R5 to R6 growth stages and corresponds with the time that Cl-toxicity symptoms became visibly evident in the soybean leaves.