CHLORIDE DEPTH IN SOIL AND UPTAKE BY SOYBEAN

Soybean [Glycine max.(L.) Merr.] yield reductions due to chloride (Cl) toxicity, as a result of KCl application, have been recognized. A fie ld study was conducted to determine whether Cl uptake by soybean could be minimized by incorporating Cl deeper in the soil profile and to pr edict the most probable depth of Cl after surface application by a wea ther-based model. Essex, a Cl-susceptible soybean cultivar, was grown on a Mexico silt loam (fine montmorillonitic, mesic Udollic Ochraqualf s) with Cl incorporated at depths of 0-15, 30-45, or 60-75 cm and anal yzed for Cl through the growing season. Chloride applied at the 0 to 1 5-cm depth significantly increased Cl concentrations in soybean leaves , stems, and roots, especially, in early stages of growth. Chloride co ncentration was higher in roots > stems > leaves. There was no signifi cant increase in Cl uptake when Cl was applied below 30 cm in the soil . Chloride added to the surface of reformed soil profiles moved with t he water front as the soil was wetted. In the field, Cl added to the s oil surface in May was uniformly distributed throughout the profile by mid-September. The weather-based model, under Missouri conditions, pr edicts a 9 out of 10 chance that Cl added in the fall would move below a depth of 50 cm by June. Results suggest that the risk of Cl toxicit y to soybean can be minimized by applying fertilizers containing Cl fa r enough in advance to insure a soil wet-dry cycle prior to soybean pl anting.