THE RELATION OF SOIL TEST VALUES TO FERTILIZER RESPONSE BY THE POTATO: III. EXCHANGEABLE POTASSIUM AND POTASSIUM FERTILIZER REQUIREMENTS

Abstract

In experiments conducted at 46 locations over a 6-year period soil potassium levels ranged from 46 to 361 p.p.m. in exchangeable potassium and from 1.1 to 9.6% K saturation. The average yield of tubers on zero-K plots was 86.2% of the maximum (yield obtained from optimum K fertilization) and the zero-K plots produced less than 75% of the maximum at only two locations.Utilization of Bray's modified Mitscherlich equations indicated high variability in c1 values and poor correlation of relative yields (as percentages of the maxima) with soil test values. Expressing soil K as % K saturation provided only slightly better correlations than exchangeable K.The polynomial response equation relating absolute yields (bu/acre) with rates of applied potassium was highly significant, and near maximum yields of tubers were obtained at 150 lb/acre of K. When soils were grouped according to exchangeable K values into "high" (> 230 p.p.m.), "medium" (230–130 p.p.m.), and "low" (< 130 p.p.m.) classes, similar responses to application of K fertilizers were obtained for each. However, greater yields of tubers resulted on the higher soil test classes throughout the entire range of treatments.A highly significant linear depression in dry matter content resulted from potassium applications, with each 50 lb/acre increment of K depressing dry matter by 0.5%.