Soil Test Values and Grain Yields during 14 Years of Potassium Fertilization of Corn and Soybean

Past fertilization may have increased the quantities of available K in many soils to levels not profitable for crop producers to maintain. To evaluate this possibility, the effects of annual K applications on soil test K (STK) values and yields of corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] grown in rotation were studied from 1976 to 1989 at two sites in Iowa. Four annual K treatments ranging from 0 to 90 lb K/acre were superimposed on plots having various initial STK values that had been established by applying 0 to 1200 lb K/acre from 1971 to 1975. The rate of annual K required to maintain STK values increased with increases in initial STK values. Significant yield increases due to annual K additions were observed frequently in plots having STK values below the medium soil test class (68 to 100 ppm) and infrequently in plots having STK values above medium. At prevailing prices of grain and fertilizer, economic returns to investments on annual K applications for both crops were positive when STK values were less than 75 to 100 ppm and negative when STK values were above this range. The results suggest that crop producers could increase profits by not applying K fertilizer to soils testing above the medium STK range.