Effects of 50 Years of Fertilization with Phosphorus and Potassium on Soil Test Levels and Yields at Six Locations

Identical experiments were started at six locations in 1929 to determine response of cotton (Gossypium hirsutum L.), vetch (Vicia villosa L.) and corn (Zea mays L.) to rates of P and K in a 2-year rotation. Rates were revised in 1949 and 1958. Crops were changed in 1968 to corn, wheat (Triticum aestivum L.) and soybeans (Glycine max L. Merr.). Original soil samples and samples taken throughout the 50 years were analyzed using the Mehlich double-acid extract. The objectives of this study are to show the effects of the rates of P and K on (i) yields of the six crops during the last 20 years and (ii) soil test levels of the six soils over the 50 years. On soils that received 14 to 18 kg/ha of P from 1929 through 1957, vetch responded to fertilizer P at six locations, corn at four locations, and cotton at two locations during the 1958-1967 period. Response to P by corn, wheat, and soybeans was about the same during the next 11 years as for the previous crops. Large response to the first 28 kg/ha increment of K was produced by all crops at most locations. Only one of the six soils produced consistent response to the second 28 kg/ha increment of K. Vetch was the most responsive of the six crops to P, and cotton was most responsive to K fertilization. Rates of 14 to 18 kg/ha P increased soil test P from an average of 19 ppm in 1929 to 33 ppm by 1957. When applications were discontinued, the level dropped to that of untreated plots by 1973. Application of 27 kg/ha or more of P rapidly increased soil test P to “high” or “very high” levels, and these have been maintained by 30 kg/ha applications during the last 20 years of higher production. Plots that received no K for 50 years dropped from an average of 54 ppm in 1929 to 40 ppm by 1957 and have changed very little in the last 20 years. Rates of 18 to 28 kg/ha of K have maintained the soil test K level over the 50 years. Higher rates have increased K, even on soils with cation exchange capacities (CEC) < 5 meq/100 g. Rates of K accumulation showed a direct relationship to the CEC. Applications of 28 kg/ha of K were adequate to produce top yields and also to increase the level in five of the six soils.