Crop Yields and Available P and K in Soil as Affected by Sixteen Annual Applications of N, P, K Fertilizers

Corn, wheat, and a red clover-orchardgrass mixture were grown in rotation on Tatum silt loam for 16 consecutive years with each crop being grown eachyear and all plant residue removed annually.

The application of P significantly increased the yields of corn, wheat, and red clover-orchardgrass throughout the experiment. Hay and wheat yields were not significantly increased by added K. Added K significantly increased corn yields only during the last three years of the experiment.

Available soil P increased rapidly with the highest rate of applied P. Available soil K decreased so that at the end of the experiment there had been about a 50 per cent depletion in available soil K with all rates of added K.

The results emphasize the importance of clay minerals in supplying certain nutrients essential for plant growth. Soils that contain K supplying minerals such as dioctahedral mica and vermiculite require less added K for crop production than do soils not containing minerals that supply available K.