RELEASE OF POTASSIUM BY 18 ONTARIO SOILS DURING CONTINUOUS CROPPING IN THE GREENHOUSE

Abstract

Eighteen Ontario soils were ranked according to their capacity to supply potassium from non-exchangeable forms to eight successive crops of alfalfa. This capacity was found to be closely related to the percentage of clay in the soils. Potassium content of the alfalfa and total uptake of potassium were highly correlated with both exchangeable soil potassium and release of potassium from non-exchangeable forms. Exchangeable potassium levels below 100 lb. per acre gave low yields and potassium content of the crop. Excessively high exchangeable potassium levels resulted in luxury consumption of potassium by the crop or in fixation by the soil. In soils at or near their equilibrium exchangeable potassium levels, i.e., not heavily fertilized, there was a close relationship between potassium-supplying power and the exchangeable potassium content measured either before cropping commenced or at the end of the cropping period.