Potassium availability with application of sewage sludge,and sludge and manure composts in field experiments

An experiment was conducted to determine the plant availability of K in organic wastes. Four materials: digested sewage sludge (DSS), digested irradiated sludge (DISS), digested, irradiated and composed sludge (DICSS), and composted livestock manure (CLM) were applied to farmland for two years at rates of 10, 20, 30, and 40 mg-solids ha^–1 yr^–1. Potassium fertilizer (KCl) was added to the control treatment (CT), to which no waste was applied, and to the low-rate waste applications, to meet crop K requirement. Equal yields within different treatments were obtained for the test crops lettuce and snap beans through application of available N at the levels balanced for crop economical production. Potassium from the wastes was evaluated based on the assumption that Crop K uptake/Available K applied was equal between the CT and waste treatments. The results indicated that, in general, K applied with wastes (waste K + fertilizer K) was equally available (101±7%) to fertilizer K (KCl), except for low rate application of DISS (10 Mg ha^–1), where the availability of K was low (89±1%).