STUDIES ON SLOW RELEASE FERTILIZERS .1. EFFECTS OF TEMPERATURE, SOIL-MOISTURE, AND WATER-VAPOR PRESSURE

Using first-order kinetics we describe the release of nutrients by coated slow-release fertilizer (SRF). Plotting the logarithm of the concentration of intact fertilizer in the soil [log(Q0− Qt), where Qt, is the amount applied and Qt, is the amount released at time t] versus the time yielded a straight line, as predicted. The rate constants of SRF release at different temperatures are linearly related to the water vapor pressure: (Equation presented) where Pw is the vapor pressure and A and B are constants. We conclude that the rate-determining step in the nutrient release out of SRF is the migration of vapor from the soil into the fertilizer granule. The very mild effect of soil moisture on the rate of nutrient release is consistent with the proposed mechanism. We discuss the possibility of predicting nutrient release from SRF using the derived relationships. © 1990 Williams & Wilkins.