A mechanism of nutrient release from resin-coated fertilizers and its estimation by kinetic methods. 5. Effect of soil moisture level on release rates from resin-coated mixed fertilizer

摘要

The release of nutrients from a thermoplastic resin-coated NPK mixed fertilizer were studied at different temperatures and soil moistures, ranging from the near-field moisture capacity to a level below air-dry soil. From the soil moisture content, water vapour pressure was estimated for the simulation studies. The effects of osmotic potential and diffusion of nutrients outside the granules of the coated fertilizer on the release rate are also discussed. Results showed that: (1) the release rates ofthe nutrients were in the order: nitrogen (ammonium and nitrate), potassium and phosphate. The pattern of each nutrient release fitted with the simulation model; (2) the release rate of nitrogen was the same as the estimated value in the solution when the soil moisture content levels were higher than 40% (0.7 MPa of water potential) of the maximum water holding capacity. However, the release rate decreased as the level of soil moisture decreased below this point, and became nil at～100 MPa of water potential; and (3) the water vapour pressure was estimated from the soil moisture content, and the temperature corresponding to the vapour pressure of pure water was calculated. Using the calculated temperature, the nutrient release was simulated with the proposed model, but the simulated values were slightly higher than the observed values. The vapour pressure close to the fertilizer granules may have been lower than the estimated value under the low moisture condition in which the solution from the granules tended to stagnate and the diffusion outside the granules decreased.