1998 Volume 69 Issue 6 Pages 582-589
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 the ievel lower than air-dry soil. From the soil moisture content, water vapor pressure was estimated to carry out the simulation studies. The effects of osmotic potential and diffusion of nutrients out side the granules of the coated fertilizer on the release rate were also discussed. 1) The release rates of the nutrients were higher in the order of nitrogen (ammonium andnitrate), potassium and phosphate. The pattern of each nutrient release fitted well with our simulation model reported previously. 2) The release rate of nitrogen was the same as the estimated value in the solution under the condition in which 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 about 100 MPa of water potential. 3) The water vapor pressure was estimated from the moisture content of the soil, and the temperature corresponding to the vapor 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. We considered that the vapor pressure in the close vicinity of 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.
