Estimation of nitrogen release from coated urea using the Richards function and investigation of the release parameters using simulation models

Based on the calculated values of four parameters in the Richards function that described the nitrogen release from coated urea at several constant temperatures, the following assumptions were made for estimating the parameters when the temperature changes. (1) The maximum amount released (N) and the curve shape parameter (d) are constant regardless of the temperature. (2) The rate constant (k) follows the Arrhenius equation, which is represented by the rate constant at 25 degrees C (k(s)) and the apparent activation energy (E-alpha) fixed for all the fertilizers. (3) The time to the inflection point (t(i)), corresponding to the time to the point where the maximum amount of nitrogen releases per unit time, is obtained by using the other parameters (d, k). When these assumptions were used, the average standard error of fitting was 1.6%. This result indicates that nitrogen release from coated urea at various temperatures can be described using three parameters (N, d, k(s)) and a constant (E-alpha). The nitrogen release characteristics of a certain type of coated urea could be described using only two release parameters (d, k(s)), because N was approximately equal to the nitrogen content of the fertilizer. Therefore, the Richards function enables to estimate the nitrogen release from coated urea more exactly and more simply than the Bertalanffy function, which had been used until now. In addition, the release parameters were investigated using two simulation models. The fluctuation model implied that d indicates the complexity of the release process. The hole model supported the close relationship observed among three parameters (d, k, t(i)) and suggested a mechanism for nitrogen release.