Abstract
We studied the release of nutrients from several resin-coated fertilizers as well as the effect of water behavior on the releasing processes. Five coated fertilizers were used ; i. e., three kinds of thermoplastic resin-coated urea (TPU) having different releasing patterns, i. e., exponential or sigmoidal, and two kinds of thermosetting resin-coated urea (TSU) having sigmoidal releasing patterns. The data were fitted to the reaction model proposed by ISHIBASHI et al. (1992) with a slight modification. 1) The nutrient release patterns of five resin-coated fertilizers obeyed the first-order reaction model with a parameter of t_<au> to correct for the initial lag-period. Zero was given to the t_<au> for the coated urea having an exponential release pattern. 2) To initiate the release of nutrients, certain amounts of water were needed to permeate into the coated fertilizers, and TSU needed more water to permeate than TPU. 3) There was no major differences in the mechanisms involved in the release between the TPU with the exponential releasing pattern and the TPUs having the sigmoidal releasing pattern. Delay in the release from the sigmoidal-type TPU was due to its greater water requirement to start the nutrient release compared to the exponential-type TPU. Accordingly, the initial lag-period was always dependent on the temperature. 4) As the coatings of the TSU were more flexible than those of the TPU, the internal pressure in the TSU granules, caused by permeating water, was alleviated by the expansion in volumes of the granules. The flexibility and the expansion caused the delay of the release (the lag-period), were also dependent on the temperature.
