Study on nitrogen characteristics of coated controlled-release fertilizer with different particle sizes

At present, although controlled-release fertilizer (CRFs) has stepped on the industrial scale in China, the technology of controlled release is behind that of developed countries such as USA and Canada. Qualities of CRF of different products are different. One of the reasons for the quality difference might come from the difference of substrate of CRF, being kernel compounds of the CRF. Currently, most of researches on CRF pay great attention to the coating materials and coating technique rather than the substrate. The previous study of the authors has found that particle sizes and its distribution of the substrate affected greatly on the longevity, being a quality parameter of controlled release fertilizers. The CRFs used in this study were a resin coated Kemira Grow-How NPK (15-15-15) granule which is manufactured by Planta Controlled Release Company. The substrate was mixed based on the real proportion of particle size distribution of the commercial products. Amount of coating material is 7.7% of the substrate. Then the CRFs were divided into three groups of 2-3mm, 3-4 mm and 4-5 mm, and incubated in distilled water with a ratio of fertilizer to water of 1∶20 for 0, 1, 2, 3, 4, 5, 6, 7, 14, 21, 28 and 56 days under constant temperature at 25℃±1℃, respectively. The ammonium and nitrate were determined at the given time to evaluate the effect of granule size on the nitrogen release rate (NRR). Results showed that the NRR and initial solubility are decreased with the increase of the substrate particle sizes. However, the longevity of CRF is increased with the substrate particle size. The NRR after 24 h incubation was 26.9%, 4.45% and 1.26% for CRF with a particle size of 2 to 3, 3 to 4 and 4 to 5 mm, respectively. The longevity is 4 d, 28 d and 56 d for CRF with a particle size of 2 to 3, 3 to 4 and 4 to 5 mm separately. During the early stage of incubation, nitrogen released mainly from CRF which particle size is smaller than 3 mm, whereas the later stage is from CRF which particle size is bigger than 3 mm. The ratio of coating material to substrate is increased with increase of the particle size. The ratio of coating material to substrate of CRF with particle size of 2 to 3 mm is decreased by 15.58% and that of 3 to 4 mm decreased by 4.29%. Nevertheless, the ratio of coating material to substrate of CRF with particle size of 4 to 5 mm is increased by 5.65％. Therefore, the substrate evenly distributed in particle size is one of the most important factors affecting quality of CRFs.