Effect of granule size and coating thickness on nitrogen release characteristics of controlled release fertilizers (CRFs)

Research of controlled release fertilizers (CRFs) has been paid great attention in China recently because of their advantages of high nutrient use efficiency, less burning damage to plants and easier to handle. However, the application of CRFs to arable crops has been restricted by high cost of the products. How to reduce the cost of CRFs becomes a crucial factor to promote CRFs in farming practice. The high cost of the CRFs is due to high value of coating material and energy consumption. A lot of researchers have paid great attention to study on manufacture CRFs by cheaper coating materials such as FMP and clay mineral, being producing slow release fertilizer. Nevertheless, longevity of such CRFs is shorter than that of the ones coated by resin or polymer. In other word, CRFs were manufactured by use of FMP and clay as coating material can not control nutrients release properly. So far, no research concerns decreasing cost of coated CRFs by reducing total surface area of a unit weight of kernel fertilizer granule in literature. In order to realize target to decrease manufacture cost by declining surface area of a unit weight of kernel fertilizer, the present study tested nutrients release characteristics of a serials CRFs with different granule diameter under several coating thickness. The CRFs in study were manufactured by coating Kemira Grow-How NPK (15-15-15) granule as kernel compound. The kernel granules divided into three groups of 2.5 to 3.0 mm, 3.0 to 4.0 mm and 4.0 to 5.0 mm in diameter. Each group granule coated by 51 g, 61 g and 71 g resin per square meter surface of the kernel granule according to a patent technique of South China Agricultural University to produce CRFs with three thickness of coating. The coating (materials) treated with adding starch and PVA or without them as additives respectively. The CRFs were incubated in distilled water with a ratio of fertilizer to water at 1∶20 for 0,1,2,3,4,5,6,7,14,21,28,35,42 days under constant temperature at 25℃1℃ and 40℃1℃ respectively. The ammonium and nitrate were determined at the given time to (evaluate) the effect of granule size, thickness of coating and additive materials on the nitrogen release rate (NRR). Results showed that NRR increased with granule size in treatment of CRFs having the same coating thickness under both normal and higher temperature. However, the effect of granule size on NRR decreased with increasing coating thickness. Surface structure of the kernel granule affecting NRR of the CRFs. Increase of coating thickness could decrease NRR. No matter what size of the CRFs, the nitrogen release rate decreased significantly with increase of coating thickness. Additives of both starch and PVA could enhance the NRR. The application amount of PVA had remarkable influence on the NRR compared to starch. To reduce amount of coating material by use of bigger granule could rely on the use of smooth surface kernel granule. Therefore, reducing cost of CRFs could be achieved by manufacturing CRFs through coating smooth surfaced big granule NPK compound.