Preparation And Nutrient Release Characterization Of Marine Polysaccharide Coating Controlled-release Fertilizers

Fertilizers have been used for many years to release nutrients for plant growth. However, most nutrients of applied traditional-fertilizers are low fertilizer utilization rate. Unabsorbed nutrients release into surface or groundwater, which lead to the eutrophication of water body. Application of slow-/controlled-release fertilizers is an effective way to solve these problems. Slow-/Controlled-release fertilizers provide a good solution, in which fertilizer components(nutrients) are released slowly to avoid wasting. It is no doubt that application of slow-/controlled-release fertilizers can enhance the efficiency, minimize environmental pollution and develop the efficient, green and sustainable agriculture.In this paper, a series of single-layer and double-layers controlled-release fertilizers were prepared. The inner coating was chitosan from the shells of crab and shrimp, sodium alginate from the seaweeds or their derivatives, and the outer coating was microcrystalline wax, aqueous polyurethane or paraffin wax. The nitrogen release characteristics of coated fertilizers were determined in water at 25°C. The release kinetics was discussed. The fitting parameters and coefficient of determination were determined by dynamic models. The morphology and composition of the products were characterized by scanning electron microscope, Fourier transform infrared spectroscopy and ultraviolet–visible spectroscopy spectrophotometer. The releasing mechanism of double-coating controlled-release fertilizers was conjectured. The main conclusions were as follows:(1) A series of single-layer and double-layers controlled-release fertilizers were prepared. The core of double-coated controlled-release fertilizers was the common fertilizer granule, the inner coating was chitosan or sodium alginate, and the outer coating was the freezing point of 70°C or 80°C microcrystalline waxes. The results showed that as the outer coating, the released effect of the freezing point of 80°C microcrystalline wax was better than the freezing point of 70°C microcrystalline wax. As the inner membrane, the released effect of sodium alginate was better than chitosan. Among them, the nutrient release rate was 13.89% and 48.72% after 1 and 30 days of sodium alginate as inner coating and freezing point of 80°C microcrystalline wax as outer coating, which possessed excellent slow-release effect.(2) Because of reducing the holes and solubility of the membrane materials, we prepared controlled-release fertilizers with lower initial release rate and longer releasing period by optimizing of the cross-linking ratio between chitosan and glutaraldehyde and the sodium alginate film. The rate of nutrient release from coated pellets decreased with the decrease in porosity of the coat. The results showed that with the increase in the degree of cross-linking between chitosan and glutaraldehyde, the porosity could be decreased. Nutrient could be slowly diffused out of the pellets through the chitosan-glutaraldehyde copolymer membrane holes. Therefore, the slow-release rate was enhanced as the degree of cross-linking increased. By using of calcium alginate instead of sodium alginate, the solubility of the inner membrane was reduced, bringing about the controlled-release fertilizer with lower initial nutrient release rate and better release effect.(3) The release properties of double-coating controlled-release fertilizers were researched by using of aqueous polyurethane or paraffin wax instead of microcrystalline wax as the outer membrane, and chitosan or sodium alginate as the inner membrane. The results showed that using aqueous polyurethane and paraffin wax as the outer membrane had played a certain role in slow-release effect. However, the slow-release property of microcrystalline waxes in freezing point of 70°C and 80°C were better than aqueous polyurethane and paraffin wax.Compared with the nutrient release characterization of various merchant slow-/controlled-release fertilizers, the slow-release property of marine polysaccharide and its derivatives double-coating controlled-release fertilizers possessed the same excellent and the membrane materials of marine polysaccharide and its derivatives double-coating controlled-release fertilizers were more eco-friendly. The research of this paper had significantly theoretical and practical means. The results indicated that the double-coating controlled-release fertilizers had extensive perspectives in agriculture and horticulture.