Preparation of composite coated urea by castor oil-based polyurethane coupling water retaining agent and its application

As a large agricultural country, China relies on chemical fertilizers to ensure food production. However, the current utilization rate of chemical fertilizers is generally low, and excessive application leads to excessive enrichment of soil nutrients in the short term, far exceeding the nutrient requirements of plant growth. This nutrient imbalance not only causes a serious waste of fertilizer resources, but also causes a series of chain environmental problems, such as soil compaction, microbial imbalance, ammonia volatilization and nitrogen leaching, which pose multiple threats to the ecological environment. In this context, it is necessary to develop a composite coated urea, which can not only match the nutrient demand law of crops through slow release, but also improve the soil water use efficiency with the help of water retention function. In this study, super absorbent polymer (SAP) made in the laboratory was used as the outer coating, and polyurethane synthesized by polymethylene polyphenyl polyisocyanate (PAPI) and castor oil (CO) was used as the inner coating. The composite coated urea (PCUB) with multi-functions such as water retention and slow release was prepared, and the effect of the amount of SAP on the slow release performance and loss control effect of the fertilizer was discussed. The number of days for 80% slow release of polyurethane coated urea without SAP (PCU) was 65 days, and PCUB with 5% SAP (PCUB5) could effectively prolong the slow release period of fertilizer to 72 days, which indirectly proved that the hydrogel layer formed by SAP after water absorption could delay nitrogen release through three-dimensional network structure. The ammonia volatilization experiment showed that the ammonia volatilization of PCUB5 treatment group was 17.65% lower than that of PCU treatment group within 30 days, and 76.67% lower than that of pure urea treatment group. At the same time, the soil added with PCUB showed a significant increase in water holding ratio and water retention ratio, indicating that the increase of SAP can enhance the ability of soil water interception, and can also extend the effective water supply cycle of soil. Through the soil column leaching experiment, it was found that the 30-day total nitrogen cumulative loss concentration of the PCUB5 treatment group was 79.03% lower than that of the pure urea treatment group. The results of field experiment showed that the PCUB5 treatment group with 20% reduction in dosage significantly improved the growth performance of summer maize compared with the pure urea treatment group, the plant height increased by 4.37%, the stem diameter increased by 5.59%, the chlorophyll content increased by 4.90%, the total dry matter content increased by 6.71%, and the yield increased by 24.7% (P<0.05), indicating that the nitrogen release cycle of PCUB was more suitable for the nutritional needs of summer maize growth. Studies have confirmed that the PCUB achieves accurate matching of nitrogen release and crop fertilizer requirement cycle through the synergistic effect of hydrogel barrier effect and hydrogen bond. Its' water-fertilizer regulation' mechanism provides an innovative solution for agriculture in arid areas. The nutrient release cycle of PCUB can meet the nutritional needs of summer maize growth cycle, and can significantly increase the yield of summer maize under the condition of reducing 20% urea. It is a kind of fertilizer product with good application prospect.