Coating agent for granular nitrogen fertilizer, slow-release nitrogen fertilizer and preparation method of slow-release nitrogen fertilizer

The invention discloses a coating agent for a granular nitrogen fertilizer, a slow-release nitrogen fertilizer and a preparation method of the coating agent. Solid powder of the coating agent is prepared from a marine bioactive extract, and a water solution is prepared from magnesium sulfate, zinc sulfate, manganese sulfate and water. According to the coating agent for the urea fertilizer, the surface of nitrogen fertilizer particles is coated with the aqueous solution and the solid powder to form a coating to prepare the slow-release nitrogen fertilizer, the surface of urea particles is coated with the solid powder with high affinity with the nitrogen fertilizer, one or more layers of envelopes can be formed, slow release of the nitrogen fertilizer can be achieved, and the nitrogen fertilizer can be effectively protected. Long-acting, multi-effect and efficient utilization is achieved, the utilization rate is increased, the effective period is as long as 60-70 days, and the pollution to the environment can b