Preparation method of slow-release selenium fertilizer

The invention relates to a slow-release selenium fertilizer and a preparation method thereof. The preparation method comprises the following steps: drying and crushing wood chips, and performing high-temperature pyrolysis to obtain biochar; the method comprises the following steps: pretreating biochar with nitric acid to obtain a pretreated product; modifying the pretreated product by using a polyethyleneimine methanol solution to obtain a primary product; modifying the initial product by using a glutaraldehyde solution to obtain a modified mixed product; mixing the modified mixed product with a sodium selenite solution, and filtering to obtain a discrete product; and drying the discrete product to obtain the formed slow-release selenium fertilizer. And packaging and storing the obtained slow-release selenium fertilizer. According to the invention, the loading capacity of sodium selenite is improved through the modified biochar, and effective components are effectively sealed and stored and can be stably rele