An organic slow-release fertilizer for garden trees and a preparing method thereof

An organic slow-release fertilizer for garden trees and a preparing method thereof are disclosed. The fertilizer is prepared from papermaking black liquid, a sulfuric solution having a concentration of 10 wt%, a formaldehyde solution having a concentration of 5%, sodium sulfite, coal gangue powder, ferrous sulfate, rice bran, soybean curd residues, rice washing water, dead leaves, a silicate microbial agent, lactobacillus planturum powder, bifidobacterium powder, ammonium polyphosphate, calcium nitrate, potassium permanganate and a proper amount of water. A cheap and easily available lignin raw material is obtained through simple modification and is as good as a purified product prepared through caustic soda, thus saving energy and achieving waste utilization. Microbial floras beneficial to soil are added, and are adopted as adjusting components used for the fertilizer for garden trees after adding organic matters and fermenting for amplification. The microbial components of the fertilizer can accelerate deco