Large-particle slow-release fertilizer as well as preparation method and application thereof

The invention belongs to the technical field of fertilizers, and relates to a large-particle slow-release fertilizer and a preparation method and application thereof.The preparation method comprises the steps that urea and monopotassium phosphate are compounded according to a certain proportion to prepare a fertilizer inner core, then a castor oil-based polyurethane prepolymer is prepared, and the castor oil-based polyurethane prepolymer is prepared; mixing the prepared castor oil-based polyurethane prepolymer with a certain amount of chain extender butanediol, spraying the mixture on the surfaces of fertilizer inner core particles in a boiling state, curing to form a film, and drying to obtain the castor oil-based polyurethane coated fertilizer. Adding bentonite, urea, modified starch and a rhizobium agent into the castor oil-based polyurethane coated fertilizer in proportion, mixing, and granulating; and finally, wrapping a sodium alginate layer, cross-linking and drying to obtain the large-particle slow-re