Abstract: A method for the manufacture of physically prepared slow-release fertilizer by coating fertilizer particles with at least one layer of a substantially water insoluble polymer possessing a three dimensional structure. The three dimensional structure results from a prepolymerized crosslinked unsaturated oil having between 12 to 20 carbon atoms and a iodine value of at least 120, the extent of crosslinking being above 5%. Optionally, the coating layer contains micronutrients and inert materials. In a preferred embodiment, the coating consists of three layers, the first being obtained with a high viscous unsaturated oil and the subsequent two layers with a lower viscosity unsaturated oil. The method is applicable for most of the known water-soluble fertilizers.