A slow release fertilizer is a granulated fertiliser that releases nutrients gradually into the soil.The slowness of the release is determined by the low solubility of the chemical compounds in the soil moisture. Since conventional fertilisers are soluble in water, the nutrients can disperse quickly as the fertiliser dissolves. Because controlled-release fertilisers are not water-soluble, their nutrients disperse into the soil more slowly. The fertiliser granules may have an insoluble substrate or a semi-permeable jacket that prevents dissolution while allowing nutrients to flow outward.

Slow release is a strategy in material science in which a chemical compound is introduced into a system at a reduced speed.

Getting a chemical at a certain spot comes at a cost because of the spraying effort. If the desired effect is to last, administering the chemical has to be repeated over and over again. Supplying a large single dose reduces that frequency but is not always an option because the large dose is toxic or has adverse side effects compared to a small dose. Slow release combines the beneficial effects of applying a single large dose distributed over a long time in small quantities. Slow-release fertilizers work by reducing the diffusion of a chemical into the system.