Slow- and controlled-release involve only 0.15% (562,000 tons) of the fertilizer market (as of 1995). Their utility stems from the fact that fertilizers are subject to antagonistic processes. In addition to their providing the nutrition to plants, excess fertilizers can be poisonous to the same plant. Competitive with the uptake by plants is the degradation or loss of the fertilizer. Microbes degrade many fertilizers, e.g. by immobilization or oxidation. Furthermore fertilizers are lost by evaporation or leaching. Most slow-release fertilizers are derivatives of urea, a straight fertilizer providing nitrogen. Isobutylidenediurea (“IBDU”) and urea-formaldehyde slowly convert in the soil to free urea, which is rapidly uptaken by plants. IBDU is a single compound with the formula (CH3)2CHCH(NHC(O)NH2)2 whereas the urea-formaldehydes consist of mixtures of the approximate formula (HOCH2NHC(O)NH)nCH2.
Besides being more efficient in the utilization of the applied nutrients, slow-release technologies also reduce the impact on the environment and the contamination of the subsurface water.[18] Slow-release fertilizers (various forms including fertilizer spikes, tabs, etc.) which reduce the problem of “burning” the plants due to excess nitrogen. Polymer coating of fertilizer ingredients gives tablets and spikes a ‘true time-release’ or ‘staged nutrient release’ (SNR) of fertilizer nutrients.
Controlled release fertilizers are traditional fertilizers encapsulated in a shell that degrades at a specified rate. Sulfur is a typical encapsulation material. Other coated products use thermoplastics (and sometimes ethylene-vinyl acetate and surfactants, etc.) to produce diffusion-controlled release of urea or other fertilizers. “Reactive Layer Coating” can produce thinner, hence cheaper, membrane coatings by applying reactive monomers simultaneously to the soluble particles. “Multicote” is a process applying layers of low-cost fatty acid salts with a paraffin topcoat.