Understanding Coating of Slow Release Fertilizers
Water-soluble fertilizers may be coated or encapsulated in membranes to slow the release of nutrients. For example, Osmocote, a controlled-release fertilizer is composed of a semipermeable membrane surrounding water-soluble nitrogen and other nutrients. Water passes through the membrane, eventually causing enough internal pressure to disrupt the membrane and release the enclosed nutrients. Because the thickness of the coating varies from one pellet, or prill, to another, nutrients are released at different times from separate prills. Release rate of these fertilizers is dependent on temperature, moisture, and thickness of the coating. Osmocote is recommended for turf, floriculture, nursery stock, and high-value row crops.
Another type of coated fertilizer is sulfur-coated urea (SCU), which is manufactured by coating hot urea with molten sulfur and sealing with a polyethylene oil or a microcrystalline wax. Nitrogen is released when the sealant is broken or by diffusion through pores in the coating. Thus, the rate of release is dependent on the thickness of the coating or the sealant weight. SCU is broken down by microorganisms, and chemical and mechanical action. The nitrogen in SCU is released more readily in warm temperatures and dry soils. SCU appears to be more effective when applied to the soil surface, rather than mixed into the soil. Any method of application that crushes the granules will increase the release rate to some extent.
SCU is best used where multiple fertilizer applications are normally necessary, such as on sandy soils or in areas of high rainfall or irrigation. SCU is used on grass forages, turf, ornamentals, and strawberries.
Though slow-release fertilizers are convenient for the gardener, there are some drawbacks associated with their use. Because the rate of release may be dependent upon soil moisture and temperature, the availability of nutrients may not be constant or predictable.
Gardeners need to be aware of exactly what they are buying when they purchase a slow-release fertilizer. If it is strictly slow-release, there may be a period of little or no release immediately after application followed by a period of heavier release that gradually decreases throughout the season. A blended fertilizer — one that mixes slow-release with soluble fertilizer and lists a Percent W.I.N. on the label — will release nutrients upon application and throughout the growing season. Use of blended fertilizers may provide an early start for young plants, giving them an advantage throughout the growing season. Apply blended fertilizers early in the growing season for best results. For summer flowers, this would be in the spring. However, for cool-season turf, fertilize in the fall when growth is beginning.
As gardeners’ responsibilities continue to increase in regard to the environment and groundwater, slow-release fertilizers are a welcome alternative to the less-convenient soluble fertilizers.