Gardening Articles: Care :: Soil, Water, & Fertilizer
Controlled-Release Fertilizers (page 2 of 3)
by Warren Davenport
Currently, there are three prominent kinds of controlled-release fertilizers marketed to home gardeners. While costs vary widely, all are relatively expensive compared to fast-release fertilizers. However accounting for the value of your time and the reduced number of applications they require, their cost is more in line with less expensive, fast-release fertilizers.
One of the best reasons for home gardeners to use these fertilizers is that most nursery plants are already accustomed to them. These are the same types of fertilizers that many professional growers use.
Compared to natural organic fertilizers, most slow- andcontrolled-release kinds are more concentrated, easier to handle, and less expensive (on a cost per nutrient basis); and they are not dependent upon soil microbes and water to make their nutrients available.
Another advantage to home gardeners is that these fertilizers have been studied extensively. As a result, their label directions are more specific and far more accurate than the directions on most fertilizer products.
The chief disadvantage of these fertilizers is the same as that of any other kind of synthetic fertilizer: They do not directly contribute organic matter to the soil or otherwise improve the soil's physical characteristics.
Nutricote. Various formulations are available, all based upon common fertilizers including ammonium nitrate, ammonium phosphate, calcium phosphate, potassium nitrate, and magnesium phosphate. Granules of fertilizer are coated with a thermoplastic resin and a proprietary chemical release agent.
Depending upon the formulation, the spherical gray pellets release nutrients for up to a year at 77oF. The release rate is influenced primarily by temperature but also by water acting on the release agent.
Nutricote is marketed to home gardeners as Dynamite Plant Food.
Nutri-Pak. Another type of controlled-release fertilizer, called Nutri-Pak, consists of specially designed plastic packets that gradually release the soluble fertilizer inside. Developed by the soil science department of the University of Wisconsin, gardeners drop one or more opened packages into the planting hole. The packs can remain effective in the soil for up to five years.
Osmocote. This industry standard for 30 years offers a wide variety of specialized formulations. All begin as a granular complete fertilizer, which is normally composed of ammonium nitrate, ammonium phosphate, calcium phosphate, and potassium sulfate. These granules are then coated with a type of plastic (an alkyd resin) to form a tiny, nearly spherical yellowish-brown spheres. The coating's thickness, along with temperature, determines how long the fertilizer lasts. For instance the 14-14-14 lasts four to five months at soil temperatures of 60°F, three to four months at 70°F, and one to two months at 80°F. Compared to other controlled-release fertilizers, Osmocote releases more nutrients in cooler soils.
Polyon. Several formulations are available, and all are based on fertilizers such as urea, ammonium nitrate, ammoniated phosphates, potassium sulfate, potassium nitrate. The green pellets have a thin, hard, and nearly break-resistant polymer shell. Nutrient release is regulated primarily by temperature, allowing extremely predictable results. Depending upon the formula, the green particles release nutrients for three to nine months at 86°F.
Polyon is marketed to home gardeners as Pursell's Sta-Green.
Other kinds. Two other controlled-release fertilizers are sulfur-coated urea 36-0-0, usually abbreviated SCU, and polymer-encapsulated SCU. Both are made by spraying molten sulfur onto granular urea. In the case of plain SCU, the thickness of the sulfur coating (along with temperature and moisture) determine the release rate. SCU is relatively inexpensive and is a component of various brands of home lawn fertilizers.
As its name states, polymer-encapsulated SCU is coated with a thin polymer layer, a step that makes it last longer. Both SCU and polymer-coated SCU are used in many lawn and garden fertilizers.