Fertilizers may be classified as either fast- or controlled-release. These terms refer primarily to the nitrogen availability of the formula.
Fast-release fertilizers include nitrogen in the form of water-soluble nitrates. Ammonium nitrate (NH4NO3), calcium nitrate (Ca(NO3)2), and potassium nitrate (KN03) are the most common forms used in home garden fertilizers, with ammonium nitrate the one most often found.
Nitrate nitrogen is quickly available to plants but is soluble and does not adhere to soil particles, so is easily leached out of the soil.
Ammonium and Urea Nitrogen
Urea is converted in the soil to ammonium very quickly, so can be thought of as a form of ammonium. Nitrogen in the form of ammonium is not available to plants until it is converted to nitrate by soil bacteria in a process called nitrification. In warm soil, most of the ammonia is nitrified in two or three weeks, so there is a release of available nitrogen over that period. Unlike nitrate nitrogen, nitrogen in the form of ammonium salts attaches to soil particles and does not leach out until it is nitrified. This makes ammonium nitrogen somewhat slow-release.
Many controlled-release fertilizers depend on bacterial activity in the soil to release nutrients. Bacterial activity is strongly dependent on soil temperature, being most active in warm (65 degrees to 75 degrees) soils. In cold or hot soil, however, microbial conversion is slowed, almost coming to a halt in soils under 55 degrees or over 85 degrees.
Most of the time, this temperature dependency works to the gardener’s advantage, with maximum nitrate release occurring at the same temperatures as maximum plant growth. But in a few cases, such as when applying late-season turfgrass fertilizer for early spring green-up, it can work against you.
Bacterial activity is also hampered by high or low pH, and by extremely dry or wet soils.
The release of nitrogen to plants is further slowed by coating dry forms of urea with molten sulfur, forming small yellow pellets. The sulfur slows the breakdown of urea over a period of many weeks, depending on the thickness of the sulfur coating. Sulfur-coated urea is dependent on soil bacteria, which break down the sulfur coat as well as nitrify the urea, so is sensitive to soil temperature.
UF (Urea Formaldehyde) and Methylene Urea
These more complex forms of urea are also broken down by microbial activity, but the rate of breakdown is slower than with uncombined urea. Like urea, they are dependent on soil microorganisms—and therefore soil temperature—for release.
UF is composed of chemical chains of different lengths; the longer the chain, the more resistant it is to bacterial decomposition. Because of the different chain lengths, about 1/3 of UF is released within a few weeks, and the remainder slowly, over a period of years. To be most effective, UF must be applied over a period of about three years to build up a steady release of nitrogen.
Methylene urea is composed of shorter chains and releases nitrogen more rapidly, over a period of months rather than years.
IBDU (Isobutylidene Diurea)
This form of urea is only slightly soluble, so nitrogen is dissolved from it slowly. The rate of nitrogen release is controlled mostly by the size of the particles, with more finely-ground IBDU releasing nitrogen more quickly. IBDU is frequently used in turfgrass fertilizers.
Polymer-Coated Pellets (Osmocote)
Another way of controlling the release rate of nutrients is to coat pellets of soluble fertilizer with a semipermeable resin membrane. As water seeps through the membrane and dissolves the fertilizer salts, osmotic action increases pressure within the pellet, forcing small amounts of dissolved nutrients out through the membrane. By varying the thickness of the membrane, these pellets can be made to deliver nutrients over a precise period of time, depending on the crop they are made for.
This method of controlling nitrogen release gives the most dependable of all methods, with little variation caused by soil temperature. Although soil moisture is necessary for osmotic activity, the level of soil moisture does not affect the rate of release.
MagAmp (Magnesium Ammonium Phosphate)
This fertilizer salt releases nutrients slowly because of its low solubility. The rate of release is controlled somewhat by the size of the pieces. MagAmp has a formula of 7–40–6, plus 12 percent magnesium. It lasts for from 4 to 12 months in the soil.
Spikes and Stakes
Spikes and stakes are pressed or driven into the ground within plant root areas. Only 2 to 3 inches long for houseplants, or several inches long for trees, they are made from a fiber impregnated with fertilizer. Inserted in the soil around a plant’s root area, they slowly release nitrogen and other nutrients as the fiber disintegrates. How often they must be replaced depends on the formulation. Handy for use around trees and shrubs or for house plants and container plantings, this form of fertilizer is expensive for larger gardening projects.
Because they may take months or years to release their nitrogen as nitrate, organic fertilizers may be thought of as slow-release. Like UF, they are most effective when used over a period of several years. The rate of release of nitrogen varies with the fertilizer and its state of decomposition. Organic fertilizers add organic matter as well as nutrients to the soil.