New Development In Slow-Release Fertilizers

There have been several recent developments in slow-release fertilizers. These materials supply nutrients continuously over an extended period in contrast to water soluble fertilizers which are immediately available after incorporation in moist soil. Slow-release fertilizers are not subject to rapid leaching loss and do not easily burn plants from over-application. The major need for slow release materials is to supply nitrogen or potassium. Phosphorus does not leach rapidly, as a rule, even from sandy soils, and calcium and magnesium are available from dolomitic limestone and sulfur from gypsum in slightly soluble forms. Long lasting characteristics for fertilizers may be achieved by the techniques listed in Table 1 (p. 58) which lists both new and some old materials. General comments on the properties of these materials and their potential utility in turfgrass management follow, 1, Membrane coated fertilizers: The Archer-Daniels-Midland Co., Minneapolis, has developed a process for coating individual granules of water soluble fertilizer. When coated fertilizer is placed in moist soil, water passes through the membrane, dissolves some of the fertilizer which in turn slowly diffuses out through the membranes. The rate of release from the coated granule is determined primarily by the thickness of the membrane. For materials having a coating thickness of about 15 per cent by weight, two to three months or longer are required to exhaust the fertilizer. The rate at which the fertilizer is released is only moderately influenced by soil temperature (in the temperature range which supports grass growth), not significantly influenced by soil moisture until the soil approaches the air dry state, and is not significantly influenced ' by soil p H or microbiological activity. Surface applications tend to last much longer than when the fertilizer is incorporated in soil because when on the surface, the fertilizer tends to be dry much of the time. The ideal way to use coated fertilizers on closely mowed putting greens would be to work it into the holes made by 1 aerification. It is possible to "mow off' a portion of an application of coated fertilizer if a dense turf is subsequently closely mowed after the fertilizer application. The use of smaller size granules now available, appears to have eliminated those problems. Heavy Applications Safe Very heavy applications of coated fertilizer may be made safely to turfgrass. Twelve pounds of actual nitrogen per 1,000 square feet may be applied in a single application, if desired. If heavy applications are made, it is necessary that a small amount of deep leaching occurs from irrigation; otherwise, salinity conditions could develop.