Roanoke Times
Copyright (c) 1995, Landmark Communications, Inc.
DATE: THURSDAY, February 27, 1992 TAG: 9202270015
SECTION: CURRENT PAGE: NRV6 EDITION: NEW RIVER VALLEY
SOURCE: Joe Hunnings
DATELINE: LENGTH: Medium
WHEN IT COMES TO FEEDING PLANTS, IT USUALLY PAYS TO GO SLO-O-O-OW
Plants take up nutrients throughout their growing season, so it is beneficial to provide them with a balanced fertilizer.Perhaps the most efficient way to achieve this is to apply a slow-release fertilizer, which releases nutrients at the same rate they are taken up by the plants over an extended period. Slow-release fertilizers contain one or more essential elements.
Slow-release fertilizers can be categorized by the way in which the nutrients are released. The three major types are: materials that dissolve slowly; materials from which the nitrogen is released by microorganisms; and granular materials with membranes made of resin or sulfur that control the rate of nutrient release into the soil.
Rock phosphate is an example of a material that dissolves slowly. The availability of the phosphorus depends on how finely the material is ground and how much phosphorus already is in the soil.
Slow-release fertilizers that provide potash are granite dust and greensand. Like rock phosphate, the availability of their primary nutrient (in this case, potassium) depends on the fitness of the material.
Dolomitic (agricultural) limestone is a slow-release source of magnesium and calcium. In addition to supplying the nutrients, it also raises soil pH.
Blood meal and cottonseed meal are two slow-release sources of nitrogen. In breaking down these compounds, soil microbes produce plant-usable forms of nitrogen. Soil temperature determines the activity of the soil microorganisms and, consequently, the availability of the nitrogen in blood meal and cottonseed meal.
Sulfur-coated urea is a slow-release fertilizer. Different thicknesses of the sulfur coating control the release rate of nitrogen, which becomes more rapid as temperature increases. Watering does not affect its release rate.
Sulfur-coated urea applied to the soil surface releases more slowly than it does if it is incorporated into the soil. This material generally costs less than other slow-release fertilizers and it supplies the essential element, sulfur, in addition to nitrogen, phosphorus and potassium.
When fertilizer products coated with multiple layers of resin come into contact with water, the layers swell and increase the pore size in the resin so the dissolved fertilizer can move into the soil.
The release rate depends on coating thickness, temperature and water content of the soil. There often is a large release of fertilizer during the first two or three days after application. Release time can be from zero to six months depending on the coating. This is noted on the package.
Sulfur-coated urea has been used as turf fertilizer, while resin-coated fertilizers are used predominantly in container growing and potted plants.
Slow-release fertilizers have several advantages for the gardener, the plants and the environment.
Because they need not be applied as frequently as other fertilizers, they save the gardener time.
They have a low burn potential, as the nutrients become available over time. Plants may use the nitrogen in slow-release fertilizers more efficiently, because it is continually being released over a longer period than conventional fertilizers.
This is particularly valuable with perennial plants. Also, when nutrients are released slowly, they are less likely to wash into water sources and pollute them.
The disadvantages of slow-release fertilizers must be considered, too.
They generally are more expensive than other fertilizer types.
Because their release rate is governed by factors other than plant need - such as microbial activity or soil moisture - nutrients may not be available during critical growing periods. This can be a factor in the cool days of early spring with slow-release forms of nitrogen, because nitrogen release depends on active microorganisms.
Caution should be used in applying slow-release fertilizers around trees or shrubs, as they may keep the plant in growth late in the summer. The late-season growth may not harden off completely and excessive winter damage may occur.