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Slow-release fertilizers are excellent alternatives to soluble
fertilizers. Because nutrients are released at a slower rate throughout the
season, plants are able to take up most of the nutrients without waste by
leaching. A slow-release fertilizer is more convenient, since less frequent
application is required. Fertilizer burn is not a problem with slow-release
fertilizers even at high rates of application; however, it is still important to
follow application recommendations. Slow-release fertilizers may be more
expensive than soluble types, but their benefits outweigh their disadvantages.
Slow-release fertilizers are generally categorized into one of
several groups based on the process by which the nutrients are released.
Application rates vary with the different types and brands, with recommendations
listed on the fertilizer label.
- Pelletized:
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One type of slow-release fertilizer consists of relatively
insoluble nutrients in pelletized form. As the pellet size is increased, the
time it takes for the fertilizer to breakdown by microbial action is also
increased. An example of this type is MagAmp, a 7-40-0 fertilizer that is
available in a coarse grade lasting two years and a medium grade lasting one
year. MagAmp is used commercially for container plants, but is appropriate for
use on turf, tree seedlings, ornamentals, vegetables, and flower borders.
- Chemically Altered:
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A fertilizer may be chemically altered to render a portion
of it water insoluble. For instance, urea is chemically modified to make
Ureaform (ureaformaldehyde) -- a fertilizer that is 38 percent nitrogen, 70
percent of which is water-insoluble. This percentage is often listed on
fertilizer labels as the Percent W.I.N., or the percent of water-insoluble
nitrogen. This form of nitrogen is released gradually by microbial activity in
the soil. Because microbial activity is greatly affected by soil temperature,
pH, aeration, and texture, these variables can affect the release of nitrogen
from Ureaform. For example, there will be less fertilizer breakdown in acid
soils with poor aeration -- an environment unfavorable to soil microorganisms.
Ureaform is used for turfgrass; landscaping; ornamental, horticulture, and
greenhouse crops.
IBDU (isobutylidene diurea) is similar to Ureaform, but
contains 32 percent nitrogen, 90 percent of which is insoluble. However, IBDU
is less dependent on microbial activity than Ureaform. Nitrogen is released
when soil moisture is adequate. Breakdown is increased in acid soils. IBDU is
used most widely as a lawn fertilizer.
- Coated:
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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.
- NOTE:
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Trade names are provided for your information only, and
usage in this article is not an endorsement of any item mentioned.
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