| Types of Fertilizers
The Basics of Fertilizers
Nutrient Requirements
Turfgrass, like all plants and crops, requires
all of the essential elements. However, the vast majority of nutritional
problems in turfgrass involve only nitrogen (N), phosphorus (P)
and potassium (K). In acid or alkaline soils, lime may be used to
adjust soil pH. Sulfur deficiencies are common and may require correction.
Nitrogen is the critical element for turfgrass.
The amount of nitrogen available will determine the rate of growth
and will greatly influence turf quality. The continuing need for
nitrogen over the entire growing season and its susceptibility to
losses make requirements higher than for other elements. General
recommendations for home lawns are: four to five pounds of nitrogen
per 1,000 square feet per year for warm-season grasses such as Bermuda
grass. Hybrid Bermuda grass and dichondra will respond to higher
amounts of nitrogen while St. Augustine grass will normally require
a lower rate.Phosphorus, although necessary for many vital growth
processes, is required in much smaller amounts than nitrogen. Much
of the phosphorus applied in fertilizer is rapidly immobilized in
the soil and becomes available only over an extended period of time.
An application of one pound of phosphorus per 1,000 square feet
per year should be adequate for most situations. In many areas,
residual soil phosphate will make regular application unnecessary.
Next to nitrogen, potassium is used in largest
quantities by all turfgrasses. It is important as a regulator of
plant growth processes and confers resistance to environmental stresses
and to disease. The recommended rate for potassium in turf is about
half that for nitrogen, about two pounds per 1,000 square feet per
year.
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All Nitrogen Sources Are Not Alike
Understanding the characteristics of today’s
various nitrogen sources is vital to making an intelligent decision
as to the best nitrogen source—or sources—for your own
particular turfgrass situation. In general, synthetic nitrogen sources
fall into one of three categories:
Quick-release, water-soluble sources
Slower-release, coated sources
Controlled-release reacted sources
Nitrogen sources can be used alone or in mixed
fertilizers, or even in combinations of quick and slower-release
sources. By understanding each source and its benefits and drawbacks,
turf managers and other professionals can adjust their fertilizer
application programs to get the most benefit out of each treatment.
Click here to download
a chart of Relative Performance Comparison of Nitrogen Sources
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Quick-Release Water-Soluble Nitrogen Sources
Urea: One of today’s
most widely used nitrogen sources, urea, is a low-cost, completely
soluble product. It is formed by reacting ammonia gas with carbon
dioxide in the presence of a catalyst. After application, an enzyme
called urease (present in plant tissue and organic matter) breaks
urea into its component parts—ammonium, carbon dioxide and
ammonia gas. This enzyme reaction, occurring in the presence of
water, is called hydrolysis.
Ammonium has a positive charge and is attracted
to negatively charged clay particles, root hairs and organic matter.
Soil microorganisms called nitrobacter bacteria then convert ammonium
ions to nitrate ions, which are readily absorbed into the root system.
However, under high soil pH conditions, more ammonia
is produced than ammonium. Since ammonia volatilizes into the atmosphere,
urea applications should be avoided on high pH soils or at the time
of lime application.
During conditions conducive to plant growth, much
of the nitrogen from urea is converted to ammonium and then to nitrate
within 72 hours following application. The nitrate N, moving freely
in the soil solution, is subject to both rapid absorption into the
root system and leaching from any moisture moving beyond that point.
So urea produces a rapid uptake and response in plants. Urea response
is generally short lived, lasting two-to-four weeks. Leaching can
be a problem, especially in sandy soils or under high moisture conditions,
and burn potential is increased due to solubility and high salt
index.
Ammonium Phosphate, Ammonium Sulfate, Ammonium
Nitrate, Calcium Nitrate:
Ammonium phosphates and ammonium sulfates are other commonly used
water-soluble nitrogen sources. Once these fertilizers solubilize
in the soil, the ammonium ions easily shift from the positively
charged sulfate or phosphate ions to the negatively charged clay
particles, organic matter, and even root hairs. Under some conditions,
ammonium ions can be absorbed directly by the roots. As with urea,
nitrobacter bacteria in the soil convert ammonium to nitrate, which
is readily utilized by the plant system.
Good rules of thumb when using soluble fertilizers
in either dry or liquid form are:
• Do not exceed 1 lb. of N per 1,000
square feet per application.
• Do not apply during high temperatures.
• Always water immediately following application to wash the
fertilizer from the leaf surfaces and reduce volatilization.
Characteristics of Water-Soluble Nitrogen
Quick-release sources:
• Soluble in water.
• Can be used immediately by the plant; plants show rapid
initial response.
• High potential for foliar burn.
• Require applications at low rates and frequent intervals
to sustain growth.
• Leach readily.
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Coated, Slow-Release Nitrogen Sources
 Sulfur
Coated Urea: Manufactured by moving granulated or prilled
urea through a stream of molten sulfur, Sulfur
Coated Urea (SCU) is a relatively low-cost, slow-release nitrogen
source. Urea gradually diffuses through the coating by way of cracks,
pinholes and imperfections occurring naturally in the surface. A
wax coating is applied to protect the surface from microbial degradation.
The rate of diffusion of urea from SCU depends
upon the thickness and integrity of the coating. The thicker the
coating, the slower and more gradually nitrogen is released. Nitrogen
release from SCU increases with
rising temperatures and soil moisture, and in general gives a good
residual color.
However, the fragility of the coating during transportation,
blending and application represents one of the major drawbacks to
this nitrogen source. The amounts vary greatly depending on the
integrity of the coating and the various attrition factors to which
the products have been subjected. Turf and ornamental managers can’t
be sure they will apply the amount of slow-release nitrogen they
originally purchased.
Polymer/Resin-Coated Urea:
Using a similar technology to sulfur coating, the resin-coating
process involves coating a soluble nitrogen source, such as urea,
nitrate, or ammonium. Resin-coated urea relies on osmosis rather
than coating imperfections to release nitrogen from the semi-permeable
coating.
Low concentrations of salts on one side of the
membrane allow diffusion of high concentration of salts on the other
side through the coating. Since the coating is semi-permeable, nitrogen
is released over a period of time. This release rate increases as
temperature increases and can result in excessive nitrogen release.
Polymer-coated urea works on the same principle
as resin coating. All coated products have limitations because they
rely on coatings for their slow release properties. If the coatings
are damaged, you have immediate release of nitrogen. Because of
this potential damage to the coating, use of coated urea products
for close-cut high maintenance turfs should be considered carefully.
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Characteristics of Coated Nitrogen
Slow-release sources:
• Insoluble or slowly soluble in water.
• Supply nitrogen gradually; plants show slow initial response.
• Can be applied less frequently vs. soluble.
• Reduce fertilizer losses from leaching.
• Produce more uniform growth response.
• Susceptible to coating fracture (damage) which affects release.
• N release from coated fertilizers is governed by the thickness
of the coating.
• Uniform particle size.
• Coated products may release
excessively at high temperatures.
Reacted Controlled-Release Nitrogen Sources
IB®: Isobutylidene
diurea (IB) is made by reacting isobutylaldehyde and urea. In
the presence of water, IB hydrolyzes
back to urea and butyric acid. As soil moisture increases, more
nitrogen is released. The rate of nitrogen release is governed by
the amount of moisture present and particle size.
The manufacturer’s screening process determines
particle size. IB is generally available
in greens grade, mid-size, coarse and extra coarse. Nitrogen release
rate increases as particle size decreases. Smaller particles hydrolyze
faster since there is a greater surface area to react with water.
Because temperature has less influence on IB
than on other nitrogen sources, the product gives good winter and
early spring color.
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Methylene Urea and Ureaform
The chemical combination of urea and formaldehyde
creates compounds commonly referred to as methylene urea (Nutralene)
and ureaform (Nitroform).
These products have varying-length polymers of
methylene urea. The smaller the ratio of urea to formaldehyde, the
longer the chain of polymers formed. As the polymer lengths and
number of longer chains increase, solubility decreases and nitrogen
is released more slowly.
The distinguishing characteristics of methylene
urea products versus all other nitrogen fertilizers is their availability
through the action of soil microorganisms.
The shorter-chained water-soluble polymers are readily digestible
by soil microorganisms and release nitrogen to the soil as ammonium
in a relatively short period of time. Some urea, naturally accompanying
the short-chained methylene urea polymers, creates an early or immediate
plant response. Residual activity from these soluble components
promotes color and growth responses in the initial four-to-six weeks
following application.
The longer-chain polymers contain water-insoluble
nitrogen, which is more slowly digested by soil microorganisms.
Unlike IB and SCU,
where nitrogen goes back to the soil as urea, nitrogen from methylene
urea and ureaform is converted directly back to the ammonium form
gradually throughout the growing season.
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Characteristics of Methylene Urea and Ureaform
Principal Characteristics:
• Supplies nitrogen gradually.
• Nitrogen release through microbial action (MU/UF).
• Reduced fertilizer losses from leaching.
• Low salt index, non-burning.
• No degradation during blending and application.
• Not dependent on coating or particle size for release.
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