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March 2002, No. 1
Nitrogen Fertilization in Early Spring
As the snow melts and the rains begin to thaw any remaining frost (not much this year), many of us anticipate spring green up and turf growth.
Some are tired of looking at the dull brown color of the turf, especially this year with the lack of snow cover and the moderate late winter temperatures.
The initial reaction is to apply a form of nitrogen (N) in order to green it up and give the turf a jump-start.
This practice may work, but it depends on the type of nitrogen one applies.
Nitrogen, like all fertilizers, is applied in several chemical forms.
Turfgrass nitrogen types can be divided into four basic groups; synthetic inorganic, natural organic, synthetic organic and coated products.
Each type is defined by the source (organic or inorganic) and whether they are natural in origin or synthesized from a chemical process.
Nitrogen Types
Synthetic inorganic
Natural organic
Synthetic organic
Coated
Synthetic Inorganic
The first group is the synthetic inorganic nitrogen carrier.
Nitrogen in these forms is developed by the combination of N2 found in the atmosphere with other substances, primarily H2, through forced chemical reactions known as the Haber-Bosch process.
The NH3 produced from this chemical reaction is used in commercial agriculture directly as a fertilizer known as anhydrous NH3 which is 82.5% N.
In the turfgrass world, NH3 is the basis for many of the synthetic inorganic nitrogen carriers.
Haber-Bosch Process
3H2 + N2 (catalyst and 1200°C at 5atm  )2 NH3
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The synthetic inorganic nitrogen carriers share similar characteristics:
· High water solubility
· Rapid initial plant response
· N availability has minimal temperature dependence
· High foliar burn potential
· Can be dissolved in water for liquid foliar or soil application
· Rapid cold temperature growth response
· Low cost per unit of N
· Limited residual response
The primary synthetic inorganic nitrogen carriers used for turfgrass fertilization are ammonium sulfate (NH4)2SO4, ammonium nitrate NH4NO3, calcium nitrate Ca(NO3)2, and potassium nitrate (KNO3).
These all produce a relatively short-lived flush of growth (often four weeks or shorter) and can burn grass leaves when applied incorrectly.
However, these carriers are often well-suited early spring and fall applications because of their nitrate base.
Nitrate is readily absorbed and used by the turfgrass plant.
Natural Organic
The second group is the natural organic nitrogen carriers.
Prior to the 1850s, virtually all of the fertilizer N in the United States was in this form (primarily animal manure and legume N).
The nitrogen occurring in these natural forms is in very complex organic compounds and is not readily soluble in water.
The N release from these carriers is dependent on microorganisms for decomposition of the organic compounds and soil weathering over time.
An important factor to remember when using these compounds is microorganism activity is often very limited when soil temperatures are below 55°F.
Even when soil temperatures are in the normal range for cool season turfgrass growth, the N release from microorganism activity on these carriers is slow and can offer a longer residual response.
However, it should be noted that certain natural organics might release nitrogen at a fairly rapid rate under high soil temperature conditions (e.g., blood meal).
Most of the natural organic carriers share these characteristics:
· Medium to slow initial release rate
· Low water solubility
· Minimum foliar burn potential
· Usually highest cost per unit of N
· Reduced nitrate leaching
· Lower nitrogen analysis
· 4 to 8 week residual period
The natural organic carriers used in the turfgrass industry can be grouped into two basic categories.
One group is manufactured from the by-products of animals resulting from meat and fish processing and the other group originates from plant and vegetable oil industries.
The primary natural organic carriers of N used in turfgrass are activated sewage sludge, corn gluten, and some animal by-products, although limited.
Synthetic Organic
The third and most complex type of nitrogen carrier used in the turfgrass industry is the synthetic organic group.
The synthetic organic group can be further split into water-soluble and water-insoluble types.
The turf industry uses two water-soluble synthetic organic nitrogen carriers; urea [CO(NH2)2] is commonly used and the other type, calcium cyanamide, (CaCN2), is applied to a lesser extent.
These two carriers are similar in characteristics to synthetic inorganic carriers including:
· High water solubility
· Rapid initial plant response
· Short residual response
· Leaching tendency
· High foliar burn potential
When using urea fertilizers, care should be taken to properly water-in after application to limit the amount of gaseous ammonia loss from the fertilizer zone.
Products that use these carriers, primarily urea, can also be used during cooler temperatures with success.
The water-insoluble synthetic organic nitrogen group is primarily composed of ureaformaldehydes (UF) and methylene ureas (MU).
These two carriers are reactant products of urea, and its availability to the turfgrass plant is measured by three distinct fractions based upon solubility.
One fraction is known as the cold water-soluble nitrogen (CWSN) and is immediately available to the turfgrass plant The CWSN includes unreacted urea plus some methylene ureas of low molecular weight.
The second fraction, the cold-water-insoluble nitrogen (CWIN), is less soluble than CWSN, but according to manufacturers can still be available to the plant throughout a typical growing season.
This fraction is composed of larger or heavier (molecular weight) methylene ureas that have to be microbially hydrolyzed to smaller units before they can be used by the turfgrass.
The third component of these nitrogen carriers is labeled hot-water-insoluble nitrogen (HWIN).
This fraction may take years to break down into a plant useable form.
These carriers have similar characteristics to the natural organics including:
· Intermediate release rate
· Low foliar burn potential
· Medium to low water solubility
· Reduction in leaching potential
· Long residual response
· High cost per unit of nitrogen
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CWSN: Cold-water soluble N
CWIN: Cold-water insoluble N
HWIN: Hot-water insoluble N
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In this category, nitrogen release is dependent on the activity of soil microorganisms, soil temperature, pH and nutrient level.
Below 50°F nitrogen release can be quite slow.
The water-insoluble synthetic organics release N faster than the natural organics initially, although not as quick as the synthetic organics, particularly in early spring temperatures. \
It is important to understand the amount of CWIN in the water-insoluble groups if using them for early spring application.
An extended cool spring may not provide the needed environment to sufficiently break down the larger CWIN components.
Another carrier also placed in the synthetic organic group is isobutylidene diurea or IBDU.
It is a product of the reaction of isobutylaldehyde and urea.
In the presence of water, IBDU hydrolyzes 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 the particle size of the carrier.
For the most part, IBDU has a slow release rate by virtue of its low water solubility.
However, the release rate increases as the temperature rises and pH falls, but low temperature does not affect IBDU as much as the sources that depend on microbial activity for release.
The CWSN-CWIN-HWIN characterization system does not apply to IBDU because of the uniform composition of the carrier.
A common practice is to apply an IBDU fertilizer in late autumn for an early spring response.
Coated Products
The last group of nitrogen carriers is the coated products.
A type of soluble nitrogen such as urea is coated or encapsulated by either sulfur (sulfur-coated urea [SCU]) or a polymer resin coating (polymer resin coated urea [PCU]).
The release rate of the soluble form of nitrogen is controlled by the coating thickness on each particle.
Soil temperature controls the release rate on both the PCU and SCU products, however the nitrogen release by the SCU products is also enhanced by soil moisture.
These slow-release forms are more expensive per pound of nitrogen than the water-soluble forms, but do provide a greater margin of safety and are usually preferred during summer periods.
It is noted that some of the coated products on the market are manufactured with a portion of urea that is not protected with coating and will have a quick release characteristic for after application.
Quick Release for Quick Results During Cool Temperatures
Turf managers have many different options available to them when deciding to jump-start their turf with nitrogen.
Carriers that are water-soluble release nitrogen more quickly to the turfgrass plant than other N types, especially during cooler temperatures and have a great chance of being utilized by a turf during a cool spring.
Care must be taken because quick release nitrogen sources are in a soluble form that enables them to leach readily.
Nitrate leaching can be an environmental problem due to ground water contamination.
In many turf situations, this is not a great problem, but can be of concern, however, when turf is grown on sandy soils, the water table is very close to the ground's surface, or excessive amounts of soluble forms of nitrogen have been applied.
| Nitrogen Carriers |
%N |
Time of year to utilize* |
Other |
| Synthetic Organics |
Ammonium nitrate
NH4NO3 |
33-34 |
Early Spring and Autumn |
Nitrogen is in plant useable form (nitrate) upon application |
Ammonium sulfate
(NH4)2SO4 |
21 |
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Becoming less common but still available |
Calcium nitrate
Ca(NO3)2 |
15 |
Potassium nitrate
(KNO3) |
14 |
| Natural Organics |
| Sewage Sludge |
4-7 |
Throughout cool-season growing cycle |
N release is microbial activity dependent |
| Animal by-products |
7-14 |
| Synthetic Organics |
| Isobutylidene diurea (IBDU) |
30 |
Late Spring/Autumn |
N release affected by pH and water solubility |
| Methylene urea (MU) |
40 |
Throughout cool-season growing cycle |
Most N release is microbial activity dependent |
| Ureaformaldehyde (UF) |
38 |
| Urea |
46 |
Spring and Autumn |
Fast acting/short lasting |
| Coated Products |
| Sulfur coated urea (SCU) |
29-40 |
Late Spring - Summer - Early Autumn |
High temperature and soil moisture increase coating degradation |
| Polymer coated urea (PCU) |
36-44 |
High temperatures increase coating degradation |
| * Based upon factors that release N to plant useable form from carrier in typical growing season. |
(Luke Cella, Editor; Tom Voigt, Extension Turfgrass Specialist)
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