Photo: Peter Landschoot
Because nitrogen fertilizer is a significant maintenance expense, some serious thought should go into choosing the most efficient product for your operation. Depending on when and how it is applied, the source of nitrogen also has implications on leaching and gaseous losses to the environment. Because of cost and environmental concerns, fertilizer companies have developed products called enhanced efficiency nitrogen fertilizers.
According to the Association of American Plant Food Control Officials (AAPFCO), an organization of state/federal fertilizer regulators, enhanced efficiency fertilizers allow increased uptake of nutrients with reduced potential of gaseous, leaching, or runoff losses when compared to soluble fertilizers such as urea or ammonium sulfate.
Enhanced efficiency nitrogen fertilizers are divided into two groups: stabilized fertilizers and slow-release fertilizers. Stabilized fertilizers are products amended with additives that reduce the rate of nitrogen loss through ammonia volatilization or nitrification. Slow release fertilizers release plant nutrients at a slower rate relative to soluble nitrogen products.
Nitrogen-containing stabilized fertilizers on the turfgrass market act on two processes: the transformation of urea to ammonia gas (urea hydrolysis), and the conversion of ammonium to nitrate (nitrification). When urea is applied to turf and watered into the soil, it undergoes hydrolysis and is rapidly converted to ammonia and then to ammonium, which is a relatively stable form of nitrogen that can be taken up by the plant. If the urea is not watered in, some of the hydrolyzed urea-nitrogen can be lost to the atmosphere through a process known as ammonia volatilization. The urea hydrolysis process is hastened by the urease enzyme, which is abundant in soil and thatch.
Chemical additives called urease inhibitors (NBPT and maleic-itaconic copolymer) inhibit urease enzyme activity and dramatically slow the conversion from urea to ammonia, thereby reducing volatilization. Products containing NBPT include LSN, UFLEXX, and UMAXX. Maleic-itaconic copolymer is found in a product called NutriSphere-N. It is important to realize urease inhibitors are not slow release fertilizers, and any potential improvement in nitrogen efficiency is a result of reduced ammonia volatilization. Benefits obtained with urease inhibitors will ultimately depend on the potential for ammonia volatilization following application. If conditions favor volatilization after application (hot days, high pH, leaving urea on the soil surface or in thatch with no follow-up irrigation or rainfall), using urease inhibitors will likely result in more nitrogen uptake and less volatilization.
The other group of stabilized nitrogen products is the nitrification inhibitors. When nitrogen fertilizer is applied to turf it is converted from ammonium to nitrate through a process called nitrification. Nitrification is a natural process in soils and is mediated by two specialized soil bacteria. One of these bacteria, Nitrosomonas spp., transforms ammonium to an intermediate nitrogen compound called nitrite, while the other (Nitrobacter spp.) converts nitrite to nitrate. Nitrification inhibitors are designed to specifically target Nitrosomonas bacteria, so that the nitrogen fertilizer remains in the more stable ammonium form. Ammonium tends to be stable because it is positively charged and adheres to clay and organic matter. Nitrate is negatively charged, and is more susceptible to leaching during rainy periods and in sandy soils.
Only one nitrification inhibitor, dicyandiamide (DCD), is currently used in turfgrass fertilizers. Products containing DCD include UFLEXX and UMAXX. The potential benefits in improved nitrogen efficiency from nitrification inhibitors will likely depend on the potential for nitrogen leaching following application. If conditions favor leaching after application, there is a higher probability of improved nitrogen retention in the soil using DCD. If conditions do not favor leaching, the benefits from DCD would be limited.
Slow Release Nitrogen Fertilizers
Slow-release nitrogen fertilizers provide a longer duration of nitrogen release than quick-release soluble fertilizers, and are usually safer to use on turf because of their reduced burn potential. Recent studies have shown that under certain conditions, some slow-release nitrogen fertilizers are less likely to leach into groundwater than soluble sources. Disadvantages of slow-release fertilizers include their higher price per unit of nitrogen and slower green-up rate. Whereas some slow release fertilizers release nitrogen to meet the needs of the growing turf, other have reduced efficiency (a lower percentage of the applied nitrogen is used by turf) compared to quick-release soluble sources. Slow-release nitrogen fertilizers can be grouped into different categories, including the coated ureas, methylene ureas, and natural organics (Table 1 below).
Coated ureas include sulfur-coated urea (SCU) and polymer-coated urea (PCU). SCU is made by spraying urea prills or granules with molten sulfur. A sealant, such as wax or a mixture of oil and polyethylene, is often applied to seal pores and imperfections in the sulfur. Nitrogen is released from SCU by microbial degradation of the sealant and diffusion of soluble nitrogen through pores and cracks in the sulfur coating. Particles within a SCU product are not identical. If they were, you might expect all of them to release nitrogen at the same time. Nitrogen release occurs quickly with imperfectly coated particles; an intermediate rate of release takes place with particles in which the sealant has covered imperfections; and the greatest delay in release occurs with the more thickly and more perfectly coated particles. The slow-release properties of SCU come from the variability in coatings among the individual particles. Although turfgrasses usually show a good response to SCU applications, sometimes under-fertilized lawns can appear "spotty" because particles are releasing at different intervals.
Green spots resulting from nitrogen release from imperfectly-coated SCU granules. Photo: Peter Landschoot
PCU fertilizers are coated with a thin layer of polymer resin. For nitrogen release to occur from PCU, water is absorbed through the coating and then gradually released with dissolved nitrogen through the coating by osmosis. Different coating thicknesses may be used to obtain different nitrogen release rates; the thicker the coating, the slower the release of nitrogen. Nitrogen release increases with higher temperatures and is less dependent on soil moisture levels, soil pH, or microbial activity.
Methylene urea fertilizers are made by reacting urea with formaldehyde in various ratios. These products typically contain 30-40% nitrogen and are classified as slow release. However, some contain enough water-soluble nitrogen to give a response closer to quick-release nitrogen sources than to slow-release sources. Others can be expected to give a relatively quick initial response, but have a slightly slower release rate than the quick-release sources. Any urea-formaldehyde product that does not claim water-insoluble nitrogen as a percentage of the total nitrogen, will probably release nitrogen quickly.
Natural organic fertilizers are typically derived from by-products of plant and animal processing industries. Considerable variation exists in the physical and chemical properties of different natural organic fertilizers and nitrogen release rates vary quite a bit among products. The natural organics can be characterized by relatively low nitrogen contents (usually below 10 percent), and almost always contain phosphorus. Because release of nitrogen is dependent on soil microbes, environmental conditions that influence microbial activity (temperature, soil moisture and oxygen, and soil pH) affect the breakdown of natural organic fertilizers.
Table 1. Some enhanced efficiency fertilizer products used for turfgrass fertilization
|Stabilized N Fertilizers|
|UMAXX||46 to 47|
|Sulfur Coated Ureas|
|Lebanon Pro SCU||39|
|Poly-S||39 to 40|
|XCU||38 to 43|
|Polymer Coated Ureas|
|Duration CR||40 to 44|
|Polyon||37 to 44|
|Sustane||2 to 8|
|Ringer Lawn Restore||10|
* % Nitrogen of product before blending with other compounds