Food for turf: Slow-release nitrogen

Synthetic controlled-release fertilizers accounted for nearly 30 percent of all fertilizers applied to U.S. golf courses in 2000. Controlled-release nitrogen (N) represented 69 percent of the total N contained in fertilizers applied to U.S. golf courses. These figures are expected to increase because of environmental concerns and the inclusion of controlled-release N in Best Management Practices (BMP) programs.

The basic concept of controlled-release or slow-release fertilizers is that they release their nutrient contents at more gradual rates that permit maximum uptake and utilization of the nutrient while minimizing losses due to leaching, volatilization or excessive turf growth. Methylene urea products are the most popular controlled-release fertilizers, accounting for 63 percent of the total of synthetically produced materials.

Controlled- or slow-release fertilizers are broadly divided into uncoated and coated products. Uncoated products rely on inherent physical characteristics, such as low solubility, for their slow release. Coated products mostly consist of quick-release N sources surrounded by a barrier that prevents the N from releasing rapidly into the environment. Different mechanisms, but similar (though not identical) end results.

The terms “controlled-release” and “slow-release” can mean different things to different people, but for purposes of this discussion, the two terms are synonymous. Except for a few slow-release K sources, almost all slow-release fertilizers are N sources. They represent a relatively small segment of the total fertilizer industry (3 to 4 percent), but their use is growing faster than soluble (quick-release) materials. This is primarily because they reduce the overall environmental impact of N fertilizers, as now mandated in BMPs.

Following is a discussion of the chemistry, mechanism of release and anticipated turfgrass response to the application of the different types of slow-release materials used on turf.

Uncoated controlled-release fertilizers

Non-coated materials of limited solubility can, for the most part, be manufactured in a smaller particle size than coated products, which makes them good choices for use on putting greens and other close-cut turf. (Although some coated-fertilizer manufacturers produce coated products of small size designed specifically for close-cut turf.)

Another characteristic of uncoated products is that they are homogenous. That is, their composition is the same throughout the particle. By contrast, coated products, which consist of a fertilizer core, or substrate, surrounded by a coating are not homogenous. This is important because a coated product's release rate depends on the integrity of its coat. If a mower or some other equipment damages a particle, it will release its nutrient almost immediately. Homogenous fertilizers, by contrast, release at a rate that is not dependent on any coating. Thus, equipment cannot destroy the fertilizer particles' slow-release characteristics.

  • Ureaformaldehyde reaction products

    Ureaformaldehyde (UF) reaction products represent one of the oldest controlled-release N technologies, having been first produced in 1936 and commercialized in 1955. Urea and formaldehyde are reacted together to various extents to produce polymer-chain molecules of varying lengths. The more these products are reacted, the longer the chains tend to be. Chain length, in turn, affects release characteristics.

    • Ureaform is the oldest class of UF reaction products. It is sparingly soluble, and contains at least 35 percent total N, with at least 60 percent of the total N as cold-water-insoluble N (CWIN). Ureaform is composed largely of longer-chained molecules of UF polymers. The unreacted (and, therefore, quick-release) urea N content in UF is usually less than 15 percent of the total N.

    • Methylene ureas are a class of sparingly soluble products that evolved during the 1960s and 1970s. These products predominantly contain intermediate-chain-length polymers. The total N content of these polymers is 39 to 40 percent, with between 25 and 60 percent of the N present as CWIN. The unreacted urea N content generally is in the range of 15 to 30 percent of the total N.

    • UF solutions are clear water solutions. They contain only very-low-molecular-weight, water-soluble UF reaction products, plus unreacted urea. Various combinations of the UF solutions are produced. They contain a maximum of 55 percent unreacted urea with the remainder as one or more of methylolureas, methylolurea ethers, MDU, DMTU or triazone.

    • Agronomic properties and nutrient release mechanisms of UF materials. The conversion of UF reaction products to plant-available N is a multi-step process, involving dissolution first, and then microbial decomposition. Once in the soil solution, UF reaction products are converted to plant-available N through either microbial decomposition or hydrolysis. Microbial decomposition is the primary mechanism of N release. Environmental factors such as soil temperature, moisture, pH and aeration affect microbial activity and, therefore, the rate of N release.

    The rate of N release from UF reaction products is directly affected by polymer chain length. The longer the methylene urea polymer, the longer it takes for the N to become available. For ureaform and methylene urea products, the rate of mineralization is reflected by the CWIN content and its Activity Index. The higher the AI value, the more rapidly the N will become available. It is questionable if the very long methylene urea polymers (HWIN) are effectively used by the plant.

  • Isobutylidene diurea (IBDU)

    Unlike the reaction of urea and formaldehyde, which forms a distribution of different UF polymer chain lengths, the reaction of urea with isobutyraldehyde forms a single type of molecule. Although similar in chemical structure to methylene diurea (MDU), its physical properties are quite different.

    IBDU is a white crystalline solid available in fine (0.5 to 1.0 mm), coarse (.7 to 2.5 mm) and chunk (2.0 to 3.0 mm) particle sizes. The product contains a minimum of 30 percent N with 90 percent of the N in water-insoluble form. The typical commercialized product contains about 31 percent N.

    • Agronomic properties and nutrient release mechanisms of IBDU. Nitrogen from IBDU becomes available to plants through hydrolysis. In the presence of water, the compound will hydrolyze (break down) to urea and isobutyraldehyde. The rate of hydrolysis is accelerated by low pH and high temperature. Unlike UF polymers that rely on soil microbial populations to make the N available, IBDU is primarily dependent on water as the critical element in N availability. Its low water solubility controls the transport of the product into the soil solution.

The rate at which IBDU dissolves in water is affected by particle size and the amount of water available. Once in the soil solution, the rate of hydrolysis is affected by soil pH and temperature. The powder form is mineralized much more rapidly than large particles under the same field conditions.

Because N release from IBDU is not microbe-dependent, it occurs readily at relatively low temperatures. Thus, it is one of the preferred products for cool-season applications. These, combined with the dependency on moisture, are the distinguishing characteristics of IBDU.

IBDU is used on turf, in commercial nurseries and in landscaping, forestry and specialty agriculture. Although some fine-size IBDU (31-0-0) is used alone for direct application to golf course greens, most turf use is in the form of blended fertilizers, often in combination with other types of controlled-release fertilizers.

Coated slow-release products

Coated products have several advantages. Some coated products offer a relatively inexpensive means to exploit slow-release characteristics. They also may offer desirable release characteristics in certain conditions.

  • Sulfur-coated fertilizers

    Sulfur-coated urea (SCU) technology was developed in the 1960s and 1970s by the National Fertilizer Development Center. Sulfur was chosen as the principle coating material because of its low cost and its value as a secondary nutrient.

    As the name suggests, SCUs are simply particles of urea coated with a layer of sulfur, and usually a sealant as well. SCUs are typically brown to tan or yellow, depending on the source of urea and whether a sealant is used. Soft sealants are used as a secondary coating over the sulfur coating to fill imperfections in the sulfur coating and to help keep the brittle sulfur shell intact during handling. The total N content of SCUs varies with the amount of coating applied. Many range from 30 to 40 percent N.

    • Agronomic properties and nutrient release mechanisms of SCU. The mechanism of N release from SCU is by water penetration through micropores and imperfections (i.e., cracks) or incomplete sulfur coverage in the coating. This is followed by a rapid release of the dissolved urea from the core of the particle. When wax sealants are used, a dual release mechanism is created. Microbes in the soil environment must attack the sealant to reveal the imperfections in the sulfur coating. Because microbial activity varies with temperature, the release properties of the wax-sealed SCUs are also temperature dependent.

    Even though the urea from a SCU single particle is released rapidly once the coating fails, SCUs provide extended, gradual N release because some particle coatings fail sooner, others fail later, and so forth. The overall result is a gradual release of N to the turf.

    The release rate of a single SCU particle is directly affected by the coating thickness and the coating quality. Particles with higher sulfur loads (thicker coatings) typically show fewer imperfections than particles with lighter coatings. You can infer, therefore, that thicker coatings may be desirable because they will extend the release rate. There is a risk, however, that particles with sulfur coatings that are too thick will exhibit lock-off, which means they may never effectively release their N.

    Depending on the coating weight, N application rate and environmental conditions, SCUs can effectively provide N for 6 to 16 weeks in turf applications. Because of the differential release of N due to the lack of uniformity in coating thickness and the influence of temperature on N release, severe mottling has been observed in turf, particularly close-cut turf such as greens, when SCU was applied during the cool-season growth period.

  • Polymer-coated fertilizers

    Polymer-coated fertilizers (PCF) represent the most technically advanced state of the art in terms of controlling product longevity and nutrient efficiency. Most PCFs release nutrients by diffusion through a semipermeable polymer membrane, and the release rate can be controlled by varying the composition and thickness of the coating. The type of fertilizer substrate also may influence the rate of N release.

    • Meister products. Meister products are produced by using thermoplastic resins as coating materials. The coatings are applied to a variety of substrates including urea, diammonium phosphate, potassium sulfate, potassium chloride and ammonium nitrate. Release-controlling agents such as ethylene-vinyl acetate and surfactants are added to the coating to obtain the desired diffusion characteristics, while coating thicknesses remain similar for most products. Release rates can also be altered by blending talc resin into the coating.

      As with other PCFs, nutrients are released by diffusion through the coating. Typical of most polymer-coated fertilizers, the release is largely controlled by temperature.

    • Reactive Layer Coating. A relatively new coating technology known as reactive layer coating (RLC) combines two reactive monomers as they are simultaneously applied to the fertilizer substrate. These reactions create an ultra-thin membrane coating, which controls nutrient release by osmotic diffusion. RLC products include coated basic fertilizer materials such as urea, potassium nitrate, potassium sulfate, potassium chloride, ammonium sulfate, ammonium phosphate and iron sulfate, in various particle sizes. Coating weights on urea vary from 1.5 to 15 percent, depending on the release duration desired.

      The RLC process permits application of ultra-thin, hence lower cost, membrane coatings, which distinguishes this technology from many other PCFs. Another advantage of RLC is that its manufacturing process costs less than many other commercial polymer-coated fertilizer technologies.

      The coating thickness determines the diffusion rate and the duration of release for RLC products. RLC-coated urea with a 4-percent coating (44 percent N) will release at twice the rate and will have half the duration as an 8-percent coating (42 percent N).

    • Multicote products. In the production of multicote products, fertilizer granules are heated in a rotating pan and treated with materials that create multiple layers of a fatty acid salt. This is followed by the application of a paraffin topcoat. Coating weights are relatively large compared to other technologies, but this is offset by the comparatively low cost of the coating materials. Substrates include potassium nitrate, urea and triple superphosphate. The various coated components are blended together into different grades.

  • Polymer-coated sulfur-coated fertilizers

    Polymer-/sulfur-coated fertilizers (PSCF) are hybrid products that utilize a primary coating of sulfur and a secondary polymer coat. These fertilizers were developed to deliver controlled-release performance approaching that of polymer-coated fertilizers but at a much-reduced cost. Sulfur is employed as the primary coating because of its low cost. Low levels of a polymer topcoat control the nutrient release rate. Unlike the soft wax sealants of SCUs, the polymers of PSCFs are chosen to provide a continuous membrane through which water and nutrients must diffuse, rather than fill in imperfections. The water permeability characteristics of the polymer controls the rate of water diffusion in and out of the particle. The combination of the two coatings permits a positive cost/benefit value over products with single coatings of either sulfur or polymer. PSCFs possess excellent abrasion resistance and handling integrity. Because the outer coating is a hard polymer, the products do not leave waxy residues on application equipment.

The nutrient-release mechanism of PSCFs is a combination of diffusion and capillary action. Water vapor must first diffuse through the continuous polymer layer. The rate of diffusion is controlled by the composition and thickness of the polymeric film. Once at the sulfur/polymer interface, the water subsequently penetrates the defects in the sulfur coat through capillary action and begins to dissolve the fertilizer core. The dissolved fertilizer then exits the particle in reverse sequence.

This mechanism provides greater uniformity in nutrient release compared to typical SCU fertilizers. The agronomic advantages of this are reduced surge growth after application and longer residual; up to 6 months. In addition, the combination coating renders the nutrient release rate much less temperature sensitive than most polymer-coated fertilizers.

Growth responses to controlled-release fertilizers

Most controlled-release fertilizers are N-based, and most of the research involving them has evaluated plant responses to N application. Additionally, because most controlled-release N sources cost several times more per pound of N than the soluble sources, most of the evaluation has been conducted on higher-cash-value crops such as ornamentals, vegetables, citrus and turfgrasses. Little research has been conducted on agronomic crops because their use in this sector is not considered economically feasible. Technologies currently under development may reduce the cost of controlled-release products to the point that they can be used on agronomic crops, but such is not yet the case.

  • Responses on turfgrasses

    Probably more research has been conducted evaluating the response of turfgrasses to controlled-release fertilizers (CRF) than any other plant. Turfgrasses respond well to N and the slow-release extended longevity of N is well suited for turfgrass growth.

The various controlled-release fertilizers respond differently to temperature and climatic conditions. In areas where different turfgrass species are grown during different seasons, it is important to know which CRF to use on the turfgrass species that is actively growing. For example, in many parts of the United States, bermudagrass is grown as the warm-season turfgrass species, and overseeded ryegrass or Poa trivialis is grown during the cool-season. Different CRF products will produce various responses in these different turfgrass species.

The cool-season turfgrasses grow best when supplied with CRFs that are not temperature-dependent for N release, such as IBDU (see Figure 1, page 16). Ureaform products tend not to produce acceptable turfgrass growth or quality ratings when applied alone during the cool season because of the slow release on N under low-temperature conditions. Generally, he coated materials are somewhat intermediate in their response on turfgrasses during the cool-season. The type and thickness of the polymer coating has an influence on the cool-season response.

In general, the various CRFs produce a different relative response when applied during the warm-season growth period. In areas of high rainfall, IBDU tends not to produce as good or as long a growth response as some of the other CRFs during the warm season, while coated products tend to produce a better, more long-lived response in these conditions (see Figure 2, above). Methylene ureas and ureaformaldehyde products produce somewhat intermediate responses. Nitrogen release rate for some of the more methylated ureaforms is still somewhat slow, even in the warm season, and the turfgrass growth rates tend to lag behind that of some of the other CRFs. This may be desirable if it helps you maintain acceptable quality with less mowing.

There is another advantage to using some of the ureaform products: with continued use, soil tends to accumulate N, which is slowly released over an extended period, perhaps enabling a reduction in the application of N over the long term.

Environmental impacts

Not only do CRFs enhance the efficiency of nutrient utilization, they also reduce the impact on the environment and the possible contamination of the subsurface water with N. Significantly, less N has been shown to leach from controlled-release products than from soluble N sources. For this reason, controlled-release N sources are being recommended as a BMP. In the future, as a result of BMPs for various crops, the demand for controlled-release products should increase.

Numerous studies have been conducted on turfgrass to evaluate the quantity of N lost through leaching. Almost without exception, application of CRFs to turf results in less total N being leached. In a grow-in study where the turfgrass was under intensive N-management conditions, approximately 25 percent of the N leached when applied as ammonium nitrate, whereas only 8 percent of the N leached when 50 percent of the N was applied as IBDU. A primary advantage to using CRFs is the reduced threat of groundwater contamination with N. This is one reason CRFs are frequently recommended as a BMP.

Dr. Jerry B. Sartain is professor of turfgrass science in the Soil and Water Science Department at the University of Florida (Gainesville, Fla.).

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