Fast Release Nitrogen Fertilizers
Fast release, or soluble nitrogen fertilizers, are mostly inorganic nitrogen salts. The exception is urea, which is considered a synthetic organic compound. The inorganic salts become plant available when they dissolve. Urea must undergo a hydrolysis reaction prior to becoming plant available, but in most situations this reaction is quite rapid and does not constitute a slow release material.
Slow or Controlled Release Nitrogen Fertilizers
The characteristics of the fertilizer material are determined by the form of nitrogen in the fertilizer and the accompanying cation or anion. Two characteristics that are important to turfgrass production are the salt index and the residual acidity/basicity of the fertilizer. The higher the salt index the greater the potential for burn. Differences in acidity/basicity of the fertilizers will also alter the liming needs of soil.
A number of slow or controlled release fertilizer materials exist. Some of these materials are natural, but many are manufactured. The mechanism by which a controlled release of nitrogen is achieved differs among the fertilizers. The influence of temperature, soil moisture, and pH on release is also fertilizer specific. Understanding the processes that govern nitrogen release from each material is important to select the correct fertilizer for each situation.
Coated Nitrogen Fertilizer sources
Nitrogen release from plastic-coated fertilizer particles depends on moisture diffusing through the coating and into the particle, dissolving the nitrogen source inside the particle, and diffusion of the nitrogen solution out of the particle. Varying the thickness of the particle coating as well as mixing particles of different coating thickness controls the rate of nitrogen release. The type of fertilizer material inside the particle also influences the rate of nutrient release.
Processed organic fertilizers
Increasing temperature from 50 to 68 of doubles the release rate of nitrogen from plastic coated particles. Beyond 68 of there is a smaller effect of temperature on release rate. Moisture is required for nitrogen release from plastic-coated materials, but the rate of release is not altered by soil moisture between field capacity and the permanent wilting point. When plastic-coated fertilizers are applied to the turf surface, moisture could be limiting at times. Soil pH and the absence of microorganisms, from fumigation for example, have no effect on the release of nitrogen from plastic coated fertilizers.
Sulfur-coated urea consists of a urea particle coated with elemental sulfur and then sealed with wax. A microbicide may also be included in the wax to delay biological degradation of the sealant. The thickness and integrity of the coating determine the rate of nitrogen release from the fertilizer. Nitrogen release in the first seven days after application is thought to arise from damaged particles where there is no coating between urea and the soil. Otherwise, the microorganisms first degrade the wax sealant, the urea diffuses through the sulfur into the soil solution, hydrolyzes, and is then made available to the plant. Turf mowers may damage larger sulfur-coated urea particles, enhancing the release of urea. Increased temperature and moisture increase the release rate of nitrogen from sulfur coated urea, but soil pH does not have an affect. Ammonia loss from surface applied sulfur-coated urea is less than that for surface-applied urea.
Many different types of organic fertilizers are available. Base materials for organic fertilizers include animal and human manure and waste products from the processing of crop plants, trees, and animals. Seaweed provides the basis for some organic fertilizers. Treatment generally includes composting which transforms readily available nitrogen into more slowly available forms and kills disease organisms. Grinding or screening of materials is often done to reduce particle size and provide a uniform range of particle sizes.
The rate of nitrogen release from processed organic fertilizers is highly dependent on the type of material. Since there are so many different products available it is difficult to generalize as to whether release rates are fast or slow. Since biological decay of the organic fertilizer is required to release the nitrogen it is certain that a healthy microbial population is required and that temperature and moisture will greatly affect the rate of release.
The performance of many of the slow release nitrogen sources is highly dependent on the quality of their manufacture. The particle size distribution of IBDU is critical to determining the release rate of nitrogen. The uniformity of coating thickness and mixtures of particles with varied coating thickness determines the rate of release of plastic coated nitrogen sources. The relative proportions of urea, dimethylene urea, and ureaformaldehyde determine the nitrogen release of ureaformaldehyde based nitrogen fertilizers. Consistent manufacturing of these fertilizers is essential for repeatable performance and beneficial use by the turfgrass manager.