Controlled and Slow Release Nutrient Products in Turfgrass Management
FERTILISERS
The use of controlled and slow-release fertilisers has dramatically increased in recent years. Turfgrass managers frequently use fast release nutrient sources including inorganic and organic fertilisers such as ammonium nitrate, potassium nitrate and urea. Such fertilisers are relatively inexpensive to purchase and give very good results, however many problems exist with them including fast nutrient release, leading to frequent light applications being required and also a high leaching potential. This led manufacturers to devise fertiliser products that, when applied to the sward, last more than a few days, which is often the case with quick release nutrient sources. Slow-release fertiliser (SRF) and
controlled release fertiliser (CRF) are often considered to be the same. However, technically, they contain different fertiliser materials. SRF defines organic fertiliser materials, such as urea formaldehyde and milorganite, as well as chemical fertilisers of low solubility such as magnesium-ammonium phosphate. CRF refers to inorganic fertilisers that have been coated by materials such as sulphur, polyethylene, and waxes. The basic concept behind controlled fertilisers is that they release the contained nutrients at a more gradual rate than conventional fertilisers such as ammonium nitrate. Benefits of controlled release fertilisers include:
• Slower nutrient availability to the grass plant, thereby reducing excessive soft growth
• Reduction in required number of fertiliser applications
• Nutrient losses to the environment may be reduced
• Fertiliser burn is not a problem
• Reduced seedling damage compared to some fast release fertilisers with high salt concentrations
In reality, controlled release fertilisers are really only readily available as a source of nitrogen, although some controlled potassium products are coming onto the
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market. Controlled release fertilisers can be divided into two categories: coated and uncoated. Slow release fertilisers belong to the uncoated category.
Coated Controlled Release Fertilisers
Sulphur Coated Urea (SCU) This fertiliser is probably the most readily known of the coated products on the market. The concept was developed during the 1960’s and 1970’s by the National Fertiliser Development Centre and basically entails coating urea particles in sulphur. This coating gives the particles a yellow colour, which I’m sure all turfgrass managers have come across on numerous occasions. Sulphur is used in the process because it is of relatively low-cost and is also a secondary element. In many situations, a wax sealant is placed around the sulphur coating, which often helps to fill-in any imperfections of the surface of the sulphur particle. Release of nutrients with sulphur- coated urea is by penetration of water through the tiny pores in the sulphur coating. This allows water to move inside the sulphur, thus dissolving the urea inside, which then leaks out through the sulphur coating. The speed at which the water penetrates the coating depends on the thickness of the sulphur layer. In some instances problems can arise with the quality of the sulphur layer as uneven sulphur coatings may be applied to the urea particle. This may lead to some areas of the sulphur coating being very thin. Such areas will release the urea very quickly, whereas urea particles with very thick sulphur coatings may require extra time to release the nutrient inside. Other problems with sulphur coat cracking have arisen. If a wax layer is applied to the sulphur coating, some microbial activity may be required to breakdown the wax, before water can penetrate the sulphur. SCU’s usually provide nitrogen for turfgrass plants for six to sixteen weeks after application.
Polymer Coated Fertilisers This technology involves the placement
of a semi-permeable polymer layer around the nutrient particle. Water diffuses through the polymer which solubilises the nutrient inside and released from the polymer capsule. This technology is more modern than sulphur coating and was designed to counter some of the problems of coating thickness with SCU’s, leading to a more controlled release of nutrients. The speed at which nutrients are
released depend upon the thickness of the coating and also the nutrient used inside of the polymer. A big problem for many turfgrass managers with this product is the cost. Polymer coated fertilisers are far more expensive than the SCUs and new polymer coated products with thinner polymer layers have emerged. An example of this is Reactive Layer Coating of fertilisers, a technology that allows very thin membrane coatings to be placed over the fertiliser materials. Other advances include fatty acid salt layering around fertiliser granules.
Polymer/Sulphur Coated Fertilisers Often known as PolyS, these fertilisers were developed to combine the more accurate release patterns of polymer- coated fertilisers with the lower cost associated with sulphur coated fertilisers. Manufacturers wanted to develop a controlled release fertiliser that would appeal to a greater number of turfgrass managers.
A thin layer of sulphur is placed
around the nutrient particle. Subsequently, a very thin layer of polymer is placed over the sulphur to act as a top coat. The combination of the two coatings gives a lower cost product than a polymer fertiliser, yet with a more controlled release pattern than a sole sulphur coated layer. The thin layer of polymer helps to prevent fast nutrient loss through thin areas in the sulphur coating and, as already mentioned, acts as a continuous layer which water must penetrate prior to nutrient availability. PolyS fertiliser nutrient release mechanisms include both diffusion and capillary action. These fertiliser types have shown to release some nutrients up
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