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 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 to six months after application.
Osmocote and Nutricote
Osmocote production involves the coating of a soluble fertiliser core with a copolymer of dicyclopentadiene and a glycerol ester, which are dissolved in a hydrocarbon solvent. Nutricote technology uses thermoplastic resins such as polyolefins as coating materials. The coatings are used with a variety of fertilisers including urea, potassium sulphate, and ammonium nitrate.
Uncoated Controlled and Slow Release Fertilisers
Characteristics of uncoated fertilisers include homogeneity and, frequently, smaller particle size than coated products. The ability to manufacture such fertilisers with smaller particles sizes aids in their use on low cut turf such as golf greens. Homogenous refers to the fact that each particle of uncoated fertiliser is the same throughout.
Ureaformaldehyde Reaction Products.
Commonly known as Ureaform, Methylene Urea, Nitroform, Nutralene and Methex. The technology employed is among the oldest for controlled-release nitrogen. Urea and formaldehyde are reacted together to form these products. This reaction causes the formation of polymer-chain molecules. The longer the products are reacted together, the longer the polymer chains are, with longer polymer chains releasing nitrogen more slowly than short polymer chains.
The conversion of ureaformaledhyde reaction products to plant available N in the soil involves dissolution and microbial decomposition. The primary release mechanism is through microbial decomposition, with carbon from the polymer acting as a food source for the microorganisms. Thus factors influencing soil microbial activity such as soil pH and temperature will affect nutrient release.
Isobutylidene diurea (IBDU)
IBDU is well known to all turfgrass managers as a white crystalline solid that comes in various particle sizes. The reaction of isobutyraldehyde and urea form this product. IBDU is available to grass through hydrolysis and the rate of availability depends upon the particle size and the amount of water available. The thicker the particle size, the slower the nutrient releases. When adequate water is available, the fertiliser particles will break down and release the contained nutrients. IBDU nutrient release is not influenced by soil microorganisms, so nutrients can be released from this fertiliser even in cold conditions, although low soil pH values have been shown to increase nutrient availability.
There is no doubt that controlled release fertiliser products are here to stay in the turfgrass market. These products are in use of all aspects of turf including golf courses and athletic fields. Such products are very useful in improving the effectiveness and performance of nutrient chemicals. Probably the main barrier for use of these products is their high price with many turfgrass managers having to keep careful account for their budgets, however in the future I believe that these fertilisers will be used on a more frequent basis in the turfgrass industry.
About the author
Tim Butler is currently studying for a doctorate degree in Sportsturf Science at both University College Dublin, Ireland and Michigan State University, USA. For further information contact Tim Butler at firstname.lastname@example.org