Understanding Slow Release Fertilizers

Slow release fertilizers release their nutrients in different ways. An understanding of the factors affecting nutrient release is needed to understand how to use these fertilizers in greenhouse or nursery production, in a landscape or to fertilize a turf area. The following is an article on the subject.

Many types of slow release fertilizers are available to the commercial horticulture industry. Slow release fertilizers are a great advantage over soluble fertilizers in that they can offer a complete nutritional package for a season or longer. Release rates vary from weeks to years. Some products have slow release nitrogen only, and some contain slow release N-P-K. They may be incorporated or topdressed. It's up to the greenhouse or nursery grower, landscaper, or turf manager to decide which product fits into the production plan best.

The following are some of the slow release materials available.

Non-coated products

Urea formaldehyde (UF) and methylene urea (MU) are polymeric forms of nitrogen. UF is a condensation product of urea and formaldehyde MU of methylene and urea. About one third of the total N in UF or MU is available in the first few weeks, another third in a few months, and the remaining portion in 1 to 2 years. There is an initial flush of nitrogen released that lasts about 6-8 weeks. The remainder of nitrogen releases very slowly over time can help even out the nutritional program, or provide long term nitrogen nutrition. UF and MU nitrogen is released primarily by microbial action; increasing temperature and low pH values in the medium increase the release rate. There are both solid and liquid products using UF and MU.

Isobutylidene-diurea (IBDU) is a condensation product of urea and isobutyraldehyde. The nitrogen in IBDU is released by a chemical hydrolysis reaction, and in contrast to UF nitrogen, the microbial degradation is minimal. Particle size, hardness, and the amount of water passing through the media have the biggest effect on the fertilizer release. Low pH increases hydrolysis, but temperature is not a critical factor. Commercial IBDU fertilizers contain multiple particle sizes to achieve an even release rate. The maximum duration of release for IBDU particles is about 5-6 months. In the some products, IBDU nitrogen is combined with a plastic coating technology to produce fertilizers that release over periods longer than 5-6 months.

Magnesium-ammonium phosphate (Mag-Amp, e.g., 7-40-6). Nutrients from Mag-Amp are released as the compound solubilizes; release is controlled much as described for IBDU. Compared to other slow release fertilizers, Mag-Amp is low in nitrogen content, and very high in phosphorus. Perhaps it is more properly considered a slow-release phosphorus.

Coated fertilizers.

Nursery and greenhouse growers often use one or more slow-release fertilizers with either a sulfur or synthetic polymer coating.

Sulfur-coated ureas (SCU). Sulfur-coated urea has long been popular and economical for incorporation and top-dressing. The release rate is controlled primarily by the thickness of the coating and medium temperature. The sulfur in the coating is often an advantage because it lowers the pH of the medium. A disadvantage of the traditional SCU products is that a significant portion of the total nitrogen is released early and rapidly due to cracks and imperfections in the coating.

This early release feature has been eliminated and a much more even release pattern has been achieved in the next generation in sulfur coatings, the polymer-coated SCUs. The polymer-coated SCUs are but one type of plastic coated fertilizers.

Polymer-coated fertilizers. Polymer coating technologies allow manufacturers to carefully manipulate release characteristics and to provide fertilizers that release over periods up to two years in duration. Really only cost limits the manufacturer's creativity. To initiate release from plastic-coated materials, moisture must first diffuse through the coating and solubilizes the fertilizer inside. Temperature is the key factor in the nutrient release from all of these products.

Polymer coated urea has a solid urea nitrogen core, coated with various plastic polymer coatings. Differences in coating chemistry affects membrane properties and release rate. Release is due to controlled diffusion, which is fairly constant over time and depends on coat thickness, chemistry, temperature, and moisture.

Osmocote has been an industry standard for years. The original Osmocote was a resin-coated prill containing N, P, and K. Release rates are controlled by coating thickness and temperature. As water diffused into the prill, the coating swelled and the membrane became thinner. Coating imperfections in Osmocote contributed to an initial flush of nutrients in the first week after application. There are now a range of products under the Osmocote label with mixtures of release technologies including the original Osmocote.

Newer coating technologies were developed that did not swell to the degree that Osmocote did. An example is Polyon fertilizers that contain polyurethane-coated urea. Like Osmocote, coating thickness and medium temperature control release.

In contrast to Osmocote and Polyon, The release of nutrients from polyolefin-coated fertilizers is affected by temperature, but not coating thickness. The amount of a surfactant added to the coating determines how rapidly the nutrients are released; more surfactant produces a faster release. Polyolefin resin chemistry came from Japan. Nutricote is an example of a fertilizer with this coating technology. Polyolefin-coated fertilizers do not provide an initial flush of nutrients that some slow-release fertilizers have.

Many products contain more than one of the above technologies in a blend to give different release patterns along with soluble sources. These blends often will have both quick release and slow release properties.

Modified from "Slow release fertilizers for container nursery production" by Mary Ann Rose, Commercial Landscape& Nursery Specialist, The Ohio State University