Applying Controlled-release Fertilizer

Richard L. Parish  |  12/6/2004 11:22:51 PM

Figure 1. Controlled-release fertilizer under agitator in drop spreader.

Most turf fertilizers contain controlled-release nitrogen fertilizer.
There are at least two reasons for using controlled-release fertilizer. First, controlled-release fertilizer is less likely to burn your turf, since less of the fertilizer is immediately available. Second, by using a controlled-release fertilizer you can make one application last longer - typically six to eight weeks between applications.
With soluble fertilizers, you might have to put down a light dose every week or two to achieve uniform greening over the same six to eight weeks.

How Does Controlled-Release Fertilizer Work?
There are two general ways that manufacturers use to obtain controlled-release fertilizer. The first, and oldest, method is through chemical formulation. The nitrogen is formulated as urea formaldehyde, methylene urea, IBDU or some other nitrogen compound that breaks down slowly over time. The second, and newer, method is to encapsulate inexpensive soluble nitrogen fertilizer such as urea with some material such as sulfur or plastic resin. In this case, the nitrogen is released only when the coating is damaged or deteriorates enough to allow water to get at the soluble urea inside.

Does Your Spreader Affect the Controlled Release?
With the chemical formulation types of controlled-release fertilizer, your spreader should have no effect on the release rate of the fertilizer. Even if your spreader breaks or chips some granules, there should be no change in overall release rate.
The situation is different with encapsulated controlled-release fertilizer. If the spreader causes any physical damage to the coating of a granule, the controlled-release function of that granule is lost and that granule becomes readily soluble (Figure 1).

How Does This Affect Your Turf?
If a significant portion of granules is damaged by spreader application, the overall release rate of the fertilizer product will be changed. There will be more soluble nitrogen for immediate feeding and less controlled-release nitrogen for delayed feeding. If enough granules are damaged, it can cause excessive initial greening response or even burning, followed by a lack of long-term greening response.

Is This a Common Problem?
In testing a wide range of spreader types, the LSU AgCenter found that little spreader damage actually occurred with a resin-coated fertilizer. It is possible that damage would be greater with sulfur-coated fertilizer. Most homeowner spreaders are plastic and do not have aggressive impellers. Significant damage with homeowner spreaders is not expected. Some larger professional spreaders may cause enough damage to be significant, and professional operators should be alert to this possible problem.

In summary, although damage to encapsulated controlled-release fertilizer granules has been demonstrated with some spreaders, it is not a major problem with typical homeowner spreaders. It can be a problem for professional applicators using larger spreaders.

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