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Slow Release Fertilizer

“What is a ‘slow-release’ fertilizer?”

Often called “controlled release” or “timed release” fertilizers, the reality is that the rate of release of nutrients is primarily controlled by temperature.

The pellets are typically produced by surrounding water-soluble fertilizers with semi-permeable polymer coatings. Once water enters the pellet, the nutrient salts are dissolved and slowly “leak” out. The “temperature control” comes from the fact that the porous polymer coatings have engineered thermal expansion coefficients, and the warmer they are, the larger the holes in the structure, allowing the nutrients to escape more rapidly.

It is important to understand the expected temperature of the container and potting medium throughout the growing season in order to properly specify the temperature profile required and amount of product to add. For example, the Osmocote Pro 20-8-4 8-month formula we use has the following time & temperature release properties:

If you were to use a low temperature in your assessment, but the plants ended up outdoors in the heat, it is possible you could inadvertently overfeed them.

Note that the release rate information provided is the approximate time for 80% of the contained nitrogen to be released. After that, the release rate drops off significantly, so for practical purposes may be disregarded.

“How much shall I use?”

That is a very common question, and one that I’ve rarely seen answered adequately. Most often, manufacturers make recommendations by pot size, but that may not be appropriate for orchids, so let’s apply a little math to it:

First – fertilizer formulas – both water-soluble and slow-release – are specified in weight percent. That is, a pound of 20-8-4 formula contains 0.20 pound of elemental nitrogen, 0.08 pounds of K2O and 0.04 pounds of P2O5.

Second – we need to decide how much nitrogen we want to apply. My recommendation for water-soluble fertilizers is to provide approximately one liter of 100 ppm N solution per week. As 100 ppm is 100mg/L, if we applied that for 6 months (26 weeks), we would apply 2.6g of nitrogen. Now we simply have to calculate how much of the slow-release product will provide the same amount.

With our 20%N slow-release formula, in order to provide the same amount of nitrogen over a 6-month period, we would need to incorporate or top-dress our potting medium with 2.6/0.2=13g if the product released 100% of its contained nitrogen, or 13/0.8=16.25g at the more-reliable 80% release specification.

This particular brand of slow-release fertilizer weighs about 6 grams per teaspoon, an addition of 2- to 2-1/2 teaspoons per pot would be appropriate.