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Understanding
Controlled-Release Fertilizers (Aug.
1, 2007)
by Fred Davis, MG, Hill
Gardens, Palermo (To view other articles, click Archives)
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Welcome through Fred’s Garden Gate! Like you and I, plants that we routinely grow in our flower and vegetable gardens require frequent nutritional stimulation and support. In short...we need to eat to remain healthy, and so do they.
Trouble is, while we routinely follow a pattern of breakfast-lunch-supper, and can hop from table to barbecue, vitamin pill to candy machine, plants—who would really prefer a similar routine—are rooted firmly in place and, in more cases than anyone would care to admit,
are nutritionally abandoned to fend for themselves. A great many plants literally starve to death. More are so malnourished that they’re stunted, pale, and bloom in a last-ditch, pitifully desperate attempt to produce seeds in the days before withering into
Plant Hell.
Emotional and overly dramatized? Only a little. It’s what a great many “gardeners” do. Feed our faces...while we callously neglect the nutritional needs of oftentimes expensive plants in our care.
Well, while strictly speaking not an end-all to this problem of neglect, controlled-release fertilizers can help make up for our horticultural shortfalls by automatically delivering measured doses of plant nutrients over the space of two or three months... sometimes longer.
Such a product is a balanced combination of primary plant nutrients (nitrogen, phosphorus and potash... sometimes with trace elements included) which have been enclosed in a slightly porous polymer coating that allows its contents to escape into soil at pre-determined rates instead of being poured out all at once. Some rely upon soil biology to assist in that release while others must wait for sufficient soil moisture and increased soil temperature to get the job done. Temperature
and soil moisture are chief controlling factors in timing fertilizer release.
In a recent issue of Greenhouse Grower magazine, Goris Passchier of Northern Star Minerals, New Albany, Indiana, reminds that, because the fertilizer formulation is actually a salt, it is hygroscopic and therefore attracts water through the coating which then turns the contents to liquid.
He tells us to think of M&M chocolate candies. Something very desirable covered in a thin candy shell. Stored in the chill of a refrigerator, the chocolate remains a solid; subjected to warmer temperatures, it becomes softer and a little can be squeezed out of the shell; but left on a hot surface, the contents become fluid and create a gooey mess even if gently handled.
Controlled-release plant foods act in very much the same way. Salts attract water through the semi-permeable coating. Pressure builds up inside as long as any salt remains, forcing some of that now-nutrient-laden water out into the soil. Warmer temperatures accelerate the process; cooler temps decelerate it. Freezing temperatures stop the action dead in its tracks.
Here’s an insight: You’ve probably noticed that in some commercial nurseries and garden centers, plant containers have significant (sometimes inordinate) quantities of small, yellowish beads on the surface. That’s the timed-release fertilizer. Pots sitting in the sun get really hot during the growing/retailing season. Heat accelerates nutrient release, rapidly exhausting the little pellets, so nursery people are obliged to add more of the product if they want the plants to continue to look good—and sell. (Great gobs of the stuff indicate that the plant was likely left over from the previous retail season.)
Differing lengths of time that a controlled-release fertilizer is claimed to be effective is usually the result of the manufacturer either making the coating thicker or less permeable.
The most common controlled-release fertilizer for home gardeners and seasonal nurseries alike is Osmokote (14-14-14). Osmokote has the ability to provide adequate nutrition for growth, flowering (or producing seeds) and general plant health, but only if adequate moisture is provided. Note: Osmokote is not an organically-approved product.
While more expensive than heavily advertised “blue-water” fertilizers and less pricey granular formulations, Osmokote can be applied (and lightly cultivated into the soil around plants) at the first sign of growth in the spring and once again a little over two months later. Then you won’t find weekly applications of some liquid “miracle” plant food necessary. And your plants will get their “3-squares” every day.
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