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Gardening Articles: Health :: Houseplants

Controlled-Release Fertilizers

by Warren Davenport

Compared to most fertilizers, controlled-release kinds are convenient and predictable

Gardeners shopping for fertilizer face a bewildering array of choices. There is such a variety of liquids, powders, and granules, all promising fabulous results, that many gardeners essentially give up and choose the most prominent package on the shelf, or the most heavily marketed one. But two types of fertilizers are increasingly important to gardeners: the kinds called slow- and controlled-release.

These fertilizers share some similarities but differ in key respects. Rather than releasing a quick rush of nutrients as liquid, soluble crystal, or granular fertilizers do, these release their nutrients slowly over a longer period. Slow-release fertilizers relinquish their nutrients at a less predictable rate that depends mainly on the activity of organisms in the soil. In contrast, controlled-release (sometimes called coated) fertilizers release their nutrients at a specific rate over a specific period of time.

A Steady Diet for Plants

While no fertilizer is ideal for every garden or situation, slow- and controlled-release kinds offer many advantages for most home gardeners. They avoid the common "feast-or-famine" syndrome that occurs when fast-release fertilizers are applied inconsistently. Roots are briefly surrounded by plenty of nutrients, but these soon wash away again leaving roots to starve. Likewise, fast-release fertilizers are easy to apply in excess consequently damaging the plant. But since slow- and controlled-release fertilizers dole their nutrients out gradually, both potential problems are minimized.

Another advantage of slow- and controlled-release fertilizers is environmental. In many areas of the country, waterway, stream, and groundwater pollution is a problem, and some of that pollution has been traced to fertilizers washing through or off lawns. Because these regulated fertilizers release nutrients slowly, they are less likely to contribute to this kind of pollution.

In most cases, temperature is the most important influence on release rates. Not only does it affect nutrient diffusion across the coating of controlled-release fertilizers, but it also exerts a major influence on microbial activity, and thus on the release of nutrients from slow-release kinds.

Slow-Release Fertilizers

Two kinds of slow-release fertilizers are currently available. Because they are less expensive than controlled-release kinds, they are best used when the precision--and higher cost--of controlled-release is not required, and where natural organics are not desired.

Synthetic organics. Several kinds of fertilizers are produced by combining urea, a common form of nitrogen, with formaldehyde. These are called urea formaldehyde or methylene urea fertilizers. One example is light blue nitroform, a 38-0-0 nitrogen fertilizer that is 70 percent "water insoluble nitrogen" (abbreviated WIN on product labels). The release rate is determined largely by bacterial activity rather than by temperature and water. Depending upon the manufacturer, nutrients may last weeks or months. Urea formaldehyde-based fertilizers are a component of many lawn and garden fertilizers, such as Jobe's Plant Food Spikes.

A similar product is isobutylidene-diurea (IBDU), a 32-0-0 fertilizer, which is 90 percent WIN. Nutrient release is controlled by moisture, pH, and fertilizer particle size (smaller ones release nutrients more quickly). It is also a component of many lawn fertilizers.

Natural organics. Many home gardeners favor natural organics, primarily for their soil-improving qualities. There are numerous kinds, and each has unique qualities. Nutrient release rates are highly variable and determined primarily by soil bacteria and fungi, both of which require warm soil temperatures to be active. The more biologically active the soil, the faster the release rate. Examples of natural organics include blood meal, cottonseed meal, soybean meal, fish emulsion, manures, and composts. All are common.

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