Controlled-Release Fertilizers

By Warren Davenport, June 23, 2008

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.

Controlled-Release Fertilizers

Currently, there are three prominent kinds of controlled-release fertilizers marketed to home gardeners. While costs vary widely, all are relatively expensive compared to fast-release fertilizers. However accounting for the value of your time and the reduced number of applications they require, their cost is more in line with less expensive, fast-release fertilizers.

One of the best reasons for home gardeners to use these fertilizers is that most nursery plants are already accustomed to them. These are the same types of fertilizers that many professional growers use.

Compared to natural organic fertilizers, most slow- andcontrolled-release kinds are more concentrated, easier to handle, and less expensive (on a cost per nutrient basis); and they are not dependent upon soil microbes and water to make their nutrients available.

Another advantage to home gardeners is that these fertilizers have been studied extensively. As a result, their label directions are more specific and far more accurate than the directions on most fertilizer products.

The chief disadvantage of these fertilizers is the same as that of any other kind of synthetic fertilizer: They do not directly contribute organic matter to the soil or otherwise improve the soil's physical characteristics.

Nutricote. Various formulations are available, all based upon common fertilizers including ammonium nitrate, ammonium phosphate, calcium phosphate, potassium nitrate, and magnesium phosphate. Granules of fertilizer are coated with a thermoplastic resin and a proprietary chemical release agent.

Depending upon the formulation, the spherical gray pellets release nutrients for up to a year at 77oF. The release rate is influenced primarily by temperature but also by water acting on the release agent.

Nutricote is marketed to home gardeners as Dynamite Plant Food.

Nutri-Pak. Another type of controlled-release fertilizer, called Nutri-Pak, consists of specially designed plastic packets that gradually release the soluble fertilizer inside. Developed by the soil science department of the University of Wisconsin, gardeners drop one or more opened packages into the planting hole. The packs can remain effective in the soil for up to five years.

Osmocote. This industry standard for 30 years offers a wide variety of specialized formulations. All begin as a granular complete fertilizer, which is normally composed of ammonium nitrate, ammonium phosphate, calcium phosphate, and potassium sulfate. These granules are then coated with a type of plastic (an alkyd resin) to form a tiny, nearly spherical yellowish-brown spheres. The coating's thickness, along with temperature, determines how long the fertilizer lasts. For instance the 14-14-14 lasts four to five months at soil temperatures of 60°F, three to four months at 70°F, and one to two months at 80°F. Compared to other controlled-release fertilizers, Osmocote releases more nutrients in cooler soils.

Polyon. Several formulations are available, and all are based on fertilizers such as urea, ammonium nitrate, ammoniated phosphates, potassium sulfate, potassium nitrate. The green pellets have a thin, hard, and nearly break-resistant polymer shell. Nutrient release is regulated primarily by temperature, allowing extremely predictable results. Depending upon the formula, the green particles release nutrients for three to nine months at 86°F.

Polyon is marketed to home gardeners as Pursell's Sta-Green.

Other kinds. Two other controlled-release fertilizers are sulfur-coated urea 36-0-0, usually abbreviated SCU, and polymer-encapsulated SCU. Both are made by spraying molten sulfur onto granular urea. In the case of plain SCU, the thickness of the sulfur coating (along with temperature and moisture) determine the release rate. SCU is relatively inexpensive and is a component of various brands of home lawn fertilizers.

As its name states, polymer-encapsulated SCU is coated with a thin polymer layer, a step that makes it last longer. Both SCU and polymer-coated SCU are used in many lawn and garden fertilizers.

Comparing Fertilizer Costs

The best way to compare costs of fertilizers is to calculate the cost per pound of its nitrogen. Multiply the weight of the material by the percentage of nitrogen (as a decimal), then divide that number into the price. For example, the nitrogen in a $3.97, 1-1/4-pound package of 14-14-14 costs $22.69 per pound: $3.97 divided by (1.25 x .14).

How to Use

The key benefits of slow- and controlled-release fertilizers are their ease of use, reduced number of applications, and more specific and accurate directions on the product label. However, if you have never used them before, you might have general questions. Here are guidelines for some common situations.

Houseplants. Actual nutrient needs depend upon the particular plant and how fast it's growing. Especially in fall through spring, when growth slows and water needs decrease, use at half the recommended rate. However, if growth is fast, as in a sunroom or under artificial lights, or if new leaves are pale yellow, increase the rate.

Outdoor container plants. Fertilize most outdoor container plants at the recommended rate. Start just before spring growth starts, and choose a fertilizer according to how long your plants will remain in the pots (which may be the same as the length of your season). In most areas, make one application of an eight- to nine-month formulation, or two of a three- to four-month one.

Lawns. Most quality brands of lawn fertilizer contain a significant proportion of controlled- release nitrogen. The most commonly used sources are SCU, methylene urea, or polymer-coated urea.

Lawn-fertilizing schedules abound. Generally it is best to fertilize lawns just prior to periods of active growth. That means spring and fall for cool-season, northern grasses; spring and summer for warm-season southern grasses.

Most northern lawns need about 2 to 3 pounds of "actual" nitrogen for every 1,000 square feet each season to remain healthy; southern lawns of Bermuda grass need twice that. Calculate the needed amount of fertilizer by dividing the pounds of actual nitrogen you want to apply by the percent (as a decimal) of the nitrogen in your fertilizer. For example, 11 pounds of an 18 percent nitrogen fertilizer equals 2 pounds of actual nitrogen.

Check with your local cooperative extension agent for specifics, and follow label recommendations.

Landscape plants. If your plants are closely grouped and the roots overlap, scatter 10 to 15 pounds of a three- to four-month 14-14-14 controlled-release fertilizer over 1,000 square feet. The best time is late winter or early spring. Repeat again in late fall. If applied only once a year, spread 17 to 22 pounds of 18-6-12 over the same area.

Flowers. For indoor flowers, follow the directions for houseplants. For annuals or perennials, apply as directed for landscape plants. Apply at planting time or shortly after. Incorporate the fertilizer into the top 2 to 3 inches of soil, or scatter it evenly over the soil surface. Don't pile it on or near the plant's base.

Vegetables. Fast-maturing cool-weather vegetables such as radishes and lettuce are best fertilized with a fast-release fertilizer, because the soils are too cool to allow nutrient release from most slow-release, controlled-release, or organic fertilizers. For warm-season crops such as tomatoes and corn, use a three- to four-month 14-14-14 controlled-release fertilizer at planting time. Incorporate at a rate of about 10 pounds over 500 square feet of soil.

Warren Davenport is president of PAT2H Consulting Services, a horticultural research company in Rydal, Georgia.

Photography by John Goodman

Today's site banner is by Dutchlady1 and is called "Plumeria 'Texas Rose'"