In recent years, several companies have introduced lines of "natural" fertilizers. What are they, how do they work, and are they worth the cost? First you need to be sure you know what you're buying.

What's in a name?

Products labeled fertilizer must show how much of the three primary plant

The word natural is more vague. Most chemical companies use it to mean fertilizers extracted from living things or from the earth with minimal processing. Synthesized ingredients, like ammonium nitrate, are usually (but not always) excluded.

From bag to plant

Most natural fertilizers contain modest amounts of mostly slow-release nitrogen, so they're unlikely to burn plants to which they're applied, and they continue working for a long time. To compensate for relatively low nutrient value, apply more to get the same results you'd see from synthesized fertilizer. In addition, over time natural fertilizer can improve soil texture by adding organic matter, which helps soil hold water, air, and nutrients.

Mined nutrients, like rock phosphate and limestone, don't contribute much organic matter to the soil. Only those derived from recently living things (cottonseed meal, for example) build soil's organic content.

The most heavily used plant nutrient, nitrogen, comes in both water-soluble (fast-release) and water-insoluble (slow-release) forms. Water-soluble nitrogen gives plants a quick dose of nutrients as it dissolves and washes down around the root system. Water-insoluble nitrogen must be slowly broken down by microorganisms for plants to use it.

Cost and sources

Natural fertilizers sell for more because they cost extra to make. Many also contain biodegradable ingredients designed to increase microbial activity in soil; such activity is said to decrease disease and improve growing conditions. Most garden centers and nurseries sell packaged natural fertilizers. If you'd like to buy individual bulk ingredients and mix your own, write or call Down to Earth, 532 Olive St., Eugene, Ore. 97401; (503) 342-6820.

Natural mined nutrients

Dolomitic lime raises the pH of acid soils, making more nutrients available to roots. It also supplies calcium and magnesium.

Langbeinite ore contains magnesium, potassium, and sulfur. Of the three, only potassium, at 20 percent, is a major nutrient. Langbeinite ore is used in blends, but rarely alone.

Limestone raises soil pH and supplies calcium.

Rock phosphate contains calcium and lots of phosphorus--20 to 30 percent--of which only about 2 percent is readily available to plants.

Natural nutrients derived from animals, plants

Alfalfa meal has about 5 percent water-insoluble nitrogen, 2 percent phosphorus, 1 percent potassium.

Bat guano contains 10 percent nitrogen (most of it water soluble), 3 percent phosphorus, 1 percent potassium.

Blood meal is dried blood from slaughterhouses. At about 12 percent nitrogen (mostly water-insoluble), it's quite strong for a natural fertilizer. In Oregon State University trials, this was the most effective natural fertilizer. (Store it away from cats and dogs, who eat it.)

Bone meal contains 3 to 5 percent water-insoluble nitrogen, 12 percent phosphorus. The phosphorus helps roots grow strong.

Cottonseed meal has 6 percent water-insoluble nitrogen, 2 percent phosphorus, 1 percent potassium.

Dried poultry waste (chicken manure) contains about 5 percent nitrogen (3 parts water-insoluble to 1 part water-soluble), 3 percent phosphorus, 2 percent potassium.

Feather meal has 12 percent nitrogen, most of it water-insoluble.

Kelp meal contains about 3 percent potassium, abundant trace elements.

Soybean meal has 6 percent water-insoluble nitrogen, 2 percent phosphorus, 1 percent potassium.

Lawn problems--and solutions

Brown spots or streaks in your lawn might make you suspect some insect or disease. Even though pests do cause problems, more often the culprit is something simpler. Here are five of the most common causes.

Compacted soil. If water and air can't move easily into the soil, roots don't grow. Use an aerator that extracts soil plugs, then apply water slowly so that it can soak in.

Dog urine. Dogs can cause 8- to 10-inch areas of dead grass. Use a spreading grass that repairs itself, or replant using seed or sod.

Excessive fertilizer. The salts in fertilizers will kill grass if you apply too much. Use only the amount recommended on the product label, and avoid accidentally overlapping passes with a spreader.

Too much shade. All lawn grasses grow best in full sun. Thin trees to allow light to reach lawn. Also raise mowing height (longer grass blades trap more light). Use grasses that tolerate some shade, such as St. Augustine and tall fescue.

Too little water. Most lawns have one area that dries out first. Adjust sprinklers to provide better coverage, or water the area by hand. Don't overwater the rest of the lawn just so the driest area will get enough.