A Quick Guide to Fertilizers

By McCAUSLAND, Jim | Sunset, March 2000 | Go to article overview

A Quick Guide to Fertilizers


McCAUSLAND, Jim, Sunset


Native Americans once put a fish in a hole, covered it with earth, planted a corn seed, then harvested the crop a few months later. A biblical parable offers this advice for reviving a failing fig tree: "Dig about it and dung it." And indeed, fertilizing plants used to be as simple as applying fish and manure. Today, things aren't that simple: Take a look at the dizzying array of packaged fertilizers on nursery shelves. How do you decide which fertilizer to buy? First, read the label.

Check out the nutrients

Nitrogen (N), phosphorus (P), and potassium (K) are the primary plant nutrients. They are always listed on the labels of packaged fertilizers (look under Guaranteed Analysis) in the same order, known as the N-P-K ratio. For example, a fertilizer that's labeled 10-8-6 contains 10 percent nitrogen, 8 percent phosphorus, and 6 percent potassium. Any fertilizer that contains all three primary nutrients, such as a 10-8-6, is called a complete fertilizer.

Of the three primary nutrients, nitrogen is generally in shortest supply in Western soils and thus needs to be replenished most often. Fertilizers supply nitrogen in water-soluble (fast-release) or insoluble (slow-release) forms. Soluble nitrogen becomes available to plants quickly. Insoluble nitrogen must be broken down slowly by microorganisms in the soil for plants to use it. Most fertilizers contain both forms of nitrogen, although labels don't always specify the percentages.

Plants also need smaller amounts of secondary nutrients--calcium, magnesium, and sulfur--and trace amounts of micronutrients, including iron, manganese, and zinc. These secondary and micronutrients are already present in most garden soils, so they're not always included in general-purpose fertilizers (check the label); they are commonly sold as separate supplements.

Natural or chemical?

You can buy fertilizers in either natural or chemical form. Plants can't distinguish nitrogen that came out of a chicken from nitrogen that came out of a chemical factory. People, however, seem to have their preferences. Some favor natural fertilizers, using them to complement organic gardening practices. Others prefer the convenience of chemical products in controlled-release form. Applied properly, both have their appropriate uses in the garden.

NATURAL FERTILIZERS are derived from dead organisms. These fertilizers include all kinds of animal manures, fish emulsion, and meals made from blood, bone, alfalfa, cottonseed, kelp, and soybeans. These products are usually more expensive, pound for pound, than chemical fertilizers.

Most natural fertilizers contain lower levels of nutrients than chemical products. Because they tend to release nutrients over a longer period of time, they're less likely to burn plants (although manures that aren't fully composted can burn plants). However, since they depend on soil organisms to break down the nutrients, they're not as effective in chilly weather, when such organisms are less active. On the other hand, the insoluble nitrogen they supply tends to stay put in the soil and not move into the water supply, where it could have harmful effects. Natural fertilizers also improve the texture of the soil and increase the amount of beneficial microorganisms. For best long-term results, dig them into the soil early in the season.

CHEMICAL FERTILIZERS are mass-produced by industrial means. They usually have higher levels of nutrients and a larger percentage of soluble nitrogen than natural fertilizers. The fast release of soluble nitrogen is a plus in chilly weather, when cool-season crops and spring-flowering shrubs and trees can use a boost. However, when fertilizer is applied too heavily and followed with excess irrigation or rainfall, the soluble nitrogen can run off the soil and pollute surface water as well as groundwater.

Liquid or solid?

You can buy natural and chemical fertilizers in liquid or solid form. …

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