Gardening Articles: Landscaping :: Trees, Shrubs, & Vines

Organic Fertilizers

by National Gardening Association Editors


Healthy soils are the basis for healthy gardens

Most gardeners know that for the best results in their vegetable gardens, flower borders, and lawns, they need to start with fertile soil. Although some soils are naturally fertile, most need the addition of some form of fertilizer. One of the key choices a gardener will make is whether to use organic or chemical fertilizers. Organic fertilizers are directly derived from plants, animals, and naturally occurring minerals, while chemical fertilizers are synthesized from various elements.

Why Use Organic Fertilizers?

Organic fertilizers dole out their nutrients as a steady diet in sync with plant needs. Because the nutrients come from natural sources, a portion of them may be temporarily unavailable to plants until released by a combination of warm temperatures, moisture, and microbial activity -- the same conditions plants need to grow. Released slowly, the nutrients from organic fertilizers are unlikely to burn plant roots or be leached away by water. And a single application may last a whole growing season. Chemical fertilizers (except time-release products), on the other hand, tend to have their nutrients available all at once. If the nutrients are not readily taken up by plants, there is the risk of them leaching out of the soil and into waterways.

While you’ll have to buy chemical fertilizers, some organic fertilizers, such as manure and compost, can be obtained or made for free. The downside is they’re harder to transport and bulkier to use than chemical fertilizers and commercially available organic products.

Much of the benefit of organic fertilizers comes not from the nutrients, but from the organic matter the fertilizers contain. Organic matter -- decomposed material that was alive -- helps soil hold water and air, makes nutrients already in the soil more available, and helps prevent diseases. While all chemical fertilizers lack organic matter, even some "organic" ones, such as rock phosphate, also contain no organic matter. When using any type of fertilizer not containing organic matter, always work materials such as straw, peat, compost, and leaves into the soil.

Types of Organic Fertilizers

In catalogs and garden centers, you can find many different kinds of organic fertilizers. Some feature one product source, such as greensand, while others are a blend of organic materials creating a complete fertilizer such as 4-3-3. All fit into one of the basic categories -- plant, animal, mineral, compost, or manure -- that are further described below.

Plant substances

These fertilizers (e.g., alfalfa meal, cottonseed meal, corn gluten meal) are often rich in specific nutrients, such as nitrogen. They can be considered renewable resources, but you should take into account the resources that are used to grow as well as process or transport them. Some, such as cottonseed meal, are by-products of other industries.

Animal processing by-products

Industries such as dairy farming and meat or fish processing generate waste materials that are dried or minimally processed into fertilizers. Some examples of these products are bone meal, blood meal, and fish emulsion.

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