Organic Fertilizers 101

By National Gardening Association Editors

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 benefit from the addition of some form of fertilizer. One of the key choices a gardener will make is whether to use organic or synthetic fertilizers. Organic fertilizers are directly derived from plants, animals, and naturally occurring minerals, while synthetic fertilizers are manufactured from various elements.

Why Use Organic Fertilizers?

Organic fertilizers, such as compost, 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. Synthetic 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.

You'll have to purchase synthetic fertilizers; however, some organic fertilizers, such as compost and manure, can be obtained or made for free. The downside is that organic fertilizers are harder to transport and bulkier to use than synthetic fertilizers.

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 synthetic fertilizers lack organic matter, even some "organic" fertilizers, such as rock phosphate, also contain no organic matter. When using any type of fertilizer not containing organic matter, work organic matter such as compost into the soil as well.

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.
  • Mineral-based fertilizers. Naturally occurring mineral fertilizers are considered organic only in the sense that they were not extensively processed. Among them are Chilean nitrate, rock phosphate, greensand, and sulfate of potash magnesia.
  • Composts

    Compost is the "Cadillac" of organic fertilizers. Although making compost from a variety of raw materials is possible, the finished products are remarkably similar in their final concentrations of nitrogen, phosphorus, and potassium. Composts generally contain a good balance and wide spectrum of nutrients, and they're rich in humus - even though their actual nutrient concentrations are relatively low.

    Composts are available commercially or can be homemade, and can be used along with other fertilizers. Making your own compost is an ideal way to recycle yard waste and make fertilizer simultaneously -- and you always know what ingredients went into the finished product. Compost also makes great tea for your plants. Watering with a compost tea is an easy way to get many of the benefits of compost, without the hassle of moving heavy materials into the garden.

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