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.
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.
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.
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.
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.
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.
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. They can be used along with other fertilizers. Commercial composts typically are made from various kinds of animal manures and lawn and garden wastes. 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. Aerating the tea to increase the oxygen content stimulates the production of the compost microbes, making for better tea and healthier plant growth.
Manure is derived from mammals, birds, and in one case, insects. Manure is available fresh or dried. Composted manure is the safest form because unlike fresh manure it doesn't burn plant roots, doesn't contain weed seeds, and plant and animal diseases have been destroyed. The composition of manure varies not only with the source animal, but also with the age of the animal, the bedding, and the method of manure storage and application.