In nature, plants don't receive an annual dose of fertilizer from a farmer or gardener; as microbes break down dead plants and animals, they become nutrients for the soil, available for uptake by the roots of living plants. Other nutrients derive from rocks, which release minerals into the soil as they erode. Organic agriculture and gardening also rely on these principles, and organic growers incorporate plant and animal matter or mineral sources into the soil rather than utilizing synthetic sources of essential nutrients. Soil testing reveals what nutrients are already in your soil and helps you determine how much and what types of organic fertilizers to use (see References 2).
Many organic fertilizers come from animal sources, either as slaughter byproducts or livestock manures. Blood, feather and fish meals serve as sources of nitrogen. Blood meal provides the highest nitrogen levels at 12 percent, although such high nitrogen levels can also burn and damage plant roots. Feather and fish meals provide between 7 and 10 percent nitrogen. Bone meal contains 12 percent phosphorus, providing a reliable source of this mineral in organic gardens (see References 1). Animal manures also enrich the soil with nutrients, including nitrogen, phosphorus and potassium. However, raw manures can damage plant roots, lower soil pH, pollute surface and groundwaters, and contaminate edibles. To reduce these risks, compost livestock manures before applying them to your garden. (See References 2)
Alfalfa and soybean meals contain 3 percent and 7 percent nitrogen, respectively. Cottonseed meal also contains about 7 percent nitrogen; however, overuse of chemical pesticides on cotton crops generally makes cottonseed meal unfit for use in organic production. Wood ash provides low amounts of nitrogen and contains about 6 percent potassium. However, it also raises soil pH, so avoid using it unless you also want to change the pH of your soil. (See References 1)
Colloidal, rock and hard-rock phosphates add phosphorus to your soil. However, soil conditions can greatly affect the effectiveness of these products; a soil test can determine the product that is best for your circumstances. Two conventional fertilizers, natural potassium sulfate and potassium magnesium sulfate, or langbeinite, are also acceptable sources of potassium in organic agriculture. Greensand is an organic source of potassium, containing about 7 percent, which it slowly releases into soil. (See References 1)
Many organic gardeners compost, hoping to reduce or eliminate the need for fertilizers in their home garden. Unless you're composting high-nutrient materials such as manures, however, compost alone might not feed your plants adequately depending on the soil's composition and the needs of the specific plants. Organic growers typically use compost to improve the structure, aeration, water and nutrient-holding capacities of their soil. (See References 3)