Fertilizing with organic nutrients

— Soil Fertility Note 12 — Fertilizing with Organic Nutrients March 1999 NCDA&CS Agronomic Division Web site: www.ncagr.gov/agronomi Serving North Carolina growers since 1 940 Phone: 919-733-2655 Some growers prefer to use organic sources of nutrients (Table 1 ) instead of chemical fertilizers to grow plants. Organic sources do have the advantages of containing slow-release nutrients, posing little risk of soluble salt injury, and contributing considerable amounts of organic matter to the soil. Some of the beneficial response that organic growers see may actually be coming from the increased organic matter they are adding to the soil. Animal Manures Animal manures are an excellent source of organic nutrients. Fresh manure may be available directly from livestock operations. Commercial, packaged manures generally contain composted material. Since manures are highly variable in nutrient content, it is advisable to purchase an analyzed manure or have an analysis done to determine nutrient content. The Waste Analysis Section of the NCDA&CS Agronomic Division can conduct tests to determine exact nutrient concentrations in either fresh manure or noncommercial, composted manure. Composted manures have several advantages over fresh manures. They are less likely to have a strong odor, attract undesirable insects, or burn plants if applied directly. Also, since they contain lower levels of moisture and higher concentrations of nutrients than fresh manures, smaller quantities are needed to supply a given amount of nutrients. In addition, composted manures usually have labels that specify the exact nutrient concentrations they contain. Compost Many materials from yard waste to sawdust to industrial by-products can be composted. Compost is generally ready to mix into the soil when you can no longer identify what the material originally consisted of. It usually has a dark brown appearance, is granular in size, and has a musty smell. Uncomposted waste, such as leaves and grass clippings, is useful as mulch but not as a nutrient source. When uncomposted organic matter is mixed into the soil, plants may become nitrogen deficient. This situation is due to the build up of microorganisms that break down organic matter and tie up nitrogen, making it unavailable to plants. Cover Crops Legumes, such as clover or vetch, make excellent cover crops. When killed or mixed back into the soil prior to planting, they can supply up to 1(X) lb of nitrogen per acre. For small gardens, this would be about 2 lb of nitrogen per 1000 ft2. Mixing a cover crop into the soil also adds organic matter to the soil, making clay soils easier to work and sandy soils more able to retain nutrients. Lime Lime is a naturally occurring material produced by crushing rock containing high amounts of calcium and magnesium carbonates. Because it is not chemically altered, it can be used by organic growers. Lime is also the primary source of calcium and magnesium. Management Considerations Never fertilize without knowing the nutrient status of your soil and of the organic source you intend to apply. The NCDA&CS Agronomic Division can perform these tests for you. A soil test report is good for at least two years and contains recommendations for lime and fertilizer applications, including micronutrients. For lawns and gardens, the fertilizer rate suggested on the soil report can be applied more than one time per growing season. For field crop situations, however, the suggested rate is for the entire growing season and should not be exceeded. The total amount of fertilizer applied during the growing season will depend on soil type, crop, and weather conditions. You can use the suggested fertilizer rate for several years until a subsequent soil test gives a new recommendation. Soil pH. As a rule, plants grow well at a pH of 6.0 although vegetable gardens do better at a pH of 6.5. In North Carolina, soil pH is usually less than optimum. Rainfall and fertilization, even with some organic sources, tend to lower soil pH over time. The chief puipose of applying lime is to raise the pH to a more desirable level. When the pH is too low, levels of aluminum and hydrogen in the soil are toxic to the root systems of plants. If the pH is too high, micronutrients may be bound in forms unavailable for plant uptake.