I don’t make compost because it makes me feel good. I do it because composting is the only thing I’ve seen in farming that costs less, saves time, produces higher yields and saves me money.
—CAM TABB, WEST VIRGINIA BEEF AND CROP FARMER
Finished composts generally provide only low relative amounts of readily available nutrients. During composting, much of the nitrogen is converted into more stable organic forms, although potassium and phosphorus availability remains unchanged. However, it should be kept in mind that composts can vary significantly and some that have matured well may have high levels of nitrate. Even though most composts don’t supply a large amount of available nitrogen per ton, they still supply fair amounts of other nutrients in available forms and greatly help the fertility of soil by increasing organic matter and by slowly releasing nutrients. Compost materials can be tested at selected commercial agricultural and environmental laboratories, which is especially important if certification is sought. Composts can be used on turf, in flower gardens, and for vegetable and agronomic crops. Composts can be spread and left on the surface or incorporated into the soil by plowing or rototilling. Composts also are used to grow greenhouse crops and form the basis of some potting soil mixes. Composts should not be applied annually at high rates. That is a recipe for overloading the soil with nutrients (see discussion in chapter 7).
DISEASE SUPPRESSION BY COMPOSTS
Research by Harry Hoitink and coworkers at Ohio State University shows that composts can suppress root and leaf diseases of plants. This suppression comes about because the plants are generally healthier (microorganisms produce plant hormones as well as chelates that make micronutrients more available) and, therefore, are better able to resist infection. Beneficial organisms compete with disease organisms for nutrients and either directly consume the disease-causing organisms or produce antibiotics that kill bacteria. Some organisms, such as springtails and mites, “actually search out pathogen propagules in soils and devour them,” according to Hoitink. In addition, Hoitink found that potting mixes containing composts “rich in biodegradable organic matter support microorganisms that induce systemic resistance in plants. These plants have elevated levels of biochemical activity relative to disease control and are better prepared to defend themselves against diseases.” This includes resistance to both root and leaf diseases.
Composts rich in available nitrogen may actually stimulate certain diseases, as was found for phytophthora root rot on soybeans, as well as fusarium wilts and fire blight on other crops. Applying these composts many months before cropping, allowing the salts to leach away, or blending them with low nitrogen composts prior to application reduces the risk of stimulating diseases.
Composting can change certain organic materials used as surface mulches—such as bark mulches—from stimulating disease to suppressing disease.