13.4: Using Composts

Composts help reduce organic waste and are universally beneficial to the soil if applied at appropriate rates and managed well. They can be used on turf, in flower gardens, on trees, 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 they 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).

Composts benefit the soil by providing nutrients, enhancing biological processes and improving the physical structure. Organic farmers are especially keen on using composts as a way to replenish the nutrients that were extracted by their crops (as they cannot use synthetic fertilizers). Although they can “grow” their own nitrogen with legume rotations and cover crops, most other nutrients need to be restocked with organic materials from external sources. Good compost is ideal because it contains the nutrients and carbon that keep the soil healthy, and compost often suppresses pathogens. Conventional farmers, especially for high-value crops, also like to apply compost as a soil amelioration method to enhance crop yields and to reduce pest pressures and environmental impacts (e.g., by improving water infiltration). Composts are also extensively used in landscaping and gardening as urban soils are often compromised by construction activities and heavy traffic (see Chapter 22 on urban environments). You don’t see a lot of compost use on crops growing on large acreage where the cost is generally too high to justify applications (animal manure application is more common). A recent trend in highway departments is to compost roadkill and apply the product to enhance roadside plantings.

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 provide relatively low amounts of readily available nutrients. During composting, much of the nitrogen is converted into more stable organic forms, although potassium and phosphorus availability remain 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.

In some cases, the repeated use of compost, especially on some organic farms, may result in buildup of certain nutrients. For example, if high amounts of compost are applied to meet a crop’s nitrogen needs (remember, compost is relatively low in available nitrogen), then nutrients like phosphorus and potassium are applied in excessive amounts and can accumulate in the soil. Also, salts may build up if there is not enough rainfall to wash them out of the soil (like under high tunnels and in greenhouses). It is recommended to monitor the soil through regular soil tests and to change the fertility strategy accordingly (for example, by using a legume cover crop as a nitrogen source and reducing compost applications).

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 are therefore 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 for 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.

Protecting Drinking Water Supplies

Composting of manure is of special interest in watersheds that supply drinking water to cities, such as those that serve New York City. The parasites Giardia lamblia (beaver fever) and Cryptosporidium parvum cause illness in humans and are shed through animal manure, especially young stock. These organisms are very resistant in the environment and are not killed by chlorination. Composting of manure, however, is an economical option that kills the pathogen and protects drinking water.