Compost and Vegetable Crops

Gordon Johnson, Extension Vegetable & Fruit Specialist;

We have had several new composting businesses come on-line in the last three years on Delmarva and vegetable growers have had questions on whether or not compost would be a good fit for their production systems.

Composting businesses provide a valuable service to the region by taking waste products that might otherwise end up in landfills at a cost to the public, and producing a product that can be used as a soil amendment.

In the composting process, organic stock material sources such as yard wastes, manure and litter, wood waste, food scraps and garbage, paper, hatchery waste, or other waste materials are combined in a proper mix to create a carbon to nitrogen ratio that will promote the growth of microorganisms that then decompose the materials, producing a dark, humus-rich end product. In addition, in the composting process, the compost piles will heat up to over 150°F, killing any pathogens in the materials. A properly produced compost can be used for vegetable production without concerns for transferring plant pathogens or pathogens of concern for food safety.

Compost will contain plant nutrients, the level of which depends largely upon the stock materials used. Nitrogen content may be significant; however, much of the nitrogen will be in organic form and will be slowly available over several years. Most of the phosphorus and potassium will be available.

While compost does contain plant nutrients, the more important benefit that it provides is stable organic matter. Because it has already been decomposed, the organic component contains humus-like materials that will decompose very slowly when added to the soil. This means that compost will immediately raise the organic matter of the soil. This in turn will increase the cation exchange capacity (CEC) of the soil, improve soil moisture holding capacity, and improve soil physical characteristics (reduced compaction, improved aeration, decreased crusting).

Research has also shown that certain composts can reduce the incidence of soil borne diseases and pests. This is most likely because the organic addition promotes more diversity in soil microorganisms that can compete with pathogens and because the improved physical properties of the soil reduce the impact of certain pathogens.

When using compost, growers should first receive an analysis of the material. From this analysis you should look at the following:

Compost Maturity – Only use mature compost that has finished the composting process. Immature compost will continue to decompose, and can cause soil imbalances in some cases.

Compost pH and Calcium Carbonate Equivalent (lime value) – Lime or high pH materials such as hatchery waste are often added to compost to achieve the best microbial activity. This means that compost will have some liming value and if used in too high of amounts, or on soils that already have a high pH, may increase the pH so that it is too high.

Electrical Conductivity (EC or salts levels) – Composts that use manure or poultry litter as part of the stock materials can accumulate salts (particularly potassium) at high levels. This is a particular problem with spent mushroom soil based composts. While the potassium certainly has good fertilizer value, the high salt content must be taken into account in application rates so that you do not cause salt injury on crops.

Nutrient Content – As previously stated, compost has a base nutrient content. You need to account for available nutrients in the nutrient management plan for the crop the compost will be used on. Much of the nitrogen will be in organic form and only a portion will be available for the growing season. You should look for estimated nitrogen release on your compost analysis to make nitrogen calculations.

Moisture Content – Compost will be partly water. With higher moisture composts, you will be paying for more water and less of the humus material and nutrients. In addition, higher moisture composts do not spread as well as lower moisture composts.

You should visit the compost facility and look at the finished product if possible before purchasing. Other compost processes that growers should ask about include screening (to achieve uniform spreading and remove clumps), removal of foreign material, and storage conditions. Improperly stored compost can absorb rain water, have weed seeds that blow into the compost, or be contaminated in other ways.

In research at the University of Delaware with several compost materials, a rate of 5-7 tons per acre showed yield benefits on sandy soils in the first year with several vegetable crops. Lower rates did not show first year yield effects and higher rates did not show additional benefits. However, specific effects on a grower’s farm will depend on soil type, existing organic matter, existing soil health, and compost source; therefore, rates should be adjusted accordingly.

Compost can be very useful in intensive vegetable production systems where tillage activities tend to cause organic matter losses. Compost is also useful for high tunnel systems where it is often difficult to use other soil improving methods once the high tunnel is constructed.

The decision to use compost is also an economic one. Compost can cost anywhere from $5.00 to $30.00 per ton depending upon the source and distance for transport. Growers need to consider the fertilizer, liming, and soil improving value of the compost and evaluate that against other soil improving programs using standard fertilizer or liming practices, cover crops and soil improving crops, manures or poultry litter, and minimum tillage practices.