Apr 20, 2022
Revisiting compost for vegetable production
Gordon Johnson, University of Delaware

Each year I field questions from vegetable and strawberry growers on the use of compost for their production systems. The availability of commercial compost has fluctuated over the years based on the companies operating in the region.

Bioenergy Devco now manages the composting facility in Seaford Delaware and produces another quality compost for use by growers on Delmarva. Expansions are planned. There are several other commercial composting companies in the region producing quality products.

Growers also can learn how to make good compost and many of our small producers make compost at some level. The following is some information on how to evaluate and use compost.

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 between 130-170° F, killing pathogens of concern in the materials. A properly produced compost can be used for vegetable production without concerns for transferring plant pathogens or human pathogens.

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 potassium will be readily available while phosphorus availability is more variable.

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 the improved physical properties of the soil (such as reduced compaction) that limits the impact of certain pathogens. Newly finished compost also contains beneficial microorganisms that directly affect plant pathogens by antibiosis or hyperparasitism. Some composts have also been shown to induce resistance to pathogens in crop plants.

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

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

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.

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 elevated levels. The elevated salt content must be accounted for when determining application rates so that salt injury does not occur with crops.

Calcium Carbonate Equivalent (lime value) – Lime is generally not added in the composting process; however, high pH materials such as hatchery waste sometimes are composted. This means that certain composts may have more liming value.

Moisture Content and Physical Condition – 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. Compost should be adequately screened so that the product spreads well.

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. 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.

The decision to use compost is also an economic one. Compost can cost anywhere from $20.00 to $50.00 per ton depending upon the source and distance for transport. Growers need to consider the soil improving and nutrient value of the compost and evaluate that against other soil improvement programs such as cover cropping and green manure crops.

For small growers, permanent composted beds can create extremely productive and resilient growing systems. Three to four-foot-wide beds 100 or more feet in length are laid out with paths between the beds. The bed surface is covered with compost and paths are covered with bark or wood chips. Crops are planted through the compost. Each year additional compost is added to the bed. Cover crops are planted on the beds between seasons and are laid down or rolled by hand and then planted through providing additional mulch that decomposes. Crop debris also remains to decompose. A rich organic layer develops that no longer requires any tillage.


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