From a food safety perspective, improperly prepared compost that contains animal manure, animal mortality or post-consumer food residue used when growing fresh produce can be a hazard to human health.
Proper composting of animal-derived materials such as manure is essential to ensure a reduced risk of food contamination.
The U.S. Food and Drug Administration as part of the Food Safety Modernization Act is in the process of establishing new guidelines for the use of animal manures in compost used on food crops.
The proposed safety rules state that all compost with animal-derived materials such as manure, be either statically composted to at least 131°F for three days then cured and insulated, or aerobically composted to at least 131°F for 15 days and turned consistent with current food safety recommendations of having the compost reach temperatures of between 100° F and 160°F for 5 to 15 days.
The new rules also require that properly composted material with animal matter cannot be placed on food crops within 45 days of harvest. It also requires compost containing animal matter be applied in such a way that minimizes contact with the crop. If the compost is not properly made, the rule states that it must be treated like raw manure. The pre-harvest application window for raw manure under the proposed rule is nine months prior to harvest.
Although these regulations are aimed at commercial growers, I believe home gardeners who use compost containing animal matter might be wise to follow the same rules.
In order for the home gardener to meet the requirements being considered by the FDA they will need to use a process called "hot composting." A hot compost pile needs to heat to a temperature range of between 120 and 150°F. These temperatures are achieved by balancing food, water and air within the pile to favor the growth of high temperature microorganisms. The advantage of hot composting is that weed seeds as well as disease organisms are killed, while many beneficial fungi such as mycorrhizae are not killed.
Once the hot cycle is completed the pile begins to cool; as this occurs creatures such as earthworms, insects, and other organisms will complete the decay process.
To construct a hot compost pile gardeners will need a combination of bulking agents called browns and energy materials called greens.
Bulking agents are dry porous materials that help aerate the compost pile. They can be such things as wood chips, sawdust, hay, straw, shredded leaves, or corn stalks. These are too low in moisture and nutrients to decay quickly on their own.
Energy materials or greens provide nitrogen and high-energy carbon compounds needed for fast microbial growth. These include grass clippings, fresh dairy, rabbit, or chicken manure, fruit and vegetable waste, and vegetable trimmings. If added without bulking agents, the greens are too wet and dense to allow much air into the pile. If you have too many greens you are most likely to get an offensive odor.
Some raw materials such as tree and shrub trimmings, horse manure and bedding, deciduous leaves, and hay can be ground up and added to the pile. They will compost readily by themselves, and can be added to an existing pile to ensure the success of a hot compost pile.
To construct a hot pile you want to add two parts by volume browns with one part greens. Chop your raw materials into small pieces. For optimal hot composting, particles should be from 1/8 to ½ inch in diameter.
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Mix the types of raw materials, rather than layering them. A large pile holds heat better than a small pile. For hot composting, make the initial pile at least a cubic yard in volume. Keep the pile moist, but not wet, the consistency of a wrung-out sponge. Turn the pile at least once a week to aerate it. Compost ingredients have their own microorganisms; there is no need to add starters or soil. Some compost mixtures need supplemental nitrogen, so add nitrogen if your pile has mostly fibrous woody materials.
Use a compost thermometer to monitor the interior temperature of your compost pile.
A pile with the right proportions of greens and browns, and that is turned regularly will produce enough heat to kill any harmful bacteria that might be passed on to the food crops that you are growing. Compost is an excellent soil amendment and made properly is quite safe to use on any type of crops.
The new FDA compost regulations are part of the Food Safety Modernization Act which is expected to go into effect in mid-2013. As a home gardener, you may want to start applying the guidelines when your composting season begins this year.