Manure is the oldest fertilizer known to civilization and can be a cost-effective soil amendment with many beneficial qualities. Many gardeners feel manure is superior to synthetic products. Careful and appropriate use of manure, especially in vegetable gardens, is important.
Avoiding the Risk of Contamination on Edible Crops
When using manure in the garden, certain precautions should be taken to prevent potential problems. The use of manure presents a risk of food-borne illness from manure contamination. Possible contaminants include salmonella, E. coli, roundworms, and tapeworms. Some people are especially susceptible to food-borne illnesses. These people should use good food handling practices or avoid eating uncooked vegetables from manured gardens. People at greater risk include pregnant women, very young children, and individuals with chronic diseases.
Fortunately, the risk of illness can be minimized in several ways. Root crops, such as radishes and carrots, leafy vegetables like lettuce, and any other vegetable where the edible part touches the soil have the greatest risk for potential problems. Manure should be applied at least 120 days before harvesting any vegetables that come into contact with soil and 90 days for other vegetables that do not contact the soil, such as sweet corn and trellised tomatoes. Additionally, washing and/or peeling will remove many potential pathogens. Thorough cooking is even more effective because most pathogens cannot survive high temperatures.
When to Apply Manure to Gardens
Because edible crops should not be harvested until 90-120 days after a manure application, spring applications are not possible as most vegetables are ready to harvest sooner than 90-120 days after spring planting. One tactic is to apply manure in the fall so there is adequate time between when the manure was applied and when the crop is harvested. As a general rule, do not apply manure after the garden is planted.
Using Manure in the Garden
Using well-composted manure rather than fresh can lower the risks involved with using manure. Composting the manure reduces the odor and weight and kills many weed seeds and pathogens (as long as the pile gets to 131°F or higher). The composting process does lower the availability of nitrogen compared to fresh, but it also reduces the possibility of causing fertilizer burn on plants as the composting process greatly reduces the ammonium content in the manure, which is particularly helpful for ammonium-rich poultry manures.
After spreading composted manure, incorporate the manure into the top 6 to 8 inches of soil. This helps prevent a loss of soluble nitrogen to the atmosphere and allows you to get the most benefit from the application. If applying manure in the fall, consider also applying mulch or planting a cover crop on amended ground to prevent nutrient runoff over the winter and early spring.
For perennial crops such as asparagus and rhubarb, fertilize after the harvest season is completed.
Do not use cat, dog, or pig manures in gardens or compost piles. Some of the parasites found in these manures may survive and remain infectious for people.
How to Compost Manure
Composting manure before spreading it in garden helps to kill parasites and reduce seeds from weeds. Composted manure is also easier and safer to use. Start by creating a pile roughly 3 to 5 feet tall and wide. While this size pile is ideal, at a minimum, the compost pile needs to be roughly 1 cubic yard in size to allow for the pile to reach the recommended temperatures.
For the composting process to be most efficient, the carbon to nitrogen (C:N) ratio must be well-balanced. A C:N ratio of 25:1 or 30:1 is best, but ratios as low as 20:1 and as high as 40:1 will be acceptable. The moisture content also needs to be considered. Manure with 50 to 60% moisture content is ideal with the acceptable range being 40-65%.
In many situations, manure with bedding mixed into it will have a C:N ratio and moisture level within the acceptable range. If the pile has a high amount of manure with little to no bedding, then add a dry carbon source such as straw, corn stover, or leaves to increase the C:N ratio and reduce the moisture content to an acceptable range.
Once piled, anaerobic organisms in the manure begin to utilize oxygen, moisture, carbon, and nitrogen to break down the organic matter and create carbon dioxide and heat. The pile should be turned frequently to introduce more oxygen and prevent excessive heat buildup that could kill the beneficial microorganisms. For home compost piles, use a shovel or garden fork to turn the pile a couple of times each month. Add moisture as needed to keep the compost at a moisture level that feels like a lightly wrung-out sponge. Always wear gloves when touching the compost and wash your hands when finished. When properly managed the pile should heat up to 130 -140°F.
Once the pile cools to ambient temperatures, the composting process is nearly finished. Manure will reduce in volume by about 30% when composted. For well-managed compost piles, this typically takes 4 to 8 months. The compost then requires an additional curing period of 2 to 4 months to create the high-quality compost that can be used in the garden. It is beneficial to use a two-bin system for composting manure. One bin can be used for the active composting of the manure and the other bin can be used for curing. When finished the compost will have a hummus, soil-like consistency. A well-managed pile started in early spring is typically ready to spread by fall.
- Using Manure in the Home Garden from the University of Wisconsin-Madison
- Turn Manure into Compost for Your Garden from Oregon State University
- Guidelines for Using Animal Manures and Manure-Based Composts in the Garden from the University of New Hampshire
- Guidelines for Using Manure on Vegetable Gardens from the University of Maine
- Garden Soil Management (pub)
- FSMA Compliant On-Farm Thermophilic Composting: A Safe Way to Enrich the Soil from North Central Region Center for FSMA Training, Extension, and Technical Assistance
- Raw Manure under the FSMA Final Rule on Produce Safety from the United States Food and Drug Administration