Animal manure has been used in vegetable gardens for centuries. It adds nutrients and organic matter, aiding in the development of healthy, living soil. However, there have been many health scares linking the use of manure as a fertilizer to breakouts of E. coli (Escherichia coli), which may make you wonder: Is it safe to add manure to a vegetable garden?
According to University of Illinois Plant Pathologist Nancy Pataky, the bacteria that is already on plant roots, as well as bacteria and fungi in the soil, would compete with E. coli and keep it in check, perhaps even feed on it. "Additionally, no research has indicated that the E. coli bacterium is anything more than a surface contaminant."
That said, it's quite possible for manure to spread disease to human beings, although there have not been many long-term studies involving manure and home gardens. According to Van Bobbitt and Dr. Val Hillers of Washington State University Extension, "Pathogens can be transferred from animal manures to humans. The pathogens salmonella, listeria, and E.coli 0157:H7, as well as parasites, such as roundworms and tapeworms, have been linked to applications of manure to gardens." As for E. coli, contamination can occur when any type of food comes in contact with animal feces.
Tips for Avoiding Contamination by Manure
There is always a risk when you use manure or manure tea in your garden, but there are some precautions you can take to stay safe.
Don't Use Fresh Manure
Aside from contamination risk, the fresher the manure, the more of a chance it will be high in nitrogen and ammonia, which can burn plant roots and even inhibit seed germination. If the manure is from a plant-eating animal, it is probably also full of weed seeds, which will not be inhibited from sprouting.
If you still want to make use of fresh manure, don't apply it after your garden has been planted. The U.S. Department of Agriculture recommends an application window of 120 days prior to harvesting and eating any vegetable where the edible part comes in contact with the ground. That includes anything grown below the ground (beets, carrots, potatoes, radishes, etc.) as well as anything sitting on the ground, like lettuce, spinach, and even vining crops like cucumbers and squash. You can apply fresh manure up to 90 days prior to harvest for vegetables that are far enough away from the soil that nothing will splash up on them but err on the side of caution.
Find Other Ways to Use Manure in the Garden
Instead of using manure as a fertilizer, use it as a soil conditioner. Add fresh manure in the fall for spring planting. It will have time to work into the soil and compost. Wait until all vegetables have been harvested before adding it to the soil.
Another option is to side-dress with composted manure during the growing season. Manure that is composted lessens the risk of contamination, especially if the pile heats up to 140 degrees or more. You can purchase composted manure or, if you have a source of fresh manure, compost it yourself. According to Stephen Reiners, Cornell University horticulturist, hot summer temperatures will usually kill E. coli.
Make Sure It's Free of Pathogens
If you are buying packaged manure, the bag should state whether it is pathogen-free. Don't assume that just because it is sold as fertilizer that it is fully composted. If you are getting your manure locally, inquire at the farm if their animals have had any health problems.
Use Common Sense Precautions
Thoroughly wash your hands and nails before and after harvesting produce grown with manure. Since root crops (beets, carrots, radishes) and leafy vegetables (chard, lettuce, spinach) are the most susceptible to contamination, wash these vegetables well and possibly peel them before eating. Cooking will also kill pathogens.
If you have been susceptible to foodborne illness in the past, avoid eating any uncooked vegetables fertilized with manure. Children, pregnant women, people with weakened immune systems, and those with chronic diseases should also avoid eating these vegetables.
Not All Manure Is Created Equal
The nutrient value of manures varies by animal. Chicken manure is considered to be the most beneficial for your garden, with an N-P-K ratio of about 1.1–0.8–0.5. Compare that with cows at 0.6–0.2–0.5 and horse manure, 0.7–0.3–0.6. Of course, even within the same species, the quality of the manure will vary.
Most sources recommend avoiding the use of pig, cat, and dog manure in the garden because it may contain parasites that can survive in the soil and infect humans.