Garden Manure And E. Coli: How To Minimize The Risk

Q: I have heard that using fresh manure in the garden can contaminate the soil with E. coli. Is this true? Can chicken manure be safely used in a veggie garden - on spinach, for instance? I'm worried about salmonella. Is composted, bagged manure safe?

A: Many gardeners are concerned about manure use, particularly the possibility of E. coli infection from manure. Our Practical Gardener question line has had numerous questions, such as the ones above, about manure safety.

E. coli grabbed headlines last year when several people died and more than 500 became sick here in the Northwest from a sudden outbreak. Before this incident, most people knew little about this bacteria and gave it little thought. Now we know that E. coli is found in most animal intestines (including humans). Although the majority of strains are perfectly harmless, certain others can cause diarrhea, vomiting, stomach cramping and high fever. A few strains can be fatal.

E. coli 0157:H7 is one of these strains. Of concern to gardeners is that the manure of infested animals can contain E. coli. Your hamburger may be well-cooked, but how about the lettuce?

Before you give up gardening, let me emphasize that to be infected the lettuce would have to come from a garden where raw, fresh manure was applied. And the source animal had to be infected with one of the nasty E. coli strains.

So how risky is the use of manure in gardens? If you use fresh manure, there is a small risk that disease pathogens may contaminate your vegetables. The risk is greatest for root crops and leafy vegetables where the edible part touches the soil.

Careful washing and/or peeling will remove most of the pathogens, and thorough cooking is even more effective.

To minimize the risk of disease, here are some precautions suggested by Van Bobbitt, Master Gardener/ Urban Horticulture Coordinator, and Val Hillers, Food Specialist, WSU Cooperative Extension.

1. Apply fresh manure at least 60 days before harvesting any crop which will be eaten raw, e.g., radishes, lettuce. Better yet, apply the manure in the fall to allow breakdown of the pathogens before spring planting.

2. Never apply fresh manure after the garden is planted. Personally, I'd stop this recommendation at "never apply fresh manure." Find a source of well-composted manure such as commercial bagged manure. Alternatively, you can layer it into your own compost pile using only mature, finished compost in the garden.

3. Thoroughly wash raw vegetables before eating.

4. Never use cat, dog, or pig manure in gardens or compost piles. Some of the parasites in these manures develop resistant life stages that may survive composting and remain infectious for a very long period of time.

5. Keep small children away from fresh manure. When thumbs go back into mouths after the "play in the dirt," it gives pathogens direct access to a susceptible host. Wash your own hands well after handling manure.

6. People who are especially susceptible to foodborne illnesses should avoid eating uncooked vegetables from manured gardens. This includes pregnant women, very young children, and folks with chronic diseases such as AIDS, cancer, diabetes, kidney failure or liver disease.

We've been talking mostly about manure as a source of E. coli. How about salmonella, which we hear is so common in chickens? Yes, it may be present in fresh manure, so follow the same precautions. According to my sources, it will not persist very long in aerobic compost piles or garden soil.

Last, but not least, are old familiar problems with manure - weed seeds, ammonia, and salts. Because bedding straw is usually mixed with manure, you may introduce lots of new weeds to your garden unless you compost the manure well. Fresh manure may give off ammonia or other fumes that could hurt tender seedlings, so again, compost. Sometimes even bagged composted manure is high in certain salts that could cause problems in the garden. (I'm not sure, but I've read that this has to do with concentrated feedlot rations.) Whatever the reason, the solution is simple - apply the manure and allow the rain a chance to leach out some of the salts before planting.

Farmers and gardeners have been using manure on vegetables for a very long time. It is an excellent source of organic matter to build soil tilth and also of many plant nutrients. If you choose to use manure, just use plenty of common sense, too. Handled with care, it can help us produce abundant quantities of healthful food for our tables.

Gardening runs Friday in Scene and Sunday in Home/Real Estate. It is prepared by George Pinyuh and Holly Kennell, Washington State University/King County Cooperative Extension agents, Mary Robson, Master Gardener program assistant, and volunteer Master Gardeners.