Good Enough to Eat: Some straight poop on using manure
Updated 10:00 pm, Wednesday, November 20, 2002
Over the nearly 20 years I've written about gardening, many readers have asked about the benefits and drawbacks of using animal manures as soil amendments and fertilizers. Here are several of those questions, the first verbatim, the others recollected.
Q: Is there any difference between steer manure and chicken manure when it comes to what good it does for the garden? I mean the weed-free stuff you buy in bags at the supermarket. A gardener friend tells me I should be buying a bag of commercial fertilizer instead, with numbers like 2-5-10 on it, but I can never remember the numbers.
CS: If you plan to buy composted manure in bags from the supermarket, chicken is the better deal for nutrients. Frequently I can buy four bags of chicken manure or six of the same size bags of steer manure for $10. The analysis of the chicken is 3-2-2; the analysis of the steer, .5-.5-.5. Those numbers represent the percentages by weight of nitrogen, phosphorus and potassium, always in that order. For Western Washington soils, the most critical nutrient is nitrogen. The chicken manure is 3 percent nitrogen compared to the steer manure's .5 percent. That means you'd have to use six bags of steer manure to provide the nitrogen you'd get from one bag of chicken. However, if your primary purpose in applying manure is to improve soil structure rather than to increase fertility, buy the steer manure and get two more bags for your money.
Synthetic commercial fertilizers are richer sources of the basic three plant nutrients than manures, and those nutrients cost less in synthetic form. The most commonly used all-purpose formulation in our area, 5-10-10, contains 10 times the nitrogen of steer manure, and you might pay $10 for a 60-pound bag of it. Moreover, synthetic formulations work faster than manures because their nutrients are already in the mineralized form that plants can use.
Cost of nutrients and how fast they work are only part of the comparison between manures and synthetic fertilizers, though. The synthetics won't improve the structure of sand or clay soils. The manures will. And with the exception of expensive, slow-release formulations, the synthetics won't linger in your soil to provide nutrients through the gardening season. The manures will. The bottom line? Most gardeners can find uses for both products.
Q: Is there a right time of year to apply manure to a vegetable garden? Some old references recommend applying it in fall or winter and letting it mellow in, but I wonder about runoff, what with all our rain.
CS: You're right to be concerned about runoff. To minimize that problem, apply manure in spring and till it in just before planting.
Q: Are there any drawbacks to using manure in a vegetable garden?
CS: There are a few. Weed seeds survive in uncomposted and partially composted manures. Horse manure can be especially seedy. Aroma can be a problem with fresh poultry manure; composting it would be neighborly. And if you rely on manure as your main fertilizer, then locating a cheap source, transporting it to your garden and storing it aesthetically might all be problems.
Then there are the health issues. All of us, theoretically, risk contracting E-coli, salmonella and listeria when we apply animal manures to our food gardens. Composting the manure destroys most of the worrisome pathogens. Still, high-risk individuals, including very young children, pregnant women and people with chronic diseases, shouldn't eat raw vegetables from manured gardens.
Even people at low risk should follow a few common-sense safety rules. Before eating root crops raw from manured soil, wash the produce carefully and peel it. Leaf crops that contact the ground may also be troublemakers. Wash lettuce thoroughly. If you want still more protection, add a splash of bleach to a gallon of water, soak the leaves for a minute and rinse.
Q: Are there any manures we should absolutely not use in a food garden?
CS: Because dogs, cats and pigs can carry pathogens and parasites communicable to humans, I don't recommend their manures for food gardens. If you need to make use of such products, offer them to your shrubberies.