Using manure in the organic garden
Consuming fresh organic vegetables is considered by many to be the epitome of good health, and growing them yourself is a step towards sustainable living. Animal manure has been used as an organic fertilizer since the dawn of agriculture, some 10,000 years ago. However, E. coli outbreaks raise concerns over the potential food safety risks of using manure in the garden. While there are risks associated with using manure, by taking the necessary precautions, you can greatly reduce these risks and enjoy the fruits of your labor without concern.
Many E. coli outbreaks on spinach and other leafy greens are caused by wild boars. These pigs find their way into California spinach fields, where they had a field day with all of that free salad! Pig manure, along with dog and cat manure, should not be used in the garden because it contains a particularly large amount of disease-causing bacteria and pathogens. Generally, manure from meat-eaters, such as cats and dogs, is more likely to carry disease than manure from vegetarian animals.
Cat scat can be especially dangerous to pregnant women or those with a compromised immune system. Cat scat is a vector for toxoplasmosis, a parasitic disease that can remain active in the soil for more than a year. This is troubling for those of us with raised vegetable beds that our cats find irresistible for use as litter boxes.
Solarizing the soil through winter and spring will kill toxoplasmosis and make the bed safe for planting. After watering the soil, place a large piece of clear plastic over the garden to heat the soil and kill the disease. You’ll get the added benefit of killing many weed seeds.
Keeping the kitties out of the vegetable garden takes mental dexterity and trial and error. Even though cats can climb, many of them can be deterred with fencing. I use willow fences for a rustic look, but chicken wire works well too. Some felines can’t stand walking on plastic mulch, and others don’t like walking on rocks. Planting a row of offensively aromatic plants may also keep some prima donna cats from prowling around.
Horse and cow manure is generally safe for the garden as long as it has been composted or aged. Cow manure is easier to find at most local nurseries than horse manure. Fresh horse and cow manure contains bacteria, and a lot of urine. Urine is very salty and can damage plants. Commercially produced steer manure can also be high in urine because at most commercial feeding operations, where the manure is collected, cows are being readied for slaughter and they are confined in pens with limited mobility. Before applying manure, test your soil for salt content. Don’t apply manure to soils that are high in salt.
Commercially composted manure has been heated to over 150 degrees to kill weeds, pathogens, and bacteria. Aged manure has been allowed to sit and dry out, typically for six months or more, but will still contain weed seeds and could still contain bacteria. Aged or composted manure helps improve soil texture and water-holding capacity.
If you are going to apply fresh manure, to reduce the food safety risks, the USDA National Organic Program recommends applying it at least 120 days before harvesting vegetables. In the short growing season of Montana, this means the best time to apply fresh manure is in the fall. This will give ample time for any bacteria to die, as most bacteria thrive in a moist environment, not in our dry, cold winter climate. However, fresh manure is an excellent source of weed seeds and will increase your vegetable garden weed woes.
Good horticultural practices will also reduce the risk of foodborne illnesses. The location of your vegetable garden can make a difference. Don’t grow your vegetables in an area where surface water will run onto it from either a manure storage area or land fertilized with fresh manure. In addition, don’t grow vegetables in the leach field of a septic system.
Mulching your vegetable beds will prevent water drops from bouncing off applied manure onto your crops. Mulch may deter cats from pussy footing around your vegetables, and will provide a layer of protection between vine crops such as squash and melons and manure-amended soil. Mulch provides additional benefits of water conservation and weed suppression.
Thoroughly washing all food grown in manure-fertilized gardens will also reduce the risk of contamination. Wash them just before eating to avoid spoilage. Scrub root crops such as carrots, beets, and potatoes, and thick-skinned vine crops, such as melons and squash, with a scrub brush and cool water. Better yet, peel them before eating. Even though you don’t eat the skins of squash and melons, when you slice them, bacteria present on the skin can be transferred to the flesh. Remove and discard the outer leaves of leafy vegetables before washing them.
Growing your own organic food supply can be very satisfying and will supply your family with top-notch produce to enjoy. Aged manure from vegetarian animals – horses, cows, goats, sheep, and chickens – is an excellent amendment for our sandy soils, and with the proper handling, poses little risk.
This is a reprint of an article I wrote when I was the commercial horticulture program coordinator for the western area of University of Nevada Cooperative Extension. For more specific gardening and horticulture advice be sure to visit your local Cooperative Extension office.
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