By S. CORY TANNER
A COUPLE DECADES AGO, USING MANURE to add organic matter and plant nutrients to vegetable gardens was a no-brainer. Many gardeners raised some sort of livestock, or knew somebody who did, and using manure in a garden or field was a good way of eliminating animal waste while improving the soil.
Today, most people don’t have their own farm animals and the practice of using manure as an organic fertilizer is becoming a lost art. A resourceful gardener can still track down manure for use as fertilizer—sometimes free, sometimes not— but there are some issues to consider before accepting that first load.
The first and most significant concern is food safety. Fresh manure may contain several different pathogens, such as E. coli, Listeria, and Salmonella, that can contaminate your produce. You obviously don’t want to sicken yourself or others, so never
apply fresh, un-composted manure to a planted vegetable garden.
A better alternative is to use thoroughly composted manures. It’s important to realize that “aged” is different from composted manure. Aged manure has usually only been piled for a prolonged period of time—this does not kill the pathogens. To properly compost manure, mix it with yard waste like leaves, straw or wood chips and allow to heat to 140 degrees. It should be stirred several times to make sure all of the material experiences the highest heat in the center of the pile. After the heat period, the pile should cure for two to four months before use.
More tips on using manure
- Only use manure from farm animals like cattle, horses, goats, rabbits and poultry, never carnivores, humans or pets.
- Most commercially available, bagged manures have been thoroughly composted or pasteurized (it should say so on the bag) and screened for herbicide contaminants and weed seeds, making them generally safe for use.
- More is not always better. Some soil nutrients, particularly phosphorous and potassium, can become excessive with repeated applications of manure. Also, excess manure is more likely to find its way into streams and ponds, contaminating water. A safe application rate is 30 pounds of manure per 100 square feet tilled into the top 6–8 inches of soil annually. Monitor soil nutrient levels with annual soil testing and stop applying manure if nutrient levels become excessive.
The second concern is herbicide carry-over. Some herbicides approved for controlling broadleaf weeds in pastures and hay fields can actually pass through an animal’s digestive tract and remain active in manure, even after it is composted. The herbicide ingredients of concern are picloram, clopyralid and aminopyralid. Using manures or mulch that contain traces of these herbicides can seriously damage or kill your garden plants. To avoid this problem ask your manure supplier if their pastures or hay fields were treated with any of these products.
The last concern is weed seed contamination. Believe it or not, many weed seeds can pass through an animal and be a nuisance for the gardener. Horse manure is particularly notorious for this. Composting will help reduce this problem, but you may need a plan to deal with an outbreak of weeds when fertilizing with manure.
S. Cory Tanner is an area horticulture agent for Clemson Extension based in Greenville County. Contact him at firstname.lastname@example.org.