By Wayne McLaurin
University of Georgia
When is it dangerous?
Not all manure carries the newer bacterial strains. But there's no way to tell without extensive lab testing. A child's death in Maine was traced to E. coli 0157:H7 from calf manure added to the family garden.
Using aged manure for turf and ornamental plants isn't a problem. Amending vegetable gardens with plant-based compost, sphagnum peat or well-composted manure, too, is still good to do.
Bacteria-contaminated vegetables fresh from the garden affect people largely because the produce is poorly washed. Adults generally become ill and recover. But the organism can be life-threatening to children and the elderly.
Lettuce seems particularly subject to carrying bacteria, because it's succulent and hard to wash.
With carrots and other vegetables, scrubbing and peeling them before eating them greatly reduces the chances of bacterial contamination. Safe food-handling practices are now more important than ever.
Using manure safely
In home gardens, make sure all manure is well-composted for at least 120 days before adding it to soil where vegetables or fruits are growing.
Bacteria will survive winter freezing. So fall garden applications and even adding "aged manure" provide no guarantee of a garden free of disease organisms.
The heat produced through proper composting will kill most pathogens. The compost should heat to 130 or 140 degrees for five days or more to be more effective. Even so, research has shown that a certain percentage of the pathogens can survive.
Following the hot-compost phase, a "curing" time of two to four months allows beneficial microbes to outcompete disease pathogens. This produces an acceptable organic soil amendment.
Note that compost made solely from plant wastes doesn't need a curing period. And it's safe to add leaves or other plant matter directly to the garden.