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Fresh Manure and Food Gardens Don't Mix

By Carl Wilson, Colorado State University Cooperative Extension Agent, Horticulture, Denver County

An E.coli illness caused by eating undercooked hamburger has also affected people eating vegetables amended with fresh manure. New strains of E.coli in cattle make questionable the practice of applying fresh manure to food gardens. The practice of using aged manure for ornamental and turf plantings is unaffected by this new development.

It should be emphasized that amending vegetable gardens with plant-based compost, sphagnum peat, and well composted manure continues to be recommended.

Though not all manure carries newer bacterial strains, there is no way to tell without extensive lab testing. A child's death in Maine was traced to E.coli 0157:H7 from calf manure his mother added to the family garden. The bacteria contaminated fresh vegetables harvested from the garden and affected people because the produce was poorly washed. While adults generally become ill and recover, the organism can be life threatening to children and the elderly.

Lettuce seems particularly subject to carrying bacteria because it's succulent and difficult to wash. With carrots and other vegetables, scrubbing and peeling before use considerably reduced the chances of bacterial contamination. Safe food handling practices are now more important than ever.

In home gardens, it's recommended that all manure be well composted before being added to soil where vegetables or fruit are growing. Bacteria will survive winter freezing so fall garden applications and even "aged manure" additions provide no guarantee of a garden free of potential 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 2 to 120 percent of the pathogens survive.

Following the hot compost phase, a "curing" period of 2 to 4 months allows beneficial microbes to outcompete disease pathogens and produces an acceptable organic soil amendment. Note that compost made exclusively from plant wastes does not need a curing period, and adding leaves or other plant materials directly to the garden is safe.

Photograph courtesy of Judy Sedbrook.

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Date last revised: 01/05/2010