The use of manure is an old practice in the Salinas Valley of California for maintaining organic matter and soil tilth. Kurt Schulbach and Richard Smith, University of California Cooperative Extension farm advisors for Monterey County, evaluated the levels of microbial contamination in fall-applied manure, and the potential of those materials to contaminate lettuce subsequently grown in the spring.

In 1998 and 1999, 10 and 13 fields were monitored, respectively. Twenty-three to 40 percent of the manure sampled (both steer and chicken) was contaminated with Escherichia coli. Over the two-year study, no E. coli bacteria was found in the soil or on the lettuce at harvest in 1998 or 1999. The lack of survival of E. coli from contaminated manure in the soil is not surprising, based on prior research evaluating waste-water applications to soil that showed relatively short survival times of bacterial human pathogens in soil.

After two years of study, it appears that the risk from the use of manure contaminated with E. coli is small to nonexistent. Given the relatively high percent of manure samples with E. coli contamination, a potentially greater risk exists from locating manure piles and manure-spreading operations upwind from, or adjacent to, fields at harvest, where dust from manure piles can blow over and contaminate the fresh product.

The anxiety over the contamination issue has caused buyers to request that growers reduce or eliminate their reliance on the use of manures as a soil amendment. As a result, 42 percent of the companies surveyed in this study in 1999 decided to eliminate the use of manure, and are using composted manures and/or yard trimmings or grape pomace to alleviate concerns and risk of microbial contamination of lettuce.