Lettuce crops in the central coastal district of California may follow each other in rapid succession on the same land with only a short period between the harvest of one crop and planting of another. Some growers in this area use soil fumigation after a series of 5 to 6 lettuce crops to improve yields and quality. The effectiveness of such fumigation has been attributed to such factors as nutrient release and the control of unidentified pathogens but specific data has been lacking.