Soil Solarization (July 8, 2015)

It is now officially summer and constant triple-digit daytime temperatures will be the norm until at least the beginning of autumn in southwest Arizona. Although we may not personally appreciate the summer heat, it is the perfect time for soil solarization. Briefly, solarization of soil is accomplished by covering moist soil with clear plastic, then allowing the sun’s energy to heat the soil over a period of time. A great deal of research in diverse geographical regions has demonstrated that soil solarization can raise temperatures to levels lethal to many different types of plant pathogenic fungi as well as weed seeds. Soil is covered with plastic to conserve soil moisture and retard heat loss. The combination of high temperature and moisture is essential for killing plant pathogens and weed seeds in soil. The use of drip irrigation if available can help maintain soil moisture and optimize the soil solarization treatment. In preformed bed solarization trials without supplemental drip irrigation conducted a few years ago in Yuma County, the average temperature of soil was 113°F at a depth of 2 inches during a 1-month summer solarization period, compared to 102°F for nonsolarized soil. The average peak afternoon temperature in solarized soil during these trials was 128°F. In these yearly solarization trials, conducted in soil naturally infested with the lettuce Fusarium wilt pathogen, disease incidence in a subsequent planting of lettuce was reduced from 42 to 91%, depending on the trial, compared to disease levels in adjacent nonsolarized plots. Soil solarization, like any other cultural practice, has its benefits as well as drawbacks. Documented benefits include significant population reductions of different soil-borne plant pathogens as well as numbers of viable weed seeds. Drawbacks include the cost of buying, laying, maintaining, and removing the plastic film.