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, which 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 effectively killing plant pathogens and weed seeds in soil. If available, the use of drip irrigation can help maintain soil moisture and optimize the soil solarization treatment. In four soil solarization trials conducted a few years ago in Yuma County on preformed beds without supplemental drip irrigation, the average temperature of solarized soil was 116°F (lows and highs ranged from 82 to 149°F) at a depth of 2 inches during a 1-month summer solarization period, compared to 102°F (lows and highs ranged from 80 to 122°F) for nonsolarized soil. The average peak afternoon temperature in solarized soil during these trials was 128°F. In these yearly solarization trials, carried out 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 nonsolarized plots. Soil solarization, like any other cultural practice, has its benefits as well as drawbacks. Documented benefits include significant population reductions of several 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.

To contact Mike Matheron go to: matheron@ag.arizona.edu.

For questions or comments on any of the topics please contact Marco Pena at the Yuma Agricultural Center.
College of Agriculture, The University of Arizona, Tucson, AZ.