2018 ASHS Annual Conference

Soil solarization employs solar radiation to heat the soil under a transparent plastic film to achieve temperatures detrimental to certain soilborne pathogens and weed seeds. Most soil solarization studies have been conducted in warm climates, but recent advances in plastic film technology have made it feasible to successfully solarize soil in regions with cooler climates such as the Pacific Northwest. In 2016 and 2017, we conducted pre-plant soil solarization trials for 3-9 weeks during each summer in three tree seedling nurseries in Oregon and Washington. Soil temperature and moisture were measured continuously in solarized and nonsolarized plots at depths of 5 and 15 cm. Mesh packets containing soil from each nursery were buried at the same two soil depths, and after the trial, soil was dilution plated onto selective media for pathogen quantification. Seeds of four weed species (Poa annua, Polygonum pensylvanicum, Amaranthus retroflexus, and Portulaca oleracea) were placed in mesh packets and buried at 5 and 10 cm. After the field trials, seeds were tested for germinability; viability was tested with the tetrazolium chloride assay. Fall and spring weed emergence in situ was also quantified, and time required for hand weeding was determined. We found significant reductions in soil populations of Pythium spp. and Fusarium oxysporum. Of the weed seeds tested, solarization was most effective on Polygonum pensylvanicum, least effective on Portulaca oleracea, and resulted in increased dormancy in Amaranthus retroflexus. Results with Poa annua differed in the two years. In the year following solarization, the time required to hand weed solarized plots in each of two nurseries was reduced by 55% or 69% relative to nonsolarized plots. In 2017, crop growth was greater in solarized plots than in nonsolarized plots in two of three nurseries; data for the 2018 crop year is not yet available. We developed an online model http://uspest.org/soil/solarize for growers to estimate the time necessary to solarize soil based on their farm location, start date, and target pest. The model forecasts soil temperatures from solar radiation and air temperature data at local weather stations, and predicts the amount of time necessary to kill target species based on results from controlled environment studies. Soil solarization is a cost-effective, non-chemical approach for managing weeds and certain soilborne pathogens that could potentially be applied to other Pacific Northwest cropping systems such as organic vegetables and berry crops.