Evaluating feasibility of solarization for organic small-scale farmers in coastal California

The main objective of our research was to conduct an on-farm trial to determine the feasibility of using solarization for pest control for small-scale organic farmers in our region. Our concept of feasibility was ultimately concerned with difference in net income between farm areas managed with and without solarization.

Project site: Our trial took place at La Buena Tierra, located at ALBA in Salinas, CA. Each year, ALBA leases farmland to 36-40 beginning organic farmers. Farmers develop their operations at this site for up to 4 years, after which they must leave to continue their farming journey elsewhere. Accordingly, there is constant turnover of farmers at ALBA’s property, with the effect that farmers do not have the opportunity nor the incentive to invest in the land in ways that would minimize weed seed banks and reduce soilborne disease pressure. This issue is compounded by the fact that the property is home to beginning farmers, many of whom are just learning about the pitfalls of poor weed seed and disease management and do not yet understand the importance of preventative practices. Weeds and soilborne diseases are both very common at this location and often produce major challenges for farmers. This context therefore provides the perfect opportunity to explore potential benefits of solarization as a chemical-free pest management practice.

Research design: Our field trial took place on one fifth of an acre of land (we had planned to do the experiment on a quarter of an acre; after measuring the field, we realized that the area designated for the project was slightly less acreage than expected). Prior to project implementation, the entire project area was managed similarly. The entire area was disked and bedded before treatments were applied. For sake of feasibility given our small scale, we used a simple experimental design in which one half of the area received the solarization treatment and the other half of the area received the control treatment. Given that the benefits of solarization are known to last a minimum of 3-4 months (Krueger and McSorley 2009), we evaluate those benefits using a fall lettuce crop, which is both highly susceptible to soilborne disease (Verticillium wilt is particularly common in our area) and is responsive to weed pressure.

Solarization treatment: The solarization treatment followed Fennimore’s (2018) guidelines for best practices. In late spring, we purchased clear, relatively thin, construction-grade plastic (6 mil = 0.15 mm) from a local supplier. In early July, the soil surface was raked smooth to improve contact with the plastic covering, effectively minimizing the presence of air pockets. We then installed and placed plastic over the soil and buried the edges to keep it in place. Given that wet soil conducts heat better than dry soil, we installed and used drip tape to moisten the soil prior to applying plastic and wetted soil to at least 12 inches in depth. Following best practices, the plastic was left in place for nearly 8 weeks and we regularly repaired any damages to the plastic during that period. The plastic was be removed in late August, at which point we took care not to disturb the underlying soil to avoid bringing up viable weed seeds. We then transplanted lettuce according to Johnny’s Seeds planting recommendations (https://www.johnnyseeds.com/). Lettuce was cultivated according to the PI’s standard growing practices.

Control treatment: One of the major barriers to adoption of solarization is the challenge it poses for cash flow. In our region, solarization must be completed during the hottest months of the year – in mid-summer – which is also in the middle of the growing season. Accordingly, an important consideration in evaluating feasibility of this practice is evaluating the opportunity cost of taking an area out of production for 6-8 weeks during the height of the growing season. To approximate this loss, we planted a 6-week crop. We had initially planned to plant baby bok choy but opted to plant radish given that the PI anticipated a better market for radish at that time.  In early July, radish was seeded according to Johnny’s Seeds planting recommendations (https://www.johnnyseeds.com/). Radish was be cultivated according to the PI’s standard growing practices. We managed the control plot such that the radish was harvested and the soil was prepared for subsequent planting (through disking and bed preparation) on the same day that the solarization treatment was terminated. In late August, lettuce was transplanted on the same day that lettuce was transplanted in the solarization treatment plot and similarly was transplanted according to Johnny’s Seeds planting recommendations (https://www.johnnyseeds.com/). Lettuce was cultivated according to the PI’s standard growing practices and was managed equally with the solarization treatment plot.

Soil sampling: Composite soil samples were taken from each treatment area and delivered to TriCal Diagnostics lab in Hollister, CA for analysis of soilborne pathogens at 3 dates throughout the trial: (1) immediately prior to application of plastic; (2) immediately following removal of plastic; (3) immediately following lettuce harvest.

Data collection and analysis: During the solarization period (when the plastic was on the soil), we collected data about: (1) amount of irrigation water applied (both treatments); (2) cost of plastic (solarization treatment); (3) cost of transplants (control treatment); (4) amount and type of fertilizer applied (control treatment); (5) amount and type of pesticide applied (control treatment); (6) number of boxes of radish harvested (control treatment). Although we did not say so in our proposal, we also collected data about (7) number of hours spent weeding (control treatment).

During the cultivation of the lettuce crop, we collected data for each treatment to indicate weed and disease pressure: (1) number of hours spent weeding; (2) as a proxy for disease pressure, we flagged 3 areas of 40” (width of one bed) by 10ft for each treatment plot (6 areas total) and counted the percentage of diseased heads of lettuce in each area (number of diseased heads/total number of heads); (3) total number of boxes of lettuce harvested.

All data was collected weekly from plastic application through termination of the lettuce crop in fall 2023 and was regularly uploaded into a Microsoft Excel spreadsheet. Data was quantitatively analyzed using simple statistics to compare the treatment and control plots.