From the Field
Dry Farmed Tomatoes are Delicious, and They Can Teach Us a Lot About Agroecological Transitions to Water Scarcity
In a new study, Berkeley Agroecology Lab researcher Yvonne Socolar examines the fungal communities in soil that help dry farm systems flourish.
If you live in the Bay Area, you might be familiar with the stand of vibrantly red tomatoes marked “Early Girls – Dry Farmed” just outside the entrance of Berkeley Bowl in the summer months. These likely came from California’s Central Coast, where growers first started marketing dry farmed tomatoes in the 1980s.
Dry farming essentially means that crops are grown throughout a dry season without external irrigation — depending instead on soil moisture leftover from a wet offseason. Of course, this isn’t a new farming method. Hopi farmers in arid northeastern Arizona have long harnessed the summer monsoons to sustain crops like corn, melons, and squash. Olive growers in Spain and Italy have also historically dry farmed, limiting the irrigation in their orchards through the dry yet cool Mediterranean summers.
But today, more farmers are paying attention to these old practices in the context of climate change. Droughts are longer and more intense. Rainfall is more variable. In California in particular, dry farming has garnered interest among farmers who are trying to maintain their operations as groundwater sources dwindle.
Despite the long history, there isn’t much in the scientific literature about dry farming and how crops interact with soil in such a system. That question set the stage for Yvonne Socolar’s research as a member of the Berkeley Agroecology Lab, led by BFI Co-Associate Faculty Director Tim Bowles.
For her doctoral research, Socolar studied dry farmed tomatoes in the Central Coast — but the tomatoes weren’t the sole focus of her study. As a soil ecologist, Socolar studies arbuscular mycorrhizal fungi, or AMF — fungi in soil that can help a plant grow by extending the plant’s roots with a network of fungal filaments and allowing it to access a larger volume of soil. Tim Bowles published research as a graduate student at UC Davis that showed that AMF can help tomatoes tolerate water stress. In the context of dry farming, as the top layer of soil dries out, plants may depend on AMF to extend deeper for soil moisture and nutrients. Socolar’s research builds on that work, and also explores the broader fungal communities that help dry farm systems flourish.
In a study published last month in the journal Environmental Research: Food Systems, Socolar set up an experiment on seven organic farms growing dry farmed tomatoes. On each farm, she added a commercial AMF inoculant to the soil on half of each test plot. The farmers then managed the farms as they typically did, while Socolar collected soil samples at different depths and times throughout the growing season. Socolar harvested tomatoes throughout the season, taking some back to the lab to dry them and measure their water content as a proxy for quality.
A video produced by Nolan Kirby of Earthworks Media documents Yvonne Socolar’s research on seven organic farms growing dry farmed tomatoes in California’s Central Coast region.
According to Socolar, much of this study was co-designed by the farmers she worked with — farmers like Jim Leap at the UC Santa Cruz Center for Agroecology, Joe Schirmer at Dirty Girl Produce, and Veronica Mazariegos-Anastassiou at Brisa Ranch, who all had a history with dry farmed tomatoes (Jim Leap had even authored a dry farmed tomato growers guide for the Central Coast). Many of the farmers are listed as co-authors on the paper, and a large portion of Socolar’s research was driven by their questions: How did AMF inoculants affect fruit quality and yield? How deep in the soil profile are the nutrients tomatoes access in dry farm systems? How does the fungal community in the soil change under low irrigation conditions?
The results shed further light on the important relationship between soil health and water.
For one, the AMF inoculants did not improve tomato yield or quality — in fact, in some cases, the introduced fungi harmed tomato quality. AMF is ubiquitous in farm soils, and farms will already have their own AMF community prior to inoculation. “Coming in with an exogenous powder is not actually an effective way to build soil health here,” says Socolar, “because these farms are already building a healthy soil microbial community through diversified farming practices like cover cropping and rotating crops.”
These healthy soil communities extend beyond AMF as well. Socolar’s study found that dry farm soils have a unique fungal signature that builds the longer they have been dry farmed. And this signature not only builds, but actually improves tomato quality. “This may be one of the most exciting results for farmers, who now have a new way to promote high quality fruits in their fields by dry farming for multiple years in a row,” says Socolar.
In a recent study, Yvonne Socolar researched the fungal communities known as arbuscular mycorrhizal fungi that may help tomatoes access nutrients under water stress. Here, Socolar collects soil samples to bring back to the lab. Photos by Nolan Kirby
In February 2023, Socolar organized a symposium that gathered farmers in the Central Coast region of California to discuss dry farming.
Socolar also noted that dry farmed tomatoes send their roots deep into the soil — usually more than two meters down, and much deeper than a farmer will typically manage for nutrients. Socolar’s study found that, because surface soils dry quickly in dry farm systems and effectively lock those nutrients away from plant roots, dry farm tomatoes use their deep roots to draw nutrients from depths of two feet and below. That knowledge signals that farmers may have success experimenting with several seasons of deeply rooted cover crops like daikon radish to build up nutrients deep into the soil.
In the context of these farms in Santa Cruz and San Mateo counties, dry farming works well not only because the conditions are right — the Central Coast rarely gets too hot; it receives a lot of winter rain; the soil has the right clay content to hold onto moisture — but also when farmers take care of the soil in a way that both primes tomatoes for low irrigation and adapts them to the conditions. As opposed to irrigated tomato agriculture across the state, diversified, dry farm systems on the Central Coast exemplify place-based, agroecological farming.
“This is a celebration of diversified farming,” says Socolar. “ Dry farming isn’t a silver bullet, but it can be a lighthouse example for how diversified farming practices can help farmers decrease water usage — even if it’s not all the way to zero irrigation — and become more suited for the environmental conditions where they’re growing.”
This research was supported by the USDA National Institute of Food and Agriculture through the Western Sustainable Agriculture Research and Education program.
Learn more about this research and dry farmed tomatoes:
- SARE News: Rediscovering Dry Farming
- Center for Agroecology & Sustainable Food Systems: Organic Dry-Farmed Tomato Production on California’s Central Coast