Publication Date

Fall 2022

Degree Type


Degree Name

Master of Science (MS)


Environmental Studies


Rachel O’Malley

Subject Areas

Environmental studies


California is a globally important site for agricultural production, but the state is currently suffering from an unprecedented drought. One strategy to conserve water is dry farming, where growers cache moisture in the soil from off-season rain or early-season irrigation for crops to use throughout the growing season. Some California growers have dry-farmed tomatoes for decades. Recently, some have begun adding co-composted biochar (COMBI) to boost productivity and stimulate soil microbiology, particularly arbuscular mycorrhizal fungi. To the best of my knowledge, no field research on dry-farmed tomato systems currently exists, let alone on the effects of COMBI amendments on yield, soil microfauna, or the relative cost of production. This on-farm collaboration between San José State University (SJSU) and an organic grower was thus designed to test the effect of COMBI on dry-farmed tomato yield, soil microbiota, and cost implications. Compost treatments with and without a softwood biochar amendment were applied to a 4-acre dry-farm tomato field at Martial Cottle Park (San José, CA). To estimate productivity two weeks before fruit maturity, I non-destructively measured plant height and per-plant fruit number and size, then harvested and measured fruit and shoot biomass after maturity. I sent soil samples for phospholipid fatty acid analysis to describe the microbial community. COMBI improved tomato productivity enough to outweigh the cost of the amendment. Still, microbial community parameters did not differ between treatments, suggesting that chemical or physical mechanisms may drive the yield effects.