Soil Chemical and Microbial Responses to Cover Crops as Alternatives to Plastic Mulch in Southern Californian Strawberry Fields

This study evaluated soil health in response to five cover treatments at Rodale Institute California Organic Center (Camarillo, CA) during the 2023–2024 strawberry growing seasons. The goal of the Institute was to identify sustainable alternatives to plastic mulch by examining edaphic properties under the different cover crop treatments: Perennial White Clover, Buckwheat and Peas, and Sorghum and Peas (112 kg/ha and 224 kg/ha); polyethylene plastic film mulch was the control. Soil sampling occurred in February of Year 2 of this study, immediately prior to the spring strawberry season.

At Pierce College, organic matter was determined by dry combustion, percent moisture by the oven dry method, N, P, and K by spectrophotometry using Hach reagents, TDS of soil filtrate using a ThermoFisher meter, and pH using a ThermoFisher meter and 1:5 water. Preliminary data analysis was carried out in Excel. PLFA was carried out by Trace Genomics, and diagnostics were visualized with TraceView.

Results showed that Buckwheat and Peas had the highest organic matter (3.2%) and favorable moisture and phosphorus levels. Sorghum and Peas at 112 kg/ha had the highest potassium (250 ppm) and phosphorus levels (45 ppm). Perennial Clover exhibited the highest nitrogen concentrations (17–19 ppm) and low pH, making it a suitable alternative to plastic where nitrogen is prioritized. Sorghum–Peas at 224 kg/ha displayed the highest mycorrhizal fungi levels, elevated pH (7.95), and high TDS, but also increased Phytophthora fragariae.

A bulked PLFA (Phospholipid Fatty Acid) analysis at the end of Year 2 indicated that the Buckwheat–Peas plots had the lowest levels of Phytophthora spp., while Plastic and White Clover showed the highest Phytophthora cactorum and Verticillium levels, respectively. Oxygen availability was similar across treatments, slightly below the benchmark (~75%).

Buckwheat and Peas and Sorghum–Peas (112 kg/ha) show promise as alternatives to Plastic for moisture retention and fertility, while Perennial Clover may be more suitable where nitrogen is a concern. Disease suppression and salt accumulation varied among treatments.