Trade-offs between winter cover crop production and
soil water depletion in tomato fields and pistachio orchards
by Alyssa DeVincentis and Sam Sandoval
The goal of sustainable agriculture is to provide food to a growing world population without degrading the environment and natural resource base. This is an important research topic in California, where half of the nation’s fruits and vegetables are grown and growers have to meet economic and sustainability goals while managing limited water resources in a changing climate. The use of cover crops is an agricultural practice that can help achieve this goal, but is currently used on less than 5% of California’s annual crop acreage1. Cover crops are planted between mid-October to mid-March, when the field would traditionally be left fallow between growing seasons, and are either managed as green manures and incorporated into planting beds or burned down using herbicides and left as surface mulch. Common cover crops include wheat, barley, triticale, and cereal rye.
Cover crops have many established benefits including reduced fertilizer, herbicide, and pesticide costs, enhanced soil health, and conserved soil moisture. These benefits are due to the cover crop’s ability to provide soil cover, scavenge and recycle nutrients, add crop diversity, and incorporate organic matter into production fields2. This practice increases water availability through decreased evaporation due to mulching effect, increased infiltration of rainfall by decreasing runoff, increased organic matter (which increases water-holding capacity), improved soil structure, and a protected soil surface134.
A growing number of San Joaquin Valley farmers recognize the value of using winter cover crops, but are not certain about the trade-offs between their ecosystem services and the water use and operational costs needed to grow them. These farmers speculate that benefits from this practice outweigh the costs, but do not yet have science-based information supporting this idea. This project will try to answer these questions through a 3-year study in vegetable crop fields and orchard crop environments. We will test the idea that winter cover crops use less water than assumed due to their lower evapotranspiration demand, the ability of the cover crop canopy to enhance soil water infiltration and shade and cool the soil, and the value of the cover crop which adds carbon to the soil which increases soil water holding capacity.
From left: Cover crops at a pistachio orchard, tomato field, and extension specialists Jeff Mitchell and Sam Sandoval taking field measurements.
This study will determine how much water is typically depleted from soils by cover crops during the winter in the San Joaquin Valley, and if the water costs and risk associated with this practice justify its use. This information is critical to understand the magnitude of cover crop water use and water-related implications under the current drought conditions and limited surface water supplies.The results could be applicable to a wide range of San Joaquin Valley specialty crop farms and orchards. If we find that cover cropping results in minimal soil water depletion compared to conventional fallow fields, cover cropping could be broadly adopted across the state.
This project includes 3 years of monitoring and data collection at tomato fields and pistachio orchards throughout the San Joaquin Valley. We are collecting soil moisture data using neutron probes and Decagon sensors, as well as measuring evapotransporation with Tule equipment. We are using drones to document the project, and plan to use infrared cameras in the next field seasons for ET estimations. We will use this information to perform a water balance and a cost-benefit analysis, which will be translated into outreach and extension education to target producers of these crops.
Our very preliminary results from one field site show that cover crops are increasing recharge, but their effects in the root-zone area are unclear.
We hope to inform and improve farmer decision-making and water management related to the value of using cover crops to improve the efficiency and productivity of cropping systems, while minimizing adverse environmental impacts.This project will be valuable for California specialty crop producers and will generate best management practice knowledge related to water and soil management and resource use efficiency under the critical water shortages that California faces.