The practice of cover cropping dates back thousands of years and is a cornerstone practice in building healthy and fertile soils. Like most aspects of farming, finding appropriate species for cover cropping and fine tuning management requires some trial and error. Many farmers have noted that while cover cropping requires adjustments in management, a number of benefits can be realized depending on the type of cover crop:
- Increased soil stability and reduced runoff
- Reduced compaction
- Weed suppression
- Pest suppression and better disease resistance
- Reduced nutrient losses via leaching
- Reduced dust
- Better percolation and infiltration rates
- Increase in soil organic matter
- Increase in beneficial insects and pollinators
- Sequestration and storage of carbon into the soil
Cover crops differ from cash crops in that they are not harvested to provide direct income; however, the long-term use of cover crops can result in increasing yields, decreasing inputs, and providing additional services like supplying forage and habitat for pollinators.
WHAT DOES THE RESEARCH SAY?
Cover crops can improve soil health in part through the addition of organic material in the form of roots as well as the whole plant when it is terminated and left to decompose in the soil. These additions help build soil organic matter, including carbon, by providing nutrient-rich food for microorganisms and soil macro fauna such as earthworms and beneficial nematodes. By keeping the ground covered and improving aggregation, cover crops also reduce erosion and improve infiltration and percolation (8). This ground coverage also aids in weed suppression, moderates surface temperature, and reduces evapotranspiration (12).
The timing of planting and cover crop termination are important considerations when weighing cover crop irrigation demands and overall contribution to water storage. Overall, studies show that cover crops can lead to a significant increase in water retention and soil moisture on the soil surface (25)(5)(10)(12). This may be due to improved infiltration and less runoff (16)(2)(9)(17) and better aggregation (22)(16)(24)(18). Research specific to the Central Valley shows that barley and vetch cover crops have little influence on winter soil storage (13), while cover crops grown in Northern California have been measured to reduce runoff by 44% and improve water storage compared to winter fallow (2)(11). In vineyards these numbers can be higher, with a reduction in runoff of 23-77% and erosion by 50-75% (4).
Nitrogen and Carbon Storage
Soil fertility, specifically the dynamics of nitrogen and carbon, are also positively affected by well managed cover crops. Leguminous cover crops contribute nitrogen to crops by fixing atmospheric nitrogen into plant-available forms, providing as much as 55% of the nitrogen they store in their biomass for the subsequent cash crop. In the Sacramento Valley, this can be in the ballpark of 80 to 180 pounds N per acre made available for the following crop. This nitrogen is also in an organic and slow-release form, which is accessible to soil microbes and reduces nitrogen losses via leaching and volatilization as greenhouse gas (6)(15)(19)(3). Soil carbon can also be greatly increased by cover cropping (23)(21)(1), with increases in soil organic carbon in vineyards by 40-50% (20). In California and Mediterranean areas in general, cover crops are shown to increase soil carbon more than conventional fallow, and even more than no-till (7)(1)(14).
CAFF’S WORK IN COVER CROPS
CAFF’s is currently working with a number of growers carrying out cover crop trials in nut orchards. For our cover crop pilot project, we have partnered with four farms in the Sacramento Valley who are each planting cover crops on about 50 acres for three years. The Climate Smart team works with partnering farms to match the grower’s priorities with the appropriate seed mix and plan for implementation. To better understand the cover crops’ impact on soil health, we conduct annual monitoring of soil health indicators, including soil carbon, of the cover cropped orchard as well as an adjacent orchard without cover crops. Along the way, we work with the growers to learn what works and what needs improvement in terms of seed selection, timing of planting, irrigation and when and how to terminate the cover crop. Each year we host field days to create opportunities for growers and local technical assistance providers to discuss what they’ve learned about cover crops, both the benefits and the challenges, and to share helpful resources for cover crop implementation.
In 2019, CAFF received a Healthy Soils Demonstration grant which will enable us to expand our work in cover crops in nut orchards with three new demonstration sites in Colusa, Yolo and Stanislaus counties.
Check our events page for upcoming cover crop workshops and field days!
CAFF’S HISTORY WITH COVER CROPS
Biologically Integrated Orchards Systems (BIOS) was a program that CAFF ran in the 1990s and early 2000s, working with a wide range of actors in the agricultural community (growers, pest control advisors, UC Cooperative Extension, researchers, certified crop advisors) to promote a “whole systems” approach to orchard management. Central to the BIOS program was Best Management Practice training on implementing cover crops, primarily as an integrated pest management strategy. The essential elements of this approach were organizing project management teams of local experts, hosting field days and farm visits, and conducting field monitoring of pest and beneficial organisms.The BIOS program was very successful, leading to the start of a related UC SAREP program, Biologically Integrated Farming Systems (BIFS). The success of both of these programs were documented in Keith Douglass Warner’s book, Agroecology in Action: Extending Alternative Agriculture through Social Networks (MIT Press, 2006).
Many growers who started growing cover crops with the BIOS program are now recognized as leaders in sustainable orchard production. We continue learning from the success and influence of the BIOS program as we endeavor to harvest the lessons of BIOS and integrate them into the current landscape of climate smart farming and sustainable pest management in California.
Colusa Almond Project
The Colusa Almond Project helped to reduce sediment and pesticide run-off from Colusa County almond orchards. Run-off from almond orchards along tributaries to the Colusa Basin Drain in Colusa County contains sediment and pesticides, and the Colusa Basin Drain flows into the Sacramento River. Best Management Practices (BMPs) such as cover cropping, insectary hedgerows, grassed swales, and streambank stabilization could significantly reduce contaminated run-off. We worked with local almond growers to implement BMPs and measured sediment and diazinon loads before and after, up and downstream of demonstration sites to assess BMP effectiveness. CAFF and the Colusa County RCD found that farmer-to-farmer demonstrations and information exchange is the most effective way of changing agricultural management.
COVER CROP RESOURCES
The Biologically Integrated Orchard Systems (BIOS) project was a collaboration between CAFF, farmers, pest control and crop advisors, and the UC Cooperative Extension in the
TECHNICAL ASSISTANCE FOR THE HEALTHY SOILS PROGRAM Are you interested in implementing cover crops, compost, hedgerows and other Healthy Soils practices on you farm? The
‘What Does the Research Say?’ References:
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