Increasing Grower Adoption of Adaptive Cover Cropping Systems: Effects on Vegetable Production and Nitrogen Cycling

Final Report for FW09-328

Project Type: Farmer/Rancher
Funds awarded in 2009: $50,000.00
Projected End Date: 12/31/2012
Region: Western
State: Oregon
Principal Investigator:
Nick Andrews
Oregon State University
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Project Information


We conducted two years of cover crop trials on participating farms and the North Willamette Research & Extension Center. We measured cover crop biomass, N content and aspects of soil quality. Findings were used to validate N mineralization estimates in the OSU Organic Fertilizer and Cover Crop Calculator. This work is included in a draft Extension publication, was presented at 13 extension events, and incorporated into beginning farmer educational material. Findings have been presented at five professional meetings. The project has helped growers estimate cover crop nitrogen contributions and understand some soil quality benefits of cover crops.


Intensive vegetable rotations can reduce soil health if soil organic matter is not replenished. Many Oregon farmers are committed to improving soil quality. However, they operate in a competitive business environment that encourages intensive rotations, and they need information to optimize their soil building practices while maintaining profitability. Cover crops have many well-documented benefits, but our ability to quantify these benefits has been limited. Cooperating growers have told us that greater confidence in the value of cover crops and support in adapting cover crops to their rotations will enhance their use, thus improving soil quality and saving money spent on fertilizers. Previous research in Washington State (Cogger, Bary and Kuo) and in Oregon’s Willamette Valley (Dick and Luna) demonstrated that when a winter cover crop is successfully established in the fall, and the cover crop contains at least 50% legume (dry matter basis at plowdown) summer vegetable crop fertilization rates can be reduced by 50+ lb plant-available N per acre the following spring (i.e. Pub EM 8803-E and EM 8704). Recently concluded on-farm research (Andrews and Stephenson, Western SARE project FW06-301) added to grower confidence in determining the amount of N taken up by a winter cover crop. The most reliable method for N uptake assessment was harvesting small areas of cover crop (2 x 2 feet) in spring, obtaining a %N analysis from a commercial lab, and then computing total N uptake (N% x biomass).

In a related project, Sullivan and Andrews, (agreement number 2008-34513-19284: Extending application of “Organic Fertilizer Calculator” to cover crops) collected laboratory data to develop a prediction equation for first-year plant-available N (PAN) following cover crop plowdown. Field data from the current project was used to validate this lab data in the field and to further support extension of the Organic Fertilizer Calculator to cover crops. Farmers in the area had little to no experience crediting cover crop N contributions in their fertilizer plans. Preliminary results from one-year studies indicated that sufficient nitrogen for vegetable production can sometimes be supplied by winter cover crops alone on silt loam soils with histories of moderate to heavy organic material amendment. It is unknown how well cover crops can continue to supply N and maintain or improve soil quality over time on different soil types. Experience shows that several years of reliance on manure to meet crop N requirements can result in excess soil N and P levels. Increased confidence in the ability of cover crops to provide nitrogen will tend to reduce the risk of phosphorus pollution from agriculture. This confidence, coupled with improved N monitoring on farms, will reduce the frequency of excess N levels. In this project we conducted collaborative work (farmers + scientists) to evaluate adaptive cover cropping systems on real farms. We included farms with different soil textures (light sandy loam to heavier silt loam), cultivation practices and field histories.

Project Objectives:

1. Collaborate with growers to identify adaptations to cover cropping systems in commercial vegetable rotations.

2. Improve our ability to quantify N and soil quality benefits of cover crops over a number of years.

3. Increase farmer confidence in, and use of, N mineralization estimates and soil and plant tissue testing methods to determine nitrogen input needs (need for fertilizers, manures, composts) for the summer vegetable crops.

4. Extend project findings to a larger audience.


Click linked name(s) to expand
  • Jeff Boden
  • Jim Bronec
  • David Brown
  • Brian Montecucco
  • Joe Siri
  • Dan Sullivan


Materials and methods:

At the beginning of each growing season, trials were planned at grower meetings and during one-on-one discussion with growers. Growers determined their cash crop rotation and cropping periods according to their normal practices and market demands. On-farm trials were conducted in the same location each year so that the time period available for cover cropping represented actual conditions within a commercial rotation. This design also allowed two years for the effects of cover cropping to increase compared to winter fallow, and forced us to operate within the confines of commercial vegetable rotations (i.e. short cover cropping periods). On most farms, cover crops were seeded after vegetable crop harvest. Seeding dates in fall 2008 were mid-late September, but the following year, three of the four fields could not be seeded to cover crops until mid-late October because of late harvested vegetable crops. This common challenge points to the limited window available for cover cropping in commercial vegetable rotations in Western Oregon. As a result, we initiated preliminary observational trials of relay cover crops seeded into winter squash at Praying Mantis Farm in August 2010. Species included annual ryegrass, cereal rye, oats, subterranean clover, field peas, common vetch, red clover and crimson clover. Each plot was replicated twice; this was an observational trial only. In 2010 we started cover crop trials at the North Willamette Research & Extension Center (NWREC). We discontinued the trials at PMF because he rotated the field out of vegetables to red clover seed production, and at WUG because of crop failure.

On-farm Experiments

Soil types in the on-farm experiments were as follows:

• Montecucco Farms (MF): Canderly silt loam
• Mustard Seed Farms (MSF): Amity silt loam and Concord silt loam
• Praying Mantis Farm (PMF): Aloha silt loam and Dayton silt loam
• Siri & Son Farm (SSF): Cloquato silt loam
• West Union Gardens (WUG): Aloha silt loam and Verboort silty clay loam
• North Willamette Research & Extension Center (NWREC): Willamette silt loam

Main plot cover crop treatments included common vetch (V), cereal rye and common vetch (RV), phacelia and common vetch (PV) and weedy fallow (F). Main cover crop plots in the on-farm experiments were split with zero N (0N) and 100lbs/ac plant-available nitrogen (PAN) treatment (100N) applied as feather meal. 1025 lbs/ac of granulated feather meal (13-0-0) provided an approximately 133 lbs total N and 100 lbs PAN per acre as estimated by the OSU Organic Fertilizer Calculator. Cover crops were seeded after cash crops were harvested and post-harvest tillage was completed. Fertilizer was applied after cover crops were incorporated and plots were laid out for the cash crops. Cash crops were determined by the farms commercial rotations in the 2009 and 2010 growing seasons.

• MF: table beets (2009) and green beans (2010)
• MSF: mixed winter squash (2009) and lettuce (2010)
• PMF: butternut squash (2009) and red clover (2010)
• SSF: lettuce (2009) and lettuce followed by kale (2010)
• WUG: mixed winter squash (2009)
• NWREC: popcorn (2010) and sweet corn (2011)

Detailed research methods are provided in Appendix 1.

Outreach Methods

On-farm trials also served as cover crop demonstration projects. Growers were able to observe the effects of cover crop treatments on subsequent crops. In March 2009 we met with all the farmers collaborating on the project to review findings from 2009, demonstrate the new calculator and discuss plans for the 2010 season. They also learned how to sample cover crops in the field and better understand the relationship between cover crop biomass, N content and release of PAN. Findings from this and related projects were presented at professional meeting and farmer conferences. Findings have also been incorporated into beginning farmer educational material, including OSU’s Growing Farms series.

The cover crop and organic fertilizer mineralization models were combined with enterprise budgeting spreadsheets in the Excel-based OSU Organic Fertilizer and Cover Crop Calculator. Cover crop sampling instructions and instructions for using the calculator were also posted on the website. Peer-reviewed publications reporting on the research and extension methods used to develop the calculator were posted on the site to provide the research background. An eOrganic webinar is also posted on the site in which Andrews and Sullivan describe the research basis for the calculator. When users download the calculator, a pop-up pre-user survey requests their name, contact details and asks some questions about their use of the calculator.

Research results and discussion:

As part of this project, and with supplemental funding from another grant (introduction), we developed the OSU Organic Fertilizer and Cover Crop Calculator (outreach methods and accomplishments). Since 2010 more than 620 people have registered to use the calculator. Over 52,000 acres are managed by people who have registered to use the calculator. If 25% of the registered users save $50/acre/year on reduced fertilizer costs or increased yields, the estimated annual economic impact of the new calculator is more than $650,000. In addition to farmers, agricultural professionals use the calculator. At the end of 2010, 19 agricultural professionals provided feedback on the calculator. The main users were extension faculty and conservation planners. They rated the overall helpfulness of the calculator at 4.4/5. Eight use it in their teaching, 11 in their extension work and 7 in their research. A UC Davis viticulturist said it has a “good overview of organic fertilizers and good estimates of N availability and release”. An NRCS conservationist from Washington uses it to “develop nutrient management plans with organic farmers that prevent water pollution.” An Oregon NRCS conservationist says the “nitrogen estimate is most useful, I love it!” A professor at NCSU uses it in her teaching “Students think this it’s great. I have them do group work conducting exercises in nutrient management and I hear a lot of “wow!” and “this is so cool” coming from the classroom. Quick and easy calculation of costs.” This professor now also uses it in her extension work. A WSU Extension agent reported that it “has a lot of info in one place. Easy to use spreadsheets. A number of the consultants I work with use the calculator.”

Three employees and two graduate students in the OSU Department of Crop and Soil Science were trained to sample cover crops and conduct various aspects of the trial work. One employee decided to pursue a Master’s degree in Horticulture at Oregon State University after working with the metro-area Small Farms Extension program on this and other projects.

On-farm trials:

Figures and tables are provided in Appendix 2. The five main findings from the field trials were:

1. Cover crops generally provided 2-4 times more biomass, 2-7 times more total N and 2-13 times more PAN than fallow plants.

2. Cover crop N was recovered in soil samples or apparent in crop yield when N was not leached below the rhizosphere.

3. 70-day cover crop incubations in the laboratory predicted N mineralized from cover crops in the field.

4. A published N-mineralization equation provides a useful estimate of cover crop N-mineralization for use in the OSU Organic Fertilizer and Cover Crop Calculator.

5. Some differences in water infiltration and soil N-mineralization potential were observed at some sites, but most measurements showed no consistent differences.

The cover crops produced more biomass in 2009 than in 2010 or 2011. In the fall of 2008 growers identified early harvested fields to host the trials. We intentionally stayed in the same fields the following year. Crops were harvested later and cover crop seeding dates were delayed. However, at most farms, cover crop treatments produced two-four times as much dry biomass as fallow treatments each year. An exception was PMF who raises red clover and crimson clover for seed in their rotation, and transitioned from grass seed production to organic vegetable production a few years before the project. In part due to the large number of volunteer plants from these seed crops, fallow plots produced about 70% of the biomass as cover cropped plots, and volunteer clover and weeds represented 50-75% of the biomass in the cover cropped plots (table 3).

Cover crop total N uptake of the different species averaged between from 80-140 lbs/ac in the on-farm trials. Laboratory incubations of the cover crops estimated that on average the different mixtures mineralized 40-70 lbs/acre of plant-available N after 70 days (figure 1).

Rye-vetch cover crops typically produced the highest dry matter (figure 2 for 2009 and figure 16 for 2010) and the lowest weed biomass (table 3 for 2009 and table 9 for 2010). Solo common vetch usually provided the most total N and PAN for subsequent crops (figure 3 for 2009 and figure 17 for 2010). Phacelia vetch cover crops performed well when the phacelia overwintered, but it was easily killed by hard freezes if it was more than about 6” tall going into the winter.

Nitrogen from the cover crop treatments was recovered in soil samples during early and mid-summer at some sites. Background soil nitrate levels during the main period of crop N-uptake in June and July strongly influenced the results we observed. This was measured in unfertilized fallow plots. When background soil nitrate levels were high (i.e. more than 20ppm) during the main period of crop N-uptake in June and July, soil nitrate results showed the differences between high N cover crop plots and low N fallow plots. But little to no differences in yield were visible because the unfertilized fallow plots produced enough N for crop production. In some situations heavy rain and/or irrigation may have leached N below 12” and concealed the differences between plots. When background soil nitrate levels were low (i.e. less than 10 ppm) during the main period of crop N-uptake, the crops rapidly took up available N as it was mineralized from cover crop residues. In these plots, treatment differences did not show up in soil samples but were visible in yield differences.

Nitrogen mineralized during 70-day cover crop laboratory incubations was fairly consistent with results from the field when conditions did not promote N leaching. When background soil nitrate levels were high, laboratory incubations predicted the amount of nitrate found in soil samples with reasonable accuracy (figure 9 for 2009 and 24 for 2010). When background soil nitrate levels were low, cover crop N was more apparent in crop yield (figures 13 and 14). N mineralization from cover crops was also reflected in the total crop N uptake at amounts that were fairly consistently with the predictions from laboratory incubations (figure 25). In this project and a related project that supplemented WSARE resources, a mineralization equation developed by Vigil and Kissel (1991, SSSAJ, 55, 757-761) was validated by laboratory cover crop incubations. The field trials described here demonstrated that the lab incubations and mineralization model provide useful predictions of cover crop PAN contributions.

Soil quality measurements showed some differences between sites and sampling dates. There were some differences in soil N mineralization potential and water infiltration at some site years. Other measurements did not show consistent differences between treatments. This may have been due in part to the short duration of the study and limited cover crop biomass during the second year of the project.

Soil N mineralization potential is expected to increase as additions of fresh active organic matter increase. In 2010 the on-farm trials had been cover cropped for two winters and the NWREC plots had been composted or cover cropped for one winter. Differences between the N mineralized from soil from rye-vetch and fallow plots were not apparent (figure 26), and some incubation samples were saturated, so in 2011 we switched to incubate samples in the laboratory. In 2011 at NWREC, the compost and cover crop treatments showed slightly higher N mineralization potential than fallow plots (figure 37).

Water infiltration is expected to increase as organic matter increases. At MSF infiltration rates were consistently higher in the cover cropped plots than in fallow plots (figure 28). The other sites showed no consistent differences between cover crop treatments and water infiltration rates.

Active soil carbon was measured at four farms in 2010 from fallow plots and rye-vetch plots. There were some differences between sites and between sampling dates, but no consistent differences between these two treatments with a farm were observed.

Bulk density was measured in 2009 (figure 15). Cover crop treatments either did not change bulk density or appeared to increase bulk density. Increased organic matter from cover crops would normally be expected to reduce soil bulk density. The sandy loam soil had a similar or lower bulk density than the two silt loam soils, rather than higher bulk density as would be expected.

In the observational relay-seeding trial at PMF all of the cover crops established successfully. The grower was unusually delayed in planting the winter squash crop, resulting in a smaller crop canopy when we relay-seeded. The cover crops did not face as much competition for light, water and nutrients as would be expected most years. The field peas and subterranean clover formed thin stands. The annual ryegrass, cereal rye, oats, common vetch, red clover and crimson clover were all vigorous. The oats and annual ryegrass competed with the crop and made the squash difficult to find at harvest.

The following discussion gives more detail about the cover crop treatments and their effects on N. Some laboratory cover crop incubation data from a closely related project is also included here.

A more detailed discussion of trial results from each year is provided in Appendix 3.

Participation Summary

Educational & Outreach Activities

Participation Summary

Education/outreach description:

The OSU Organic Fertilizer and Cover Crop Calculator website is online at:

A draft OSU Extension publication has been accepted for publication by OSU Extension and Experiment Station Communications and will be jointly published as a Pacific Northwest Extension Publication by WSU and U of I:

Sullivan, DM and Andrews ND (in press). Estimating plant-available nitrogen release from cover crops.

Andrews and Sullivan have presented findings from this project and related grants at five professional meetings:

1. Andrews, N., D.M. Sullivan, J.W. Julian and K.E. Pool (March 3-4, 2011). Development and Use of the OSU Organic Fertilizer and Cover Crop Calculator. Proceedings of the 2011 Western Nutrient Management Conference, Reno, NV. 30 participants.

2. Sullivan, D.M., R. Datta, N. Andrews and K. E. Pool (March 3-4, 2011) Predicting Plant-available Nitrogen Release from Cover Crop Residues. Proceedings of the 2011 Western Nutrient Management Conference, Reno, NV. 40 participants.

3. Andrews, N. and D.M. Sullivan (October 31-November 3, 2010). In Long Beach, CA. Estimating Cover Crop Nitrogen Contributions and Management Costs with a New Online Calculator. Soil Science Society of America Conference. Oral paper. About 40 participants.

4. Sullivan D.M. and N. Andrews, J. Luna, A. Garrett, R. Datta, and K. Pool (October 31-November 3, 2010). In Long Beach, CA. Monitoring Crop Nitrogen Status in Organic Vegetable Cropping Systems. Soil Science Society of America Conference. Oral paper. About 50 participants.

5. Sullivan, D.M., N. Andrews, J. Luna and J. McQueen (August 1-6, 2010). In Brisbane, Australia. Estimating N contribution from organic fertilizers and cover crop residues using online Calculators. 19th World Congress of Soil Science.

Aspects of this work have been described in two Oregon Small Farm News articles:

1. Andrews, N. (2010). Establishing Winter Cover Crops. Oregon Small Farm News. 5(4), pp 17-20. Online at:

2. Andrews, N. (2010). OSU Cover Crop Calculator to be Launched in Early April. Oregon Small Farm News. 5(2), pg 12. Online at:

Findings from this project and the calculator were introduced at an eOrganic webinar and have been incorporated into beginning farmer educational materials at OSU. This work has also been presented at thirteen Extension or farmer conferences:

1. eXtension webinar: Estimating Plant-Available Nitrogen from Cover Crops (April 13, 2010). Broadcast from OSU in Corvallis. eOrganic webinar. 35 participants. Online at:

2. Growing Farms: Successful Whole Farm Management content for beginning farmers: Grow It! soil fertility module (February 24, 2010 and February 9, 2011). 65 participants to date.

3. Extension presentations: This work has been presented at 13 extension events to an audience of over 700 people. In addition to being presented by Andrews and Sullivan, David Brown (MSF) and Jim Bronec (PMF) have presented information from a grower perspective. Extension presentations:

a. Organicology: Soils Intensive (February 10, 2011). “Within Season Nitrogen Management on Organic Vegetable Farms”, and “Nutrient management tools” (Andrews and Doug Collins from WSU). About 60 participants.

b. Estimating Cover Crop Nitrogen Contributions & Using the OSU Organic Fertilizer &Cover Crop Calculator (January 6, 2011). Annual Organic Grower’s Meeting, GS Long. Yakima, WA. About 70 participants.

c. Nitrogen Management and Cover Crops in Organic Vegetables: OSU Organic Fertilizer and Cover Crop Calculator (November 13, 2010). Port Townsend, WA. Washington Tilth Producers Conference. 80 participants.

d. OSU Organic Fertilizer and Cover Crop Calculator (October 5, 2010). At NWREC. Shade Tree Grower’s Meeting. North Willamette Research & Extension Center. 18 participants.

e. OSU Organic Fertilizer and Cover Crop Calculator (September 7, 2010). At NWREC. Nursery Field Day. 40 participants.

f. OSU Organic Fertilizer and Cover Crop Calculator: A New Tool for Estimating Cover Crop Costs and Benefits (August 19, 2010). Philomath, OR. Presenter at Benton County SWCD Soil Quality Workshop. 50 participants.

g. Estimating N From Cover Crops and Choosing Organic Fertilizers (July 30, 2010). At OSU in Corvallis. Master Gardener Mini-College. 60 participants.

h. Soil Fertility and Cover Crops (June 10, 2010). Near Salem, OR. Willamette University Student Farm. 8 participants.

i. Nitrogen Management and the OSU Cover Crop Calculator (April 13, 2010). At OSU in Corvallis. Department of Horticulture Seminar. 10 participants.

j. Nitrogen Management on Organic Farms (April 6, 2010). In Albany, OR. Natural Resource Conservation Service, Animal Waste Management Workshop. 40 participants.

k. Cover Crop Research and Fertilizer Calculators (March 6, 2010). Kamloops, BC. Certified Organic Associations of British Columbia annual conference. 50 participants. Presentation posted online.

l. Using Cover Crops and the OSU Cover Crop Calculator (February 27, 2010). At OSU in Corvallis. Organized session and delivered presentation at OSU Small Farms Conference, session repeated twice. 170 total participants.

m. OSU Cover Crop Calculator (January 12, 2010). In Canby, OR. Presentation at Organic Crops Section of the North Willamette Horticulture Society Meeting. 165 participants.

Project Outcomes

Project outcomes:


This project has led to new partnerships with the Natural Resource Conservation Service and Soil and Water Conservation Districts in Oregon, as well as strengthening partnerships with Oregon Tilth, WSU Extension and farmers in the area. West Multnomah SWCD has partnered with Andrews to work with growers in their district to optimize N fertilizer use by increasing adoption of cover crops and crediting non-fertilizer N, including cover crop N and use of the Organic Fertilizer and Cover Crop Calculator. As a result of the research and outreach activities of this project, NRCS and some Soil and Water Conservation Districts have adopted the Calculator in their nutrient management programming. When growers develop nutrient management programs under the new Organic EQIP program, they are instructed to use the Organic Fertilizer and Cover Crop Calculator. A new partnership with Washington State University is also developing in part from this project. Andrews and Doug Collins (WSU Extension) collaborated on development of a one-day Soils Intensive workshop at Organicology (sponsored by Oregon Tilth) in 2011. We also strengthened partnerships with the collaborating growers. MSF and PMF and other farmers are collaborating with us on a new Western SARE funded R & E Project to develop an online degree-day modeling system for vegetable growers (SW12-037).

New Educational and Decision Making Tool:

As part of this work we developed an online calculator and website (OSU Organic Fertilizer and Cover Crop Calculator) to help growers and agricultural professionals predict N contributions from cover crops The calculator estimates N mineralization rates from fertilizers, compost and cover crops. It also allows users to determine the most appropriate fertilizer program for other macro and micro-nutrients based on nutrient content and cost. The calculator includes integrated spreadsheets that provide the PAN estimates for cover crops and organic fertilizers, help growers match their crop nutrient requirements with fertilizer rates, determine the cost of various fertilizer programs and cover crops. (Cover crop sampling methods were identified during a previous Western SARE-funded project (FW06-301: Estimating Nitrogen Contribution from Cover Crops on Organic Vegetable and Cane Berry Farms). Detailed sampling instructions and a Quick Guide are included on the website. Recitation exercises and an answer sheet have been developed for use in undergraduate training. These exercises have been used with one Crop Production class at Oregon State University; in this class, about 60 students were taught to use the calculator.


Potential Contributions

This project and the Organic Fertilizer and Cover Crop Calculator have improved grower and agricultural professional understanding of N mineralization from cover crops, and the costs and benefits associated with cover cropping. Farmers now use the calculator to develop cost effective cover cropping and supplemental fertilizer programs. Agricultural professionals in Oregon and Washington have been introduced to the Organic Fertilizer and Cover Crop Calculator and the research behind the mineralization predictions. They can use this tool with their clients to quantify cover crop PAN contributions and refine fertilizer programs to include cover crop PAN.

This project may increase adoption of cover crops and, therefore improve nutrient management on farms, reduce erosion and protect water quality. In our work, legume cover crops proved to be one of the most cost-effective sources of N for organic growers and can sometimes be a cost competitive N source for conventional growers. This understanding may increase adoption of cover crops on farms and allow growers and the public to realize some of the other benefits of cover cropping. Fertilizer programs based on manure and compost may supply excess phosphorus in order to meet crop N demand. Legume cover crops are an important source of N that does not increase soil phosphorus levels.

Future Recommendations

The Organic Fertilizer and Cover Crop Calculator has generated interest among agricultural professionals and farmers outside of Western Oregon. The mineralization models were developed in the context of annual vegetable rotations in our climate. There is a need to validate mineralization predictions and develop decision tools for farmers and agricultural professionals in other climates.

In commercial vegetable rotations, crops are often harvested too late in the fall to establish over-wintering cover crops. Research and extension work is needed to help growers maintain and improve soil quality under these conditions. Practices such as relay-seeded cover crops and fall application of organic amendments with relatively low N content may be helpful.

Cover crops over two years did not appear to influence the soil quality attributes we measured. Future work to evaluate the influence of organic amendments on soil quality may need to include higher application rates of organic amendments (i.e. cover crops and compost) and/or be continued over a longer time frame.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.