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

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.

• Appendix 2: Figures and Tables
• Appendix 3: Outcomes