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

• 2010 annual report

Commodities

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.

Introduction

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.

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.