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Several cover crops were selected by organic growers in the North Willamette Valley of Oregon for use in this study. Four on-farm methods for estimating nitrogen contributions from cover crops were compared with a standard laboratory method. Regression analysis was used to compare estimates of total nitrogen contribution from each on-farm method with estimates from the standard lab method. First-year results showed that lab analysis of bulked species samples gave comparable estimates of total nitrogen and plant-available nitrogen contributions to the standard lab method when the same fresh weight ratios of cover crop species were tested and weeds were omitted. Second-year results were very similar when grab samples of the cover crop mixture including weeds were tested. The published fresh weight method was inconsistent with the standard lab method, but we were able to adjust the method with 2007 data and found a reasonable fit between the adjusted fresh weight method and the laboratory method in 2008. Estimates based on dry weight of the cover crop gave moderate to strong correlations with estimates using the standard lab method. Again, data from 2007 was used to develop this new method and the correlation to the laboratory method was strong with the 2008 data. Measurements of canopy height and density provided little to no correlation when compared to results from the standard laboratory method.
Based on the results of this study we advise growers to submit bulked cover crop samples to the laboratory to estimate total nitrogen contributions. If a laboratory is unavailable, species could be separated, and the dry weight of each species can provide a reasonable estimate of nitrogen contributions. If drying is not feasible the fresh weight of separated species can be used to provide what may be a satisfactory estimate of nitrogen contribution. We do not recommend using canopy height and density to estimate nitrogen contribution.
Organic and ecological farmers aim to improve soil quality and fertility. Certified organic farmers are required by the USDA National Organic Standards to implement a crop rotation that “maintain(s) or improve(s) soil organic matter content” and “manage(s) deficient or excess plant nutrients.” They must “maintain or improve the physical, chemical, and biological condition of soil and minimize soil erosion.” Cover cropping is listed as a central method to reach these objectives (§205.203 and 205.205). These rules are consistent with the values of many non-certified farmers who use ecological methods, and with the values of their customers.
Nitrogen management in organic or ecological systems is complicated by difficulties involved in measuring various nitrogen sources. These include soil mineralization, slow release of N from organic amendments and N contributions of cover crops. Various methods have been proposed in the extension literature for estimating total N contributions from cover crops. A preferred accurate method is to cut a known area of the above-ground cover crop, separate and weigh individual species, then dry and re-weigh them and submit them to a laboratory for C/N analysis. This method is thought to be reasonably accurate, but is not widely used by farmers in the Pacific Northwest. Sattell and Dick (1998) describe a method that estimates total N content on a per acre basis using the fresh weight of individual species from a 16ft2 quadrat. Sarrantonio (1994) described a method that estimates N content from the dry weight of cover crops. Most of the farmers we work with use seed mixtures of grasses and legumes. The methods listed above require that growers harvest the cover crop and separate the individual species. Collaborating growers and others have explained that they don’t have time to separate cover crop samples especially in the spring. Two additional methods were included that do not require separating cover crop species. First, an unseparated sample of the cover crops from the quadrat can be submitted to the lab for C/N testing. Second, the canopy height and canopy density method can be used to generate estimates (Sarrantonio 1994). This does not require separating species, or submitting samples to a lab. To our knowledge, none of these methods have been compared.
The main objective of this project was to compare on-farm measurement techniques for estimating N contributions from cover crops using 1) separated species lab tests (a standard experimental method), 2) bulked species lab tests, 3) fresh weight, 4) dry weight and 5) canopy height and density. We also recorded relative weed biomass in different cover treatments. After the project started, collaborating growers said they were interested in predicting available N from cover crops. We used a mineralization model to compare the ability to predict plant-available nitrogen (PAN) from bulked species and separated species sampling methods.