- Author: Yoni Cooperman
- Contributor: Jordon Wade
A variety of cover crops exist, way too many to be fully covered in this blog post. Generally speaking, cover crops tend to be grasses or legumes, and many growers utilize mixes to achieve targeted results. Legumes can be a source of N fertilization, though they can also contribute to N pollution if N levels exceed crop needs. Grasses have the potential to hold on to excess soil N and limit losses through nitrate leaching. Mixes of multiple cover crop types with different uses are used to maximize inputs of organic matter in hopes of building soil carbon.
While cover crops can have many potential benefits, like any other management decision tradeoffs are involved. While competition for soil water and nutrients can be used to control vigor, under certain conditions this can be harmful for vine development. Another possible downside to using cover crops, their potential to increase the production of greenhouse gas emissions, was the focus of our study conducted in a three year old Merlot vineyard in Lodi, CA. The vineyard soil is classified as a Devries sandy loam.
In our two year study, we compared rates of greenhouse gas (GHG) emissions from vineyard alleyway soil grown under three different cover crop mixes: a legume mix, a “soil builder” mix, and a ryegrass treatment all planted at 100 lbs/ac.
These three treatments were chosen to represent three reasons growers might utilize cover crops in a vineyard. The legume mix was chosen to be a “green manure” and increase soil nitrogen. The “soil builder” mix was meant to maximize plant biomass and increase soil organic matter. The ryegrass was chosen as a “catch crop” that can take up large amounts of soil N, limiting N losses through nitrate leaching.
After our two year monitoring period, we found that cover crops had little effect on soil N2O emissions, while they increased soil CO2 emissions. While CO2 emissions were higher when cover crops were used, there were no differences between the different cover crop types. These findings suggest that during drought years, growers are free to choose the cover crop mixes they think will best serve their needs, without being overly concerned about excess N2O emissions stimulated by cover cropping. However, the legume mix did result in higher levels of soil N and the ryegrass treatment did decrease leachable soil nitrate. It is unclear if the "soil builder" mix resulted in increased soil organic matter, although that is to be expected, considering it takes several years to substantially increase soil organic matter content.
For more information about utilizing cover crops visit the Solutions Center for Nutrient Management page on cover crops.
- Author: Sara Tiffany
- Author: Sara Tiffany
- Author: Dr. Martin Burger
Soil nitrate sampling protocol
For maximum accuracy that can reliably predict nitrate availability in the soil, growers should sample according to the following protocol:
- For fields with 60-inch beds: soil cores should be taken at 3 lateral distances from drip tape, in at least 4 locations within a field.
- For fields with 80-inch beds: soil cores should be taken 2 lateral distances from drip tape, in at least 3 locations within a field.
click here to read full summary (or scroll down)
Research on agricultural greenhouse gas emissions in tomatoes
- The adoption of subsurface drip irrigation substantially reduces greenhouse gas emissions in tomato production (compared to furrow irrigation).
- Use of nitrification inhibitors lowers nitrous oxide emissions in tomato fields with subsurface drip irrigation.
click here to read full summary (or scroll down)
UC Davis researcher Dr. Martin Burger presented the results of a survey conducted by post-doctoral scholar Cristina Lazcano on pre-plant nitrate, phosphorus (Olson-P), and exchangeable potassium levels in 16 processing tomato fields in Yolo, San Joaquin and Fresno counties. The purpose of the study was to develop an economical sampling protocol that reliably predicts nitrate availability and allows growers to adjust fertilizer rates taking the residual soil nitrate into account.
While the conversion to subsurface drip irrigation has enabled growers to precisely deliver water and nutrients close to plant roots, there is still pressure for growers to increase nitrogen use efficiency, for example to reduce the risk of nitrate leaching. Previously, the spatial distribution of macronutrients in fields under drip irrigation was not well known. One concern has been that nitrate may accumulate at the periphery of the wetted soil volume, whereas the less mobile nutrients phosphorus and potassium may be depleted near the drip tape where roots can be expected to proliferate.
According to the survey encompassing more than 1000 soil analyses, pre-plant nitrate levels in the 16 fields varied widely, ranging from 45 – 438 lbs NO3- - N per acre in the top 20 inches of soil, with higher levels of nitrate found in fields under consecutive tomato cultivation. No depletion of Olsen-P or potassium in the root feeding areas close to the drip tape was detected. The majority of the fields showed phosphorus concentrations lower than 15 ppm, which based on earlier research is the threshold below which a yield response can be expected from a P addition. In contrast, potassium levels were higher than previously reported values, ranging from 293 ppm on average in Yolo County to 468 ppm in Fresno County.
The nitrate sampling protocol was based on a Minimax analysis by selecting the minimum number of samples within the field and locations within the beds (i.e. lateral distance from the drip tape). The combination of samples with the lowest relative error across all fields (< 5% from the field average) and the lowest number of samples taken was selected as the best sampling procedure to estimate average soil NO3-N. The analysis showed that soil cores should be taken at three (60-inch beds) or two (80-inch beds) lateral distances in at least four (60-inch beds) or three (80-inch beds) locations within a field.
Table 1. Pre-plant nitrate sampling protocol for 60-inch beds in Yolo (Y), San Joaquin (SJ), and Fresno (F) County SDI tomato fields.
Table 2. Pre-plant nitrate sampling protocol for 80-inch beds in Yolo (Y), San Joaquin (SJ), and Fresno (F) County SDI tomato fields.
***The full article about this study will appear in the Oct-Nov-Dec 2015 issue of California Agriculture.
An update on agricultural greenhouse gas emissions research included results of field studies testing a nitrification inhibitor for mitigation of nitrous oxide in subsurface drip irrigated tomato.
Nitrous oxide (N2O) is arguably the most important greenhouse gas produced in the agriculture sector, with its global warming potential 300 times that of Carbon Dioxide. N2O is produced by soil microbes during N transformations. N2O is a by-product of nitrification and denitrification.
Recent studies have shown that N2O produced during nitrification can be as important as that resulting from denitrification (Zhu et al., 2013). The highest rates of N2O emissions typically occur shortly after N fertilizer applications when soils are re-wet. The main regulatory factor is the availability of oxygen since microbes use nitrate (denitrification) and nitrite (nitrification) as electron acceptors of respiration when oxygen is in short supply. Soil processes that consume oxygen, such as the presence of a carbon source, and conditions that limit replenishment of oxygen levels in the soil, such as high soil water content, promote N2O production in soil. Compacted soils lead to rapid depletion of oxygen because of the reduced air spaces and greater tortuosity of pathways of oxygen diffusion.
Although the use of the nitrification inhibitor significantly lowered nitrous oxide emissions in SDI tomato in one of the two years of the study, the reduction in absolute values is rather small (64 lbs carbon dioxide per acre) to make a significant contribution to California's greenhouse gas inventory. With the adoption of subsurface drip irrigation, tomato growers have already lowered the impact of greenhouse gas emissions from tomato production substantially as furrow irrigation generated leads to greater nitrous oxide emissions than SDI.
Zhu, X., Burger, M., Doane, T.A., Horwath, W.R., 2013. Ammonia oxidation pathways and nitrifier denitrification are significant sources of N2O and NO under low oxygen availability. Proceedings of the National Academy of Sciences of the United States of America 110, 6328-6333.
- Author: Ryan Murphy
One of the most useful features of The Solution Center for Nutrient Management is the Nutrient Management Research Database, which holds concise summaries of California specific agricultural nutrient management research. The entries can be searched using the interactive map feature (see pic below) which allows the user to select a county they are interested in and see the related summaries of research conducted in that county.
The database can also be searched by keyword, soil type, crop type, management practice, environmental consideration or irrigation system, making it easy to find the California nutrient management research that you are most interested in!
Click here to test it out and let us know what you think in the comments section below. We are always on the lookout for new research that is relevant to the California agricultural community, and new ideas to improve the database and website.
- Author: Ryan Murphy
2015 promises to be a dynamic and challenging year for California agriculture. While we've had some rain, the drought is still at the front of everyone's minds, and new nutrient management regulations are becoming a reality for many. To succeed in these conditions, California growers will have to continue to grow the crops that feed California and the world with less water and a careful attention to nutrient applications, all in a climate that seems to serve up one curve ball after another.
A small army of scientists, farm advisors, extension specialists and others are working hard on nutrient management research. California's farmers are also experimenting with new technologies, finding their own innovative solutions to nutrient management issues. There is a lot of great research out there that can help farmers grow crops sustainably and many examples of growers using best practices to conserve water and nutrients, and decrease the impact of agriculture on the climate. However, there is also a need to sift through the mountains of available information to highlight the resources that are most relevant to California agriculture, and to help strengthen the link between the work researchers do and the challenges agricultural practitioners face in their fields.
That's where the Solution Center for Nutrient Management comes in. The following explains what the Solution Center is, what this blog will do and how you can get involved:
What is the Solution Center for Nutrient Management?
The Solution Center for Nutrient Management (SCNM) was created by UC SAREP to increase access to California specific agricultural nutrient management resources and serve as a platform for conversation on important nutrient management issues. The broad goal for the SCNM is to provide resources and knowledge sharing tools that support the CA agricultural community in minimizing adverse environmental impacts of agriculture while maximizing efficiency and economic gain.
To do this, the SCNM is developing the following approaches:
- An online information portal with case studies on innovators in agricultural nutrient management, a database summarizing California nutrient management research, and content on nutrient management principles, best practices and regulations.
- Online discussion forums to focus conversations on key nutrient management issues, in partnership with FarmsReach and Sustainable Conservation.
- In person field days bringing current research on nutrient management and greenhouse gas emissions to the California agricultural community and providing the opportunity for feedback from practitioners. Click here for an events schedule
Check out the solution center website here, and stay tuned for frequent updates!
What can you expect to find on this blog?
This blog will get the word out about updates to the Solution Center website, outreach and education events we are working on, and new California nutrient management research. It's also one place to share your ideas and comments on the content we develop and the nutrient management issues that you're thinking about.
How can you get involved?
- Comment on this blog with suggestions for nutrient information issues you would like to know more about.
- Join our mailing list here or check back frequently to hear about new features of the Solution Center Website, upcoming online forums and field days.
- Send us an email with your thoughts and suggestions!