Mustard Green Manures for Potato Production

2006 Annual Report for SW03-018

Project Type: Research and Education
Funds awarded in 2003: $45,653.00
Projected End Date: 12/31/2007
Matching Non-Federal Funds: $18,324.00
Region: Western
State: Washington
Principal Investigator:
Andrew McGuire
Washington State University Extension

Mustard Green Manures for Potato Production



Washington potato growers, although producing record yields, are experiencing reduced profits due to decreasing market prices and increasing production costs. A major part of production costs for most growers is fumigation. They also have to spend more on inputs due to the decreasing soil quality.

Washington growers are increasingly interested in growing mustard as a green manure preceding potatoes to help control of nematodes, weeds, and soil-borne diseases. This practice has the potential to replace some soil fumigation and to improve soil quality. We will conduct on-farm research aimed at reducing the risk to farmers using mustards to replace fumigation.

Objectives/Performance Targets


Objective 1: Determine the measurable factors related to suppression or non-suppression of potato early dying in processing potatoes.

Objective 2: Determine the species and variety of mustard, or other Brassica, that will most benefit Washington potato growers.

Objective 3: Increase the adoption of mustard green manures by potato growers through an integrated program of on-farm research and education.

Performance Targets

Increasing the Producer’s Knowledge Base: production of research summaries containing the results from +/- fumigant trials, wheat straw management trials, and green manure variety trials
Dissemination of Information: records of website users/hits, number of articles published, and attendance at presentations and the field day.
Number of Acres Impacted: increase acres planted to mustard green manures in Central Washington by 25% every year for the next five years (three covered by this proposal), from 9,260 acres in 2001 to 28,000 acres being planted the fall of 2006.
Actual Positive Economic Impact: if we feel that a significant number of potato growers are replacing their fumigant with mustard green manures, we will conduct a survey to estimate this impact.


Objective 1: Soil samples were taken from 9 fields, at four locations in each field, before mustard green manure crops were planted in 2005 and again, from the same locations in March 2006. These samples are being analyzed for various potential indicators of pest suppression: FDA, b-glucosidase, active C, SOM, soil texture, and pH. Some but not all of this analysis has been done. The delay is due to the departure of the soil technician who was originally going to do the work. We have had difficulty finding someone else to do the work and even when we did, they have not been able to do the work quickly. Because of this delay, we have not had any additional information on which to base further soil sampling and analysis and so have chosen to not do anymore until these data are available for analysis.

Objective 2: We conducted an early variety trial (July 12 planting) with 10 entries including sudangrasses, teff, mustard, arrugula, and turnip. A heavy downpour just after planting decreased or eliminated the broadleaf’s emergence, and those plots were abandoned. The grass entries, however, did much better, and biomass samples were taken on Sept. 8, analyzed, and the results posted on the green manure website. The varieties of sudangrass that were replicated (we had limited seed for some entries as they are experimental) yielded significantly more biomass than the teff. However, we are having the teff tested for its host status of the root-knot nematodes, which affect potatoes. If it is a non-host it may be a good green manure crop or an even better rotation crop (hay) for potatoes.

We also conducted a late variety trial (August 12 planting) with 18 entries of two species of mustards, mustard blends, arrugula, arrugula/mustard blends and oilseed radish, all with 4+ replicates. The arrugula did not emerge well in most of the plots, or in the surrounding field, and so it was not harvested. The mustard, however, did well, and 4 replications of each entry were harvested on Oct. 18th. The data were posted on our website, They showed that in a year with good growing conditions, many of the mustard varieties tested can produce high amounts of biomass. In agreement with past trials, the variety that bloomed first, Ida Gold, produced less biomass, although all the differences were not statistically significant. 3.5 tons/ac of dry biomass seems to be a target that growers should aim for with August planted mustard green manures.

Objective 3

A mustard green manure workshop was held on Feb. 2, 2006, in Moses Lake, Washington. It attracted 29 people and covered the basics of mustard green manure management, potential pest suppression, soil quality improvements, and potential risks.

On April 5th, a session on Potential Mechanisms of Organic Matter Mediated Pest Suppression was held at the National IPM symposium in St. Louis. I organized this session, and although it did not address mustard green manures directly, research on the mechanisms occurring with other organic soil amendments could be similar to those with the mustard. Interestingly, one researcher has found evidence that the glucosinolates in Brassicas, the biofumigation effect, may not be a major mechanism. We had been thinking similarly, which is why we had discontinued the expensive glucosinolate analysis of mustards in our variety trials.

In late June, I attended the 2nd International Biofumigation conference in Moscow, Idaho. There I facilitated a discussion among the presenters about the state of our knowledge and where we should go in the future. One important theme that evolved from this session is the need for a set of basic information and treatments in every biofumigant related experiment. This, probably more than anything else, would help move the science forward in this area. The details can be found in the well-written review, Matthiessen and Kirkegaard, 2006, Biofumigation and Enhanced Biodegradation: Opportunity and Challenge in Soilborne Pest and Disease Management, Critical Reviews in Plant Sciences, 25:235–265. If the many Pacific Northwest researchers attending this meeting adopt this paper’s recommendations, it could have significant impact on the use of green manures before potatoes. This meeting also highlighted that the questions that we have about these practices will not be answered soon, but only after years of continued carefully designed experiments.

Two mustard green manure field days were held in October 2006; one each in the Northern and Southern Columbia Basin. Total attendance was about 57, and many of them were first-timers at these field days.

A website developed to disseminate information on mustard green manures received over 31,290 hits from January through November, 2006. This is a 5% increase over 2005. Mustard green manure publications were downloaded 2,993 times in the same period, a reduced number from 2005. This reflects either the cooling interest among farmers from past years (those that want to know about this may have already educated themselves) and/or the remaining number of farmers interested in practice but not yet using it is decreasing.

We are now working with a WSU economist and a cooperating farmer to do an economic analysis of mustard green manures within a potato rotation. When this is finished (early 2007) it will be posted online and allow farmers to do their own analysis using their data.

Impacts and Contributions/Outcomes

The challenges of leased land, crop rotation, and lack of time for management of the mustard in the fall limit the number of acres of potatoes that could possibly be preceded by mustard green manures. However, among those farmers that can grow the mustard, we have made mustard green manures an accepted practice to consider.

We have extensively tested mustard varieties, both those commercially available and experimental, and have identified those that work well in this system. Commercial seed companies have used this data to select the varieties that they sell and also the price at which they are sold. Mustard from the Columbia Basin of Washington is now being sold for green manure use throughout the West and even in states in the Midwest and Eastern U.S.

Acreage of mustard green manures in the Columbia Basin for 2006 is estimated at 24,400. The acreage seems to have reached a plateau (annual average since 2002 is about 21,000 acres) and probably will not increase unless the risk of using these green manure crops as a replacement for metam sodium fumigant can be decreased, the original goal of this project. Why this project could not achieve this goal and possible ways to accomplish this in the future will be discussed in the final report.

A WSU nematologist is now working extensively with green manures, including mustards and arrugula, to suppress pest nematodes in conjunction with nematicides. This probably would not have occurred without this ongoing mustard project and without the documentation of a significant number of farmers using mustard green manures, as this work was opposed by some Washington State Potato Commission board members.

The economic impact of the use of mustard green manures is very difficult to estimate. Only a minority of potato farmers are replacing their fumigant use with mustard green manures, a fact that will only change with advances in the science behind suppression by green manures of Verticillium and the associated potato early dying. Until then, the economic benefits are related to soil quality improvement and rotation effects, which are difficult to determine and vary widely by farm and location. If farmers currently using mustard green manures were all using the practice to replace their normal metam sodium treatment, they would save an estimated $1 million annually, or $41 per acre. This does not include the value of soil quality improvements and erosion control.