Mustard Green Manures for Potato Production

2005 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 of 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.

Increasing the Producer's Knowledge Base:
This project will increase the knowledge base of potato growers by providing them with information on 1) the potential for mustard green manures to replace the fumigant, metam sodium, in diverse potato cropping systems, 2) management of mustard green manures for optimum results, 3) the best species/varieties to use in Washington. This outcome will be measured by the production of research summaries containing the results from +/- fumigant trials, wheat straw management trials, and green manure variety trials.

Dissemination of Information:
The target audience is potato growers and their crop consultants. Targeted meetings will be the Washington Potato Conference, the Columbia Basin Crop Consultants Association, and the Pacific Northwest Vegetable Association. Targeted media outlets include Potato Country, Potato Progress, Agri-Focus Newsletter, Columbia Basin Farmer, and the Capital Press. The actions leading to this outcome are detailed under Objective 4. The outcome will be measured by keeping records of website users/hits, number of articles published, and attendance at presentations and the field day.

Number of Acres Impacted:
We aim to 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, or over 15% of Washington's potato acreage. We will continue to estimate the mustard acreage from the seed sales of the three seed companies providing mustard green manure seed in Washington.

Actual Positive Economic Impact:
If 28,000 acres of mustard green manures are used to replace the metam sodium fumigant treatment, then potato farmers would save from $1.4 – 3.0 million dollars annually, depending on the production costs of the mustard crop and the amount of nitrogen cycled to the following potato crop. This replacement of fumigant will happen more slowly than the adoption of the mustard green manures, most likely after the project is over. Nonetheless, 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 sampling and analysis: Soil samples were taken on March 3, before spring tillage, from replicated plots in three fields characterized by different soil textures (82, 49, and 31% sand) where the treatments were mustard and no mustard in 2004. These samples were analyzed for fluorescein diacetate (FDA) hydrolysis, -glucosidase, both estimates of microbial activity (FDA has been correlated with disease suppression), active C, total C and % sand, silt, and clay. FDA, b-glucosidase, and active C were significantly higher in the mustard plots in the silt loam, but no significant differences were found in the loamy sand or sandy loam. All three measurements were correlated to total soil C, with correlations following this pattern: FDA>active C>b-glucosidase. b-glucosidase and active C were highly correlated to each other. We were able to observe the following potato crop in only one of these fields, that with the loamy sand soil, but this did not yield any valuable information (see below)

Additional soil samples were taken from 9 fields, at four locations in each field, before mustard green manure crops were planted. These samples will be analyzed with another set taken in March, 2006, for various potential indicators of pest suppression: FDA, -glucosidase, active C, and possibly others. We will then see if these measurement correlate with disease suppression at these sites.

Of the five fields where soil samples were collected in 2004 and potatoes planted in 2005, we were able to measure potato yields on just three fields, comparing mustard with the fumigant metam sodium. Of the two missing fields, one did not receive metam sodium and the other had unrelated problems that severely affected production. There were no significant treatment differences on the remaining three potato fields when considered as individual replicates. This lack of differences did not allow us to correlate previous soil measurements with the success or failure of mustard as a replacement for metam sodium. We are investigating the use of bean root rot ratings as a better way of assessing disease suppression in soils receiving mustard green manures.

Objective 2

We conducted an early planted (July 12) variety trial of 9 entries (replicated, mustard and other species) and 5 unreplicated plots of high and low dhurrrin sorghums to compare with the mustards. Buckwheat and proso millet were found to be much more competitive and productive than mustard, or sorghum, when planted in July and grown into September at this location. However, several grain sorghums hold potential for the planting window.

A later planted variety trial (Aug. 12) contained 11 varieties of mustard, both B. jucea and S. alba, arrugula, broccoli, oilseed radish, Typhon (turnip x rape cross), and winter and spring canola varieties. Some varieties from the early trial were also planted in this later trial but did not produce well.
Most of the mustard plots lodged during a heavy rain and were not harvestable. Plots of the remaining entries were harvested for yield estimates, but no significant differences were found. Yields from Brassica species in the later planted trials and those from buckwheat and proso millet of the early planted trial were similar. Data are posted at

Glucosinolate levels from last year’s trial are also posted here. We opted to not analyze glucosinolate levels from this year’s trial as it would not have yielded further useful information.

In trying to determine what significant differences might exist between cover crop species that produce differences in disease suppression in the field (Neeno-Eckwall et al, 2001) we measured whole plant brix levels in various variety trial entries. We found some differences, with differences generally arranged: grasses>Brassicas>buckwheat. The Brassica entries did not differ from each other. Data can be found at the website mentioned above.

Objective 3

The annual mustard green manure field day was not held this year, but will be replaced with a workshop on Feb. 2, 2006. This will allow for more in-depth learning about this practice and the related topics of soil quality and biology. Six other presentations were given in 2005 on mustard green manure management.

A website developed to disseminate information on mustard green manures received over 27,609 hits from January through October, 2005. This is an 89% increase over 2004. Mustard green manure publications were downloaded about 3745 times in the same period.

Impacts and Contributions/Outcomes

The use of mustard green manures increased from 1,800 acres in 1999 to 20,630 acres in 2004, but then decreased to an estimated 17,000 acres in 2005. A majority of these acres will be planted to potatoes in 2005. The decrease coincides with large increases in the cost of fuel and fertilizer needed to implement this practice. Where fumigant is not being replaced by the mustard, it may be looked at as too costly, especially because it is difficult to attach an economic value to the practice’s non-pest supression benefits. Although I know of at least five large farming operations that are not using the fumigant metam sodium on their field receiving mustard green manures, it is difficult to estimate the acres on which this is occurring. If farmers replaced the fumigant metam sodium on all of these acres, farmers would save over $1.1 million (~$66 per acre) while improving the quality of their soil. The growers using green manures on these acres are beginning to see improvements in their soil quality and their ability to manage soil-borne pests, but it remains to be seen if the value of these benefits will outweigh high input costs.