Mustard Green Manures for Potato Production

Objective 1.

In 2003, an on-farm trial was organized where fumigant was applied through the center-pivot irrigation system on only parts (wedges) of the field. Previously, such research had been done using ground application equipment, which made randomization and replication relatively easy. With fumigant application through the irrigation system, both randomization and replication were much more difficult, and impossible in some cases. After the 2004 yields were measured, this type of testing was abandoned as almost all farmers were applying the fumigant through their irrigation water and the one farmer who was using ground application equipment replaced his fumigant use completely with mustard green manuring.

The focus was then shifted to analyzing the changes in the soil resulting from mustard green manures. In 2004, soil samples were taken in May from 12 locations representing three conditions: fields receiving mustard green manures, neighbor fields not receiving green manures, neighboring native soils. These soils were tested for Zn, Mn, CEC, pH, organic matter, soil texture, and fluoride levels. These results were analyzed for trends and differences which led to a second round of soil sampling and testing in late summer. We took samples from five fields that were being planted to mustard green manures before 2005 potatoes, where the farmer cooperators had agreed to apply a fumigant treatment on only a portion of the field. The soil samples from these fields were tested for soil organic matter, pH, soil texture, nematode counts (for Pratylenchus sp.), active C, soil stability, soil respiration, and levels of Verticillium dahliae.

In 2005, 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. 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 nine fields, at four locations in each field, before a mustard green manure crops were planted. Another set of samples was taken from the same locations in March, 2006, to be analyzed for various potential indicators of pest suppression: FDA, b-glucosidase, dehydrongenase, active C, pH, % sand, silt and clay, and N, C and S contents. The idea was to determine if any of these measurements correlated 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.

Objective 2

Variety trials were randomized with four or more replications of each entry. Trials were planted in August (see trial results for each year’s exact planting date) with a walk-behind planter and were fertilized and irrigated according to WSU recommendations. Biomass samples were taken at the end of August. Oven-dried subsamples were used to determine moisture content.
Samples taken for glucosinolate analysis were placed in mesh bags (large sand bags) and immediately placed in coolers containing dry ice. They were then transported to the lab doing the analysis the same day and placed in walk-in freezers until they could be freeze dried and prepared for analysis. Dr. Matt Morra at the University of Idaho did the glucosinolate analysis.

Objective 3

2003: A mustard green manure field day in late October attracted 78 producers, researchers, and crop consultants.

2004: The results of the variety trial, past research, and basic principles were disseminated to farmers and crop consultants at a Mustard Green Manure field day held Oct. 26th. Handouts from this event can be found at:
http://www.grant-adams.wsu.edu/agriculture/covercrops/green_manures/index.htm
The field day was attended by approx. 45 people.

A website developed to disseminate information on mustard green manures received over 16,000 hits in 2004 through November. Mustard green manure publications were downloaded about 3,100 times in the same period. Through direct contact by presentations or posters, 1,324 people were made aware of the benefits and uses of mustard green manures. The results of this work were published in the Australian newsletter, Biofumigation Update, and in Potato Progress, the grower newsletter of the Washington State Potato Commission.

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

The website received over 27,609 hits from January through October, 2005. This is an 89% increase over 2004. Mustard green manure publications were downloaded about 3,745 times in the same period.

2006: 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, but many of them were first-timers at these field days.

The website received over 31,290 hits from January through November, 2006. This is a 5% increase over 2005. Mustard green manure publications were downloaded about 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.

A current partial budget analysis was posted online comparing mustard green manures to metam sodium fumigation.