My first contact with the Brassica plant genus as a soil fumigant took place a dozen years ago. At the time, several associates and I were making biodiesel. Our most common raw material for this alternative fuel was waste vegetable oil donated by local restaurants. Most of this cooking waste product was corn oil and canola oil. Canola is a specially bred strain of rapeseed (Brassica rapa).
About 50 years ago, plant breeding Canadian scientists invented canola (an abbreviation for Canadian oil), with low erucic acid. Pressing regular rapeseed was okay for making food for diesel engines as well as meal for livestock, but its erucic acid wasn’t so good for humans. Through selective plant breeding, scientists took care of that issue, creating a new cousin to B. rapa.
In studying how to make better biodiesel, I came across research showing that, according to North Carolina agronomists, rapeseed grown before wheat benefited those cereal grains yields by about 20%, compared to wheat following wheat. According to Dr. Angela Post, North Carolina State University small grains specialist, the large diameter canola root makes it an excellent cover crop that can be harvested for oilseed.
Additionally, the vertical hole left by the decaying root greatly assists soil percolation. The other factor is the soil fumigant benefit provided by this Brassica member.
Brassica – specifically mustard – cover crops are known for rapid autumn growth, great biomass production and nutrient scavenging ability. They also deserve attention because of their possible pest management characteristics. Most Brassica species release chemical compounds toxic to soil-borne pathogens and pests such as nematodes, fungi and some weeds. The mustards have higher concentrations of these chemicals.
Brassicas are increasingly used as winter or rotational cover crops in vegetable and specialty crop production. Some Brassicas have large taproots that can break through plow pans better than the fibrous roots of cereal cover crops. Those Brassicas that winterkill decompose very quickly and leave a seedbed that is mellow and easy to plant in.
With a number of different species to consider in this genus, growers will likely find one or more that can fit their farming operation. Don’t expect members of this genus to eliminate all pest problems; however, they are a good tool and an excellent rotation crop, but pest management results are inconsistent.
More research is needed to further clarify the variables affecting the release and toxicity of the chemical compounds involved. Since these cover crops share insect pests and diseases with Brassica vegetables like cabbages and cauliflower, in the crop rotation, growers are advised not to follow one Brassica with another.
Brassicas provide excellent soil coverage and up to 8,000 lbs. biomass/acre. Because of their fast autumn growth, members of this genus are well-suited to capture soil nitrogen (N) remaining after crop harvest. The amount of N captured is mainly related to biomass accumulation and the amount of N available in the soil profile. Because they immobilize less N than some cereal cover crops, much of the N metabolized can become available for uptake by main crops in early to late spring. Brassicas can root to depths of six feet, scavenging nutrients from below the rooting depth of most crops.
To maximize biomass production and autumn nutrient scavenging, members of this genus must be planted earlier than winter cereal cover crops in most regions, making them more difficult to fit into grain production rotations.
Let’s examine more closely factors supporting the fumigant personality of these plants. All Brassicas have been shown to release biotoxic compounds that exhibit broad activity against bacteria, fungi, insects, nematodes and weeds. Brassica cover crops are best mowed and incorporated to maximize their natural fumigant potential. This is because the fumigant chemicals are produced only when individual plant cells are ruptured.
The timing and method of incorporation is a key factor in how effective they are at suppressing pests. UMass studies have shown Brassica green manures to suppress Rhizoctonia, Scab and Verticillium in potatoes and Phytophthora blight in cucurbits. Studies have also shown efficacy in reducing nematode and weed populations in various cropping systems. The use of Brassica cover crops for disease and insect control is still very much a work in progress.
In New Brunswick, Canada, we find agronomists addressing this subject in further depth. They write that biofumigation is the suppression of soil-borne pests and diseases through the use of plants that produce inhibitory chemicals, known as secondary metabolites. They explain, “In most cases these biofumigant plants are chopped and incorporated into the soil, so they can release inhibitory chemicals. Mustard is a well understood biofumigant. Its biofumigation properties have been studied for a number of years and scientists have developed a method to fully use these properties. Mustard and most other plants from the Brassica family produce chemicals called ‘glucosinolates.’ When glucosinolates come in contact with water and a family of enzyme myrosinase, contained in plant cells, they are transformed into another group of compounds called ‘isothiocyanate.’ These isothiocyanates give mustard its biofumigation power.”
Isothiocyanates give many Brassicas their bitter/hot/spicy taste. The isothiocyanate that is produced by mustard is called allyl isothiocyanate (AITC). AITC is a compound very similar to the compound sodium methyldithiocarbamate (contained in the commercial fumigant Vapam®).
These agronomists offer the following cultural pointers:
1) Soil incorporation should be done before the mustard crop reaches full bloom, and should be done in the morning or evening, avoiding hot sunny days.
2) Incorporation should be done when soil has plenty of moisture; do not incorporate mustard when the soil is dry.
3) Prior to the actual incorporation, chop and crush as much plant material as possible to release the fumigant from plant cells. This can be done with a flail mower.
4) Mustard must be incorporated immediately after mowing, since 80% of the fumigant gas will be released in the first 20 minutes after mowing.
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