Soil fumigants are pesticides that, when applied to soils, form gases to control pests which, if unchecked, disrupt plant growth and crop production. Fumigation suppresses weeds and soilborne plant pathogens in crops like potatoes, onions, sugar beets, tomatoes, mint, carrots and strawberries.

Chemical fumigation is widely practiced in many cropping situations. Globally, ag producers applied 2.17 million tons of pesticide active ingredients to crops in 2020, of which U.S. producers applied 0.37 million tons. While chemical fumigation has been shown to be effective, it is expensive. Recent research also shows that it impacts soil health.

Another way for producers to use fumigants does not involve synthetic chemicals. Bio-fumigation is an eco-friendly alternative employing cover crops like mustard, canola and oilseed radish. With bio-fumigation, producers learn how crops, like those listed, incorporated into the soil at the right time and in the correct manner can preserve and enhance soil health while combating pests, diseases and weeds.

According to plant scientists at Oregon State, bio-fumigation is the process of incorporating glucosinolate-containing crops, such as Brassicas, to suppress soilborne pests. Glucosinolates are diverse secondary metabolites found in various Brassica species, such as mustard, oilseed radish, arugula and related plants, playing a crucial role in plant defense mechanisms.

My first contact with Brassicas as soil fumigants occurred 14 years ago, when 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 cooking waste product was vegetable oils of corn and canola. Canola is a precisely bred strain of rapeseed (Brassica rapa). Over 50 years ago, Canadian plant breeding scientists invented the name canola (as 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 was hard on human digestive systems. Through selective plant breeding, scientists solved that issue, creating a new cousin to B. rapa, one with low erucic acid, hence canola.

In studying how to make better biodiesel, I encountered research in trade journals showing that, according to North Carolina State University agronomists, rapeseed grown before wheat benefited those cereal grain yields by about 20% – compared to wheat following wheat.

According to these NCSU scientists, 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.

One other benefit is the soil fumigant trait provided by these Brassica members. One of them, specifically mustard, is a cover crop known for rapid autumn growth, great biomass production and nutrient scavenging ability.

This plant grouping also merits attention because of its promising pest management characteristics. Most Brassica species release chemical compounds toxic to soilborne pathogens and pests, like nematodes, fungi and some weeds. Mustards usually have higher concentrations of these chemicals. Thus, Brassicas are increasingly used as winter or rotational cover crops in vegetable and specialty crop production.

Some Brassicas have a large taproot that can break through plow pans better than fibrous roots of cereal cover crops. Brassicas that winterkill decompose very quickly, leaving mellow seed beds, easy to plant in. Growers often discover at least one member of this genus fitting into their particular cropping situation.

More research is needed to further clarify variables affecting the release and toxicity of the chemical compounds involved. Since these crops share insect pests and diseases with Brassica vegetables (like cabbages and cauliflower), growers are advised not to follow one Brassica with another in crop rotations.

Brassicas provide excellent soil coverage and up to 8,000 lbs. of biomass/acre. Because of their rapid autumn growth, members of this genus – even though they’re not legumes – are well-suited to capture soil nitrogen (N) after crop harvest. The amount of N captured is mainly related to biomass accumulation, as well as the amount of this nutrient available in the soil profile.

Because they immobilize less N than some cereal cover crops, much of the N metabolized becomes available for uptake by the main crops in early to late spring. Brassicas can root down to depths of six feet, scavenging nutrients from below the rooting depths 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. This may make them more difficult to fit into grain production rotations.

Let’s examine more closely some factors supporting the fumigant personality of these plants. All Brassicas are known to release bio toxic 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 key in determining how effective they are at suppressing pests.

UMass studies have shown that Brassica green manures suppress Rhizoctonia, scab and Verticillium in potatoes, as well as Phytophthora blight in cucurbits. Studies have shown efficacy in reducing nematode and weed populations in various cropping systems. But the use of Brassica cover crops for disease and insect control remains a work in progress.

In New Brunswick, Canada, Extension 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 the soil has plenty of moisture; do not incorporate mustard into dry soil.

3) Prior to 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) Incorporate mustard immediately after mowing: 80% of the fumigant gas will be released in the first 20 minutes following mowing.