Gordon Johnson, Extension Vegetable & Fruit Specialist; firstname.lastname@example.org
In fields with heavy vegetable rotations that have built up diseases (including nematodes), or fields known to have problems in the past, use of a biofumigant mustard crop planted in March ahead of spring planted vegetables can help reduce disease levels.
Mustard family plants produce chemicals called glucosinolates in plant tissue (roots and foliage). These glucosinolates are released from plant tissue when cut or chopped and then are further broken down by enzymes to form chemicals that behave like fumigants. The most common of these breakdown products are isothiocyanates. These are the same chemicals that are released from metam-sodium (Vapam) and metam-potassium (K-Pam), commonly used as chemical fumigants.
Three mustard varieties that have been successfully used for this purpose are Pacific Gold, Idagold, and Caliente 99.
You should plant these mustards as soon as the ground is fit in March. They take 50 days to produce full biomass. Planting rates are 10-15 lbs/ A for Pacific Gold and 15-20 lbs/A for Caliente 99 and Idagold. Add 40-80 lbs of Nitrogen per A to grow the crop (the higher N level on sandy soils).
The goal is to produce as much biomass of the biofumigant crop as possible. This requires that you have a good stand, fertility, and sufficient growing time. The more biomass that is produced and that is incorporated, the more chemical is released.
The plant material must be thoroughly damaged so that enzymes can convert glucosinolates into isothiocynates. This means that you need to chop the material as much as possible and work it into the soil as quickly as possible, so as not to lose the active compounds to the air. A delay of several hours can cause significant reductions in biofumigant activity. The finer the chop, the more biofumigant is released. A flail mower is ideal.
The material should be incorporated as thoroughly as practical to release the biofumigant chemical throughout the root zone of the area that is to be later planted to vegetables. Poor distribution of the biofumigant crop pieces in the soil will lead to reduced effectiveness.
Sealing with water or plastic after incorporation will improve the efficiency (as with all fumigants). Soil conditions should not be overly dry or excessively wet.
Allow 2-3 weeks after incorporation before planting the next crop.
A March 15 planting will be ready to incorporate in early May and can be planted with the vegetable crop in late May (around Memorial Day).