Joe Nunez

UCCE Farm Advisor, Kern County

Vegetable Crops - Plant Pathology

January 17, 2003

There has been a lot of interest the past few years on the use of mustards as green manures by growers and researchers. Using green manures to add organic matter to the soil has been long known to increase soil quality. But the use of mustards as a source of green manure is an added twist that makes it uniquely functional.

In general, green manures can benefit the soil and ultimately economic crops several different ways. As a rotation crop it benefits the soil by reducing the population of pathogens that normally buildup by continuous planting of the same crop. The added organic matter from green manure is also a source of energy for soil microbes. Soils with an abundant organic matter generally have large and diverse groups of soil microbes, many of which are beneficial. Other benefits of added organic matter include improved water penetration, reduced soil compaction, less soil erosion, and better soil tilth.
But the use of mustards as a green manure for organic matter is intriguing because it also has the added benefit of biofumigation. Mustards, as well as many other brassicas, are able to produce chemicals called glucosinolates. When mustard green manure is incorporated into the soil, these glucosinolates are converted to isothiocyanates (ITC). ITCs are well known to kill or suppress many soil-borne diseases, nematodes, and weeds. The active form of metam sodium, a widely used pesticide, is methyl ITC.

Mustards as green manure have been shown to help control several soil-borne pests, by not only the effect of crop rotation and added organic material, but also by its biofumigation effect. It is not likely that biofumigation will provide the level of control as synthetic chemical fumigants. But may be another useful tool growers can use to maintain soil-borne pests to acceptable levels.
Many researchers are trying to learn more about these plants and how to best use them. Further research is needed to find how to best take advantage of the biofumigation effect, which cropping systems it may fit into, identifying the best varieties, and how to manage these plants as a green manure crop. Growers are just finding about these advantages and are working them into different cropping systems. Perhaps planting mustards will become another pre-plant routine that growers do to prepare the ground for spring planting.