Biofumigant Performance of Four Brassica Crops in an Irrigated Chile Pepper Rotation System

Biofumigants are biologically active cover crops that can be used as an alternative to chemical fumigation for agricultural soil management and pathogen control.  When incorporated as a green manure, biofumigants have the potential to increase soil organic matter and alter soil pH.  Both of these changes are desirable in southwestern United States soils.  Biofumigants also contain naturally occurring biocidal compounds, called isothiocyanates (derived from glucosinolates), that can suppress soilborne pathogens.  In the present study, a field project was conducted over two years to evaluate the biofumigant performance of three Brassica juncea cultivars (‘Caliente 61’, ‘Caliente 199’, ‘Pacific Gold’) and one broccoli cultivar (Brassica oleracea var. botrytis ‘Arcadia’).  Brassica crops were seeded in the fall, flail mowed and incorporated into the soil in the winter.  Green chile pepper ‘AZ-20’ (Capsicum annuum) was direct seeded in the spring into drip irrigated plots.  Biofumigant performance was evaluated based on soil changes including organic matter, pH, E.C., biomass production, and glucosinolate concentrations in Brassica crop tissues.  Chile pepper crop yield and stand establishment in biofumigant plots were also measured and evaluated.  In both years, all mustard treatments produced significantly higher biomass than broccoli, but were not significantly different from one another.  In year one (2011), biofumigant plots produced an average of 104.6 metric t·ha^-1 of fresh biomass.  Biofumigant biomass in year two was less than year one (49.6 metric t·ha^-1 of fresh weight) due to delayed planting.  In 2011, after incorporation of the biofumigants and before seeding the chile pepper, soil organic matter showed a temporary increase of more than 24% (‘Pacific Gold’) to 9% (mean of all other treatments).  After incorporation, the pH of the three mustard treatment plots was significantly lower (‘Caliente 61’, pH 7.54; ‘Caliente 199’, pH 7.5; ‘Pacific Gold’, pH 7.5) than the pH of the control (bare soil, pH 7.7).  ‘Caliente 199’ and ‘Pacific Gold’ treatments had significantly higher glucosinolate levels than all other treatments.  Chile pepper plants growing in the ‘Caliente 199’ and ‘Pacific Gold’ treatment plots produced significantly higher red pod yield than the other two biofumigant treatments and the control.  Biofumigants have potential to improve the characteristics of southwestern soils.  These effects may be cumulative and have a greater positive impact on chile pepper growth and yield in the second season.