On-farm Evaluation of Six Commercially Available Biofertilizers on Organic Tomato and Broccoli

Biofertilizers contain microorganisms which promote plant growth by one or more mechanisms, including increasing nutrient supply and/or availability. Native soil microorganisms clearly play critical roles in crop and soil health, and a growing body of evidence suggests that inoculation with specific microorganisms can dramatically affect plant growth in controlled environments. The purpose of this study was to test the effects of biofertilizers in a variety of production environments involving on-station and on-farm plots and high and low fertility conditions. Four commercially available inoculants (Azospirillum brasilense, Pseudomonas protogens, Bacillus amyloliquefaciens, and a 29-strain mix) and a control were applied to newly-transplanted tomato seedlings at four Ohio organic farms and a field station at OARDC in Wooster, Ohio, in 2015, followed by assessments of plant growth and measures of fruit yield. Slight increases in early season growth, fruiting, and flowering were observed for some inoculants on several farms. However, these differences did not lead to significant or even numerically consistent yield increases. A second experiment was conducted on broccoli at a farm in Fremont, Ohio in the fall of 2015 to test the combined effects of nitrogen fixing inoculants and fertilizers. Four inoculants (Azospirillum brasilense, Azotobacter spp., a 12-strain mix, and a 29-strain mix) and a control were tested in a full factorial design with two organic fertilizers (composted chicken manure, and a mixed fertilizer), and an unfertilized control. Yield was lowest in plots receiving neither fertilizers nor inoculants, highest in the plots receiving fertilizers, and intermediate in plots that were inoculated, but not fertilized. No additive effects of inoculation and fertilization were observed. Both fertilizers significantly (P < 0.05) increased yield compared to the control. When no fertilizer was added, the inoculants increased yield by 12.9%-64.6%, with the only the best performing biofertilizer resulting in a significant yield increase over the control. Biofertilizers are only a fraction of the cost of organic fertilizers, so their potential return on investment is high. However, their performance is clearly context-dependent, and conditions under which they perform most consistently are not well understood. In this study, no effects of inoculation were observed under high fertility conditions, suggesting that biofertilizers may have the greatest potential in low-input systems.