Amendment Composition and Location Affect Soil Microbial Activity, Nitrogen Availability, and Pest Dynamics on Organic Vegetable Farms.

Effectively managing nitrogen in organic farming systems is challenging, because organic fertility amendments must mineralize before they are available for plant uptake. Mineralization is a microbially-mediated process, and differences in amendment composition as well as resident microbial community structure regulate this process. Amendment composition could also alter the structure and activity of resident soil microbial communities, affecting pest dynamics. The objectives of this project were to: 1) determine how composition of four organic fertility amendments interact with resident soil microbial communities to affect nitrogen (N) availability and pest dynamics on organic vegetable farms, 2) identify amendments that improve soil biological quality, provide plant available N during periods of critical N uptake, and reduce pest outbreaks, and 3) demonstrate and increase the practice of on-farm research. Experiments were conducted during 2013-2015 on three vegetable farms. Growers participated in development of the study design, data collection and outreach events. Repeated applications of the amendments were applied to each plot annually and cabbage, squash, and tomato were planted in years 1-3 respectively. Plots were monitored regularly for N availability, soil chemical and biological properties, pest incidence and severity, and crop yield. Nitrogen availability, severity of black rot, root rot and white fly, as well as crop yield varied given amendment composition and initial soil quality characteristics. Some variability in soil quality improvement due to amendment composition was observed within each farm, though all amendments increased labile soil carbon, microbial activity, and pathogen suppressiveness over time. Results of this study demonstrate that soil biological quality is intimately tied to N availability, pest severity and yield on organic vegetable farms. On-farm research is a valuable tool that can provide new insights into soil microbial ecology and help growers identify fertility amendments that improve soil quality and optimize crop productivity on their farms.