Abstract
Prohibited from using synthetic fertilizers and herbicides, organic agriculture systems depend on different management strategies, like tillage and organic amendments, to control weeds, provide nutrients, and sustain yields. Operating under this new management framework can create soil health challenges that may keep farmers from transitioning into certified organic production; tillage can degrade soil structure, increase erosion, and damage soil biological communities. To reduce the key barriers to a successful transition to certified organic, more research is needed on how producers in the Inland Pacific Northwest (IPNW) can potentially offset the negative effects caused by tillage. Being able to track changes how soil health properties respond to management practices is also key in meeting soil health goals. However, with so many soil measurements to choose from, organic farmers would greatly benefit from a select few that respond to management practices in the short term, are practical to measure, and relay important soil health information. The overall goal of this project is to facilitate the development of resilient certified organic farming systems in the IPNW through enhanced knowledge of the most useful soil health indicators and the impacts of farm management on soil health properties.
In the second chapter of this thesis, the research objective was to provide farmers in the Palouse bioregion of the IPNW with strategies for transitioning into organic production while maintaining soil health and crop yields. In 2021 and 2022 on newly cultivated sites within two certified organic farms, we studied the effects of three tillage intensities and three amendment additions, in combination with an alfalfa-grass perennial cropping mixture. Compared to granular bone meal fertilizer, compost additions reduced soil bulk density, increased water infiltration, and increased microbial biomass in the top 10 cm of the soil. Although roto-tillage reduced alfalfa germination success due to the subsequent formation of soil crusts, compost additions offset some of this negative effect. More time may be needed to see amendment or tillage impact on aggregate stability, soil respiration, and labile carbon pools. Based on these preliminary results, we suggest the combined usage of compost additions and perennial cropping to offset the negative effects of tillage in systems that are transitioning into certified organic production.
In the third chapter of this thesis, our research objective was to provide organic farmers in the IPNW with a selection of soil health measurements that are responsive to management practices like tillage, organic amendments, and irrigation in the short term, are practical to measure, and relay important soil health information. Across five certified organic farms in the region, we used two years of soil data on 42 different soil health measurements, the variability of those data, and farmer management survey information to determine which measurements were responsive to management practices and most reflective of important soil physical, chemical, and biological health. While soil bulk density reacts too slowly to management and water infiltration rate is too variable, aggregate stability effectively responds to management and conveys important soil physical, chemical, and biological information. Soil respiration and earthworm density are highly variable measurements (CV > 50%) that are overly sensitive to climatic conditions. Phospholipid fatty acid content, however, appears to be a useful biological soil health indicator that is responsive to tillage and amendment management. While total soil organic carbon changes too slowly and water-extractable carbon and nitrogen are too volatile, we found that total nitrogen and permanganate oxidizable carbon are effective soil health indicators of the soil organic matter pool. Together the data provide a preliminary list of the soil health indicators that may be most informative to organic growers as they strive to improve soil health through conservation management.