A fresh soil health perspective: Soil health dynamics and improved measurement techniques

Encouraging greater implementation of conservation agriculture practices such as reduced tillage and cover crops may require better understanding of the effect of these practices on soil health. The overall objective of this study was to quantify soil health dynamics due to conservation agriculture practices and address methodological gaps in terms of measuring soil health parameters. We developed five sites across the state of Virginia; each site had replicated plots with combinations of reduced tillage versus disk tillage and wintertime cover crops versus no cover crops as experimental treatments. Soil and plant samples were collected 1-2 times per year for 3 years, and were analyzed for 30 soil health parameters. The parameters were first evaluated to determine if any consistently detected treatment differences. We then quantified the temporal dynamics of the eight most responsive soil health parameters, while considering influences of soil water content at time of tillage, cover crop biomass, and previous land management history.

Of the analyzed parameters, only 2-4 mm aggregate stability and magnesium showed high responsiveness and consistency in identifying tillage and cover crop effects. None of the parameters detected treatment differences in all sites or at all times, yet samples collected after high biomass cover crops or after tillage in wet conditions tended to show significant treatment differences for multiple indicators. The previous history of management in each site may have affected trends in aggregate stability, but did not appear to influence other indicators.

As soil aggregate stability was found to be the most important soil health parameter, our third study developed an improved method for measuring soil aggregate stability. This new method, Integrated Aggregate Stability (IAS), interprets aggregate stability using a laser diffraction machine. Overall, IAS showed higher correlation with the wet sieving method (R2 = 0.49 to 0.59) than widely used median aggregate size (d50) (R2 = 0.09 to 0.27). IAS can also quantify stability of macro- and micro-sized aggregates, which d50 cannot. When comparing between IAS and wet sieving, IAS requires considerably less time and sample amounts.

Our fourth study focused on creating an inexpensive yet accurate tool for measuring soil respiration, as microbial assessments based on respiration rates have great potential for detecting rapid changes in soil health. Using an Arduino-based infrared gas analyzer (IRGA) sensor, we developed the Soil Microbial Activity Assessment Contraption (SMAAC) for less than $150. Our results show that SMAAC provided consistent readings with a commercial IRGA unit when tested using three different configurations.

Altogether, the research presented in this dissertation identifies important soil health parameters and quantifies their temporal and between-site dynamics. Using this narrower set of indicators can help producers and practitioners save resources when conducting measurements to assess soil health effects of agricultural practices. Further, this work also provides improved measurement techniques for useful soil health parameters like aggregate stability and soil respiration. These findings and innovations should help to encourage greater adoption of agricultural management practices that build and preserve soil health.