By Sally Brown, University of Washington

We’ve been studying the potential connections between use of amendments and soil health for a few years.  That study has a second component; the relationship between soil health and the plants that grow in the soil.  We are testing whether the health of the soil has any impact on the yield of the vegetables and their nutritional quality.  The study was designed to focus on urban agriculture.  Three field sites have been used, one at the treatment plant in Tacoma, another at the Renton treatment plant in King County, and finally a site at the Monroe Correctional facility where inmates produce a range of soil amendments from food scraps.  While on paper a UW study, we’ve had to reach across the aisle for assistance on numerous occasions.  Andy Bary was a veteran of all three sites with Diedre Griffin helping sample soils and take measures for bulk density and water infiltration rates at the Monroe site. 

The last portion of the study took soils from the field and used them in a greenhouse study.  Anna Beebe has been the graduate student on this section of the project.  She grew swiss chard and kale in pots at the UW greenhouse and then after Covid shut that down, in her backyard.  She has been able to measure yield, nutrient content of the plants, and many of the relevant soil parameters.  But we were stuck on the biological aspects of the soils.  The biological aspects of soil health focus on the microbes that live in the soil.  By improving soil health, you also improve soil as a type of habitat.  A healthy soil will have a larger and more diverse microbial population.  There are a range of options to measure differences in soil biology as indicators of soil health.  Some of these involve testing for enzymes that microbes will secrete.  You can also measure CO2 coming off of the soil as the microbes need to breath.  Finally, you can use special assays to measure what portion of the soil carbon is likely available for the microbes.  At the University of Washington, we don’t have the set up to easily measure any of these.  In addition, because we aren’t a land grant university, Covid still has much of the research shut down. 

Again, we turned to WSU for help.  And they came through with flying colors!

At Deirdre Griffin’s lab in Mt Vernon, they are up and actively running samples (following all Covid safety protocols).  They are also experts in several of these extractions.  Dr. Griffin kindly offered to have her students work with Anna to test for CO2 evolution and active carbon on the soil samples collected from the greenhouse trial.  As an extra bit of kismet, Toby Una, who worked on the field component of this study is now working on her MS with Dierdre and is the resident lead on one of the extractions.  Anna took her soil samples to Mt Vernon where she was able to learn the procedures and run the analysis.  The extracts for mineralizable carbon are done and Anna is analyzing the data.  She will have to make another trip up north to measure CO2 evolution. 

While I don’t have data to share from the most recent example of cooperation, samples from the field trial had previously been run for Active carbon by the same group.  Results from 2019 shown below suggest that adding Tagro to the control soil from Tacoma increased the active carbon content.  For the control soil from Renton, adding the GroCo had no impact.  The worm compost did the trick for the soils at Monroe with no impact seen from use of Bokashi fermented food scraps or compost tea.


Active Carbon 2019