By Sally Brown and Ande Niedzwiecki, University of Washington; Toby Una, Andy Bary and Deirdre Griffin LaHue, Washington State University
This study was set up to see if the use of amendments made soils ‘healthier’ and if the vegetables grown in those soils were more abundant and better for you. The study has three sites and a number of amendments. The first site is at the City of Tacoma's Central Treatment Plant where we are comparing the soil at the plant with fertilizer to soil with a history of Tagro potting soil. The second site is at City Soil Farm, located at King County's South Treatment Plant. Here we are testing GroCo biosolids compost alongside soil with synthetic fertilizers. The last site is at the Monroe Correctional Facility, where inmates make vermicompost, bokashi and compost tea from food scraps. Each of these amendments have been added to the soil along with a fertilizer control.
Results from the first year showed a response beyond expectations at Tagro and mixed responses at the other sites. To see if we can get more consistent results we’ve added replicates and kale and swiss chard (healthier) and taken away the onions (less nutrient-rich). We’re also getting ready to start a greenhouse trial where each amendment can be compared side by side. What we’ve seen so far is that not all soils or amendments are created equal. Here is a summary of what we’ve seen.
The control soil at Tacoma was the ‘worst’ soil in the study. It had the lowest concentration of active carbon of any of the soils/ amended soils tested. Adding Tagro to the soil doubled the concentration of active carbon. It also decreased the bulk density and increased the infiltration rate. In other words all of the soil parameters we have data for to date are much better as a result of the Tagro.
At Renton, it was a different story. The control soil there was good to start with- with about the same amount of active carbon as the Tagro amended soil. Adding GroCo did not increase the active carbon. It did decrease bulk density and also really increased the water infiltration rate- both good things.
Finally, at Monroe, the control soil was much better than Tacoma but not as good as Renton in terms of active carbon. The worm castings (vermicompost) increased the active carbon concentration with the other amendments showing slight, but not significant improvements. Bulk density was similar across all treatments. Water infiltration rate was lowest in the worm compost in comparison to the other amendments but not statistically different from the control.
We are waiting for results on the other soil measures but the big conclusion so far is that amendments work best where the soil needs the most help.
So far, we have yield data for the 2019 crop. The plants are currently being digested for trace nutrients and we are starting to test for a range of vitamins. On a broad scale, the yield data mirrors the soil data. Yield for all vegetables from the controls at Monroe and Renton where the control soils were high in carbon were similar except for broccoli (Figure 1). Broccoli at Monroe was bigger than the broccoli at Renton. Yields in the control at these sites were much higher than from the Tacoma control. We had no harvestable carrots or chard in Tacoma.
Figure 1. Yields for vegetables grown in the control soil at all three sites.
The response to amendments was greatest in Tacoma. There was basically no comparison. If you want vegetables to eat and you start with the control soil in Tacoma you had better add lots of Tagro.
At Renton, yields in the GroCo and fertilizer amended soils were similar. We will see if there are differences in the nutritional quality of the food but the take-home seems to be that adding the compost is about as good as using fertilizer for your salads.
We don’t have pictures yet from the prison, but generally the controls out-yielded the other amendments across all the vegetables we grew.
The greenhouse study will tell us if all of the amendments are as effective at improving the Tacoma control soil. We’ll also be able to see if the Tagro that worked so well in Tacoma can improve growth and soil characteristics in the Renton soil. More data on soils and plants is upcoming, so stay tuned.