By Sally Brown and Sally Landefeld, University of Washington; Dan Eberheardt, City of Tacoma-TAGRO and Juan Carlos Hernandez, Monroe Correctional Facility
Last year was the first year of a study testing the relationship between soil health and the yield and nutritional quality of the plants grown on soils. The study is funded by NW Biosolids, King County Resource Recovery and Tagro. We have three sites for the study. At the Tagro treatment plant we are comparing the long-term Tagro amended soil to a soil that had landscape plants. At City Soil Farm at the Renton treatment plant, we are comparing GroCo and soil that had been under turf grass. Finally, at the Monroe correctional facility we are comparing soil that had been under turf to vermicompost and bokashi amended soils. Bokashi is an anaerobic fermentation process that can be used to treat food scraps. It produces a liquid and a sludge material. Control treatments at all sites were fertilized with N, P, and K at 200 lbs N per acre. The treated soils did not receive any additional fertilizer.
We grew broccoli, carrots and onions at each site. Plant yield, trace nutrients, B vitamins, and some phytochemicals were measured. We measured total soil carbon and nitrogen, available and total nutrients, bulk density, and water infiltration rates on the soils. Results from the first year did not show a clear, across all sites, answer. The Tagro treated plots outperformed the control to a ridiculous extent. At City Soil farm, there was very little difference between the GroCo amended soil and the control. Total soil zinc (you would expect big increases from the biosolids) was only mildly elevated in the GroCo plot, suggesting that very little material had been applied over time. Rabbits were the major factor at that site. Monroe also told a mixed story. At the Monroe site the two treatment plots were located between buildings with the control on the far side of the buildings. Here are a summary of the results from the Monroe site. Following the summary are a list of the changes planned for this year’s trial. Hopefully with a year of results behind us, we can figure out the kinks and get some clearer answers.
We were expecting the vermicompost and bokashi soils to have higher organic carbon content than the control soils. Higher organic matter (OM) is typically the key to soil health. If a soil has high organic matter, typically water will infiltrate more quickly, the soils will be lighter and there will be a better supply of the range of nutrients needed for plant growth.
We did see higher carbon in the vermicompost and bokashi soils compared to the control plots.
The vermicompost soil had much higher carbon than the control with the bokashi right in the middle. There was also a small increase in soil nitrogen. Remember here that we fertilized the control with nitrogen and we didn’t fertilize the other soils.
However we did not see a change in bulk density (how light the soil is- literally how much soils weigh per unit of volume). We also did not see the difference in water infiltration rate that we expected for the vermicompost soil. All of the soils that we measured had low bulk density and good infiltration rates.
We also tested the soil for both total and available nutrients and other trace elements. Here the compost and bokashi as they are plant based, would be expected to add micronutrients to the soil as these micronutrients are in the plants themselves. We did see increased total copper and zinc in both the bokashi and vermicompost soils. We also saw higher lead. Lead is not likely from the compost and most likely comes from some outside source. Here it might be that the soil had old construction materials and some had lead based paint. We are thinking that the soil in the middle of the buildings where the vermicompost and bokashi beds are is potentially from a different material or has developed differently than the control soil. At a recent meeting at the Monroe facility, we identified an area near the two treated plots that we can use as a control for the remainder of the study.
One of our hypotheses was that healthy soils will grow healthier plants. Here healthier plants could mean higher yields as well as plants that are higher in nutrients. We did see higher yields for broccoli in the vermicompost. We did not see generally higher yields in the bokashi for any of the plants grown. We also saw about the same yield in carrot for all the treatments and higher yield for onion in the control soil. Rabbits were a factor as they were out in force.
We haven’t done the full statistical analysis on plant micronutrients but we did not see the changes that we were expecting. Zinc is an important trace nutrient in plants. Soil zinc for both the vermicompost and bokashi was higher than the control. We did not see any real changes in plant zinc as a result.
We also looked at phytochemicals and B vitamins in the plants. Here we saw more encouraging results. The differences in B2 for carrot were small, with the control and vermicompost higher than the bokashi. The carrots grown in the vermicompost had higher beta carotene than the other two treatments.
For next season
• More amendments will be added to the soils at all sites
• We are going to eliminate the onions (not nutrient rich enough) and substitute in kale and swiss chard (both leafy greens, both higher in nutrients)
• We have to plan early and take defensive action against the rabbits.
• We will set up a treated plot next to the control at the Tacoma site and add additional control and treated plots at the City Soil site.