Author: Madeline Desjardins, PhD candidate, Washington State University
We tend not to want to think too hard about the contents of our toilets, much less where they go after we flush. While our waste may be out of sight and out of mind after this point for the average person, in reality, the problem of what to do with our waste isn’t as “flushable” on a larger scale. Before I started my PhD work at Washington State University’s Northwestern Washington Research and Extension Center in Deirdre Griffin LaHue’s Soil Health Lab, I can honestly say that this was an issue that I thought about very rarely. But now my eyes have been opened to the problem of and the promise of our wastewater. So, bear with me as I tell you more about what happens after you flush your toilet!
What are biosolids?
After toilets are flushed, the solids and liquids meander under the city to go through the wastewater process. Where solids are separated from liquids and bacteria are utilized to digest and transform the solids. Once these solids are cleaned and transformed, the products left over are Biosolids. They look a lot like soil, and they are high in organic matter and plant essential nutrients. One of the beneficial uses of biosolids is to land apply them as alternatives to synthetic nitrogen fertilizers.
Land application of biosolids is a great way to close the nutrient cycle by recycling urban waste and redistributing nutrients and organic matter from densely populated areas where food is being consumed back into the less populated agricultural areas where food is being produced. This allows the nutrients being taken out of agricultural landscapes to be replaced. In Washington State there is a law that requires that all biosolids be used beneficially, instead of being landfilled. King County Department of Natural Resources and Parks –Waste Water Treatment Division has a biosolids program, which supplies biosolids for land application, in farms and forests. This is where the biosolids for our research comes from. Our research is funded in part by King County as well.
How are biosolids used?
In Dr. Deirdre Griffin LaHue’s Soil Health Lab, my research focuses on how biosolids in dryland grain systems influence the physical, biological, and chemical soil health properties and whether biosolids can help growers establish cover crops in low rainfall environments. Dryland grain systems in Central Washington face some specific soil health challenges related to water availability that can severely limit crop yields. Annual precipitation (through rain and snow) in this area is low (~10 inches a year). These systems rely solely on precipitation to meet the water needs of the crop. Increasing the soil’s ability to hold water is an important management strategy in these areas. Organic matter can improve soil physical properties like water holding capacity and bulk density. Due to the high organic matter content in biosolids, we hypothesized that long-term biosolids applications would improve soil function in areas and help farmers to better manage soil moisture.
Tracking biosolid’s impact over time
Our long-term biosolids site in Central Washington has been going since 1996. This means we’ve been able to track changes in soil health on this site for many years. In 2019 The Soil Health Institute took samples from our long-term site as a part of their North American Project to Evaluate Soil Health Measurements. We were able to use this data to try to address some of our questions. Like, how biosolids affect soil physical properties relating to water holding capacity and bulk density
When we analyzed the data from The Soil Health Institute’s project, we found that biosolids applications increased available water holding capacity and decreased bulk density. Available water holding capacity is the water held between what’s drained by gravity (field capacity) and a defined permanent wilting point. We saw that biosolids increased water holding capacity at the two highest application rates (3 and 4.5 dry
tons/ac) compared to unfertilized and synthetic fertilized plots (Figure 3). In this dryland system, having higher water holding capacity is better, but lower bulk density is ideal. Bulk density, an indicator of compaction, decreased at all three biosolids application rates (Figure 4). This indicates that in the soil surface biosolids decreased compaction and created more soil pore space for root growth and water storage.
While we saw biosolids affect these two soil physical properties, there were other physical properties that did not change. Hydraulic conductivity (similar to infiltration) and aggregate stability (the soil’s resistance to disintegration) remained consistent. This leads us to believe that the increase in organic matter is what improved water holding capacity and bulk density rather than organic matter improving soil structure, therefore, improving water holding capacity and bulk density. This makes sense since we added organic matter, which itself has a higher water holding capacity and a lower bulk density than mineral soil.
In 2020 we started a new research trial in the same area of Central Washington where our long-term biosolids trial is located. One of the objectives of our research at this new trial is to explore relationships between biosolids and cover cropped and grazed systems (Figure 5). One of our driving questions, as we began this trial, is whether the improvements in soil health we saw with biosolids in our long-term site could help farmers establish cover crops in this region.
3 key takeaways about biosolids in dryland grain systems:
- Biosolids can redistribute nutrients from densely populated areas to where food is being produced
- Biosolids increase water holding capacity and decrease bulk density in dryland grain systems
- Research is being done to connect biosolids, cover cropping, and grazing systems
When you head to the bathroom next, take a second to think about how you, with the help of wastewater treatment, transportation trucks, innovative farmers, and Maddy, are working to close the nutrient loop.
Follow Madeline on Twitter @MadelineDesjar1 to keep up with the project, and while you are at it, share this blog post on your own page!
References:
Agriculture. NW Biosolids. (n.d.). Retrieved October 25, 2022, from https://nwbiosolids.org/product-use/agriculture/
Cogger, C. G., Bary, A. I., Myhre, E. A., & Fortuna, A.-M. (2013). Biosolids Applications to Tall Fescue Have Long-Term Influence on Soil Nitrogen, Carbon, and Phosphorus. Journal of Environmental Quality, 42(2), 516–522. https://doi.org/10.2134/jeq2012.0269
Hudson, B.D. (1994). Soil Organic Matter and Available Water Capacity. Journal of Soil and Water Conservation, 49, 189-194.
Soil Health Institute (April 24, 2018). North American Project to Evaluate Soil Health Measurements. https://soilhealthinstitute.org/news-events/north-american-project-to-evaluate-soil-health-measurements/