by Deirdre Griffin LaHue

In 1994, Craig Cogger and Andy Bary established a long-term biosolids trial in central Washington that, with support from NW Biosolids and King County, is still on-going and has allowed us to quantify numerous benefits of biosolids applications for soils and crop yields in these low rainfall, dryland (non-irrigated) systems. Over the years, we’ve wanted to keep the design of the experiment consistent so that we could track these long-term changes, but as farmers and their management have innovated and evolved, new research questions have arisen. 

Many farmers in this area of central Washington are interested in integrating winter cover crops into their systems to improve the soil and diversify inputs. However, establishing cover crops can be challenging when the soil is dry and coarse and there is low rainfall. We’ve found that over time, land application of biosolids increases soil water holding capacity and improves plant growth. Can biosolids give cover crops the boost they need to get established? 

In summer 2020, we started a new long-term trial on a farmer’s field in Douglas County, WA to try to answer this question. We have replicated areas with and without cover crops, and within those we have treatment plots that receive either biosolids, synthetic fertilizer, or no fertilizer (as a control). The cover cropped areas are also grazed, which is becoming a common practice for farmers in the area. Grazing has the potential to provide multiple system benefits: managing residues, reducing soil moisture depletion by cover crops, and creating an additional source of income.  

We just completed our first full season of this project, and we’re excited to delve into our recent data. Biosolids were applied in August 2020, and a mixture of cover crops and triticale (the cash crop) was planted in September 2020. The cover crop was a combination of multiple species including Austrian winter pea, sunflower, forage radish, and purple-top turnip. In the spring, we collected soil samples and measured the amount of biomass that had been produced by each plant type. Cattle were put on to the grazing plots in mid-May and grazed for about 3 weeks. We also collected triticale grain yield measurements in August 2021.  

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We found that, in the cover cropped areas, the biosolids application did indeed increase triticale plant biomass in spring compared to the fertilized and unfertilized treatments (Figure 3). However, in the non-cover cropped plots, triticale biomass was similar between the biosolids and fertilizer treatments. None of the cover crops produced very much biomass in any treatment due to the very dry conditions in fall 2020 and spring 2021. Going forward, we will adjust the cover crop mixture to choose species that are likely to be more successful. We also expect that treatment differences will become more pronounced over time with repeated applications of biosolids.

At the end of the season, triticale grain yields in the areas without cover cropping/grazing were ~12% higher than the synthetic fertilizer plots and 66% higher than the unfertilized controls. In the grazed areas, yields were more similar between the biosolids and fertilized treatments; yields with biosolids were 4.5% higher than with fertilizer and 34% higher than with no fertilizer. Yields in the grazed areas were lower overall, which is to be expected, but the grower we work with reported that the weight gained by cattle during grazing offsets this yield dip.

PhD student Madeline Desjardins has joined our WSU NWREC Soil Health Program to work on this project, and she is currently analyzing soil samples that were collected this spring. We are excited to dive into that data and to see what findings this project will bring in the coming years. Stay tuned!

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