By Deirdre Griffin LaHue - Assistant Professor, WSU Mount Vernon

The Long-Term Dryland Biosolids Research site in Douglas County, WA (a.k.a. GP-17) has been an extremely valuable experiment over the last 27 years to study the effects of biosolids land applications on crop productivity, nutrient availability, fertilizer value, soil organic matter storage, and microplastics. The trial looks at 3 biosolids treatments (applied every 4 years) compared to synthetic fertilizer and an unfertilized control. In 2019, this experiment was selected to be one of the 120 sites sampled for the Soil Health Institute’s North American Project to Evaluate Soil Health Measurement (Figure 1). To be considered for this project, sites had to have been going for at least 9 years and have replicated soil management treatments. In addition to GP-17, one other long-term biosolids experiment in Colorado was selected. 


Figure 1. The Soil Health Institute selected 120 long-term experimental sites for their North American Project to Evaluate Soil Health Measurements. The long-term biosolids trial in WA is circled in red. Figure from Norris et al., 2020.

We worked with scientists from the Soil Health Institute on the sampling and in-field soil health measurements like saturated hydraulic conductivity (how well water moves through water). Their project team then conducted analyses on over 30 different soil properties related to biological, chemical, and physical aspects of soil health (for more background on the concept of soil health, see previous Research Short Stories here and here). The Soil Health Institute shared all of the data from the long-term biosolids site with our team, and a publication will soon be written to share all of the results. Here is a sneak peek with some of the major findings.

Available water holding capacity in the soil is crucial in this low rainfall region with dryland (non-irrigated) agriculture. Farmers count on stored soil moisture to help their crops grow, and organic matter additions from biosolids can help to make the soil hold on to more water, like a sponge. Results from this study show that biosolids applications at the 3 and 4.5 dry ton/acre rates have indeed increased available water holding capacity (Figure 2), which is likely one reason that crop yields have been higher in these treatments compared to unfertilized and fertilizer controls.


Figure 2. The soil’s available water holding capacity is increased by additions of biosolids at the 3 and 4.5 dry ton/acre rate. Different letters indicate statistically significant differences.

We also found that these two higher rates of biosolids increased soil protein content (Figure 3a). Soil protein is a relatively new soil health indicator measurement and represents a major pool of organic nitrogen in the soil. It is indicative of the quality of soil organic matter and the potential of the soil to store N and release it to plants and microbes. Soil protein may also help form aggregates and thus help with water movement and compaction resistance.

Higher soil N availability may affect soil pH over time, however, as the conversion of ammonium to nitrate can acidify the soil. We may be seeing evidence of this in the pH results as the higher biosolids rates have dropped the pH to below 5.0 (Figure 3b), a level at which issues with aluminum toxicity may arise. Thus, liming or another method of soil pH management may be required over time. Soil acidification is also common with synthetic fertilizer applications, particularly in no-till systems where fertilizer is concentrated in one area of the soil profile.

fig 3

Figure 3. Soil protein (left; A.) and soil pH (right; B.) results from long-term biosolids applications.

With results from the Soil Health Institute’s project, we see that biosolids applications to agricultural fields have improved some key properties that relate to healthy soil functioning, particularly when applied at a rate of 3 dry ton/acre or more (note that application rates must be determined based on soil nutrient levels; see the Worksheet for Calculating Biosolids Application Rates in Agriculture). Available water holding capacity and soil protein are both related to storage potential of water and nitrogen, two key resources essential for healthy and productive cropping systems. Stay tuned for more information from this rich dataset from our collaboration with the Soil Health Institute.



Norris, C.E., G. Mac Bean, S.B. Cappellazzi, M. Cope, K.L.H. Greub, et al. 2020. Introducing the North American project to evaluate soil health measurements. Agron. J. (March): 3195–3215. doi: 10.1002/agj2.20234.