High rate of compost application showed stable yields in dryland wheat systems: a long-term study

Authors: Shikha Singh & Ian Burke, Washington State University

Compost is used as a nutrient source as well as soil conditioner with a number of noted benefits in agricultural production systems. Compost application improves soil structure, soil moisture retention and builds up soil organic matter (Doran, 1996).

Compost application has also been shown to activate soil microbial communities that help in nutrient cycling. Past studies have reported that compost helps improve soil water infiltration and binds the soil particles, helping reduce erodibility (Nigusse et al., 2015). All in all, compost helps provide nutrients to plants, improve soil health, and restore degraded soils.

Despite the direct and indirect benefits of applying compost to soils, its adoption among dryland wheat farmers is not very popular yet for three primary reasons:

1. Transportation cost—It costs money to move anything from one location to another, and the dryland farmers in rural Eastern Washington must foot the bill to get the compost to their fields. This can get pricy, especially when you add to the cost of the compost itself. However, Washington has taken steps to increase the availability of compost in rural locations by increasing the number of composting facilities across the state, which will increase availability and reduce transportation costs. Producers can also receive reimbursement for up to 50% of their compost costs through the Washington State Department of Agriculture.
2. Lack of research in dryland wheat cropping systems—This article covers, to the best of our knowledge, the longest-running compost application trial in dryland wheat systems, so more studies are needed to determine the benefits and challenges in other cropping systems and precipitation regimes. We are conducting research that will help inform dryland wheat producers about the rates necessary for effective compost applications.
3. Compost quality—The variety of feedstocks, composting methods, and potential contamination sources from municipal compost pose risks for compost quality. If the source has not been composted well, potential risks include weed seeds that create a headache and economic burden for producers, and other contaminants like PFAS, which are chemicals that aren’t broken down during the composting process. More rigorous testing in composting facilities will improve compost quality over time and reduce contaminants.

You can read more about the use of compost in Washington in a report to the legislature from Washington State University’s Center for Sustaining Agriculture and Natural Resources.

A long term study

Researchers are interested in learning about the effects of one-time applications of compost on the soil and crops. To test the effects of different one-time rates of compost applications, a long-term trial was established at the Wilke farm in Davenport, WA (close neighbors to the Dryland WaSHI LTARE site).

Details about this field site:

• No-till dryland wheat-based rotation
• Silt loam soils, including Broadaz and Hanning Silt Loam.
• 13” of annual precipitation (much of which comes in the winter months)

Seven years ago, in the Fall of 2016, four different rates of compost were applied: 0, 4, 10, and 20 tons per acre. A control was also established where typical fertilizer rates were applied. The compost was sourced from municipal composting in Spokane.

These different rates of compost were evaluated under two cropping systems: Winter wheat/Chem fallow and Winter wheat/Chem fallow/Winter pea/Chem fallow, which are both typical rotations of the dryland farmers in the region. The winter wheat cultivar used in this study is Otto.

To evaluate the effect of each rate of compost, winter wheat yield and quality data were collected, and soil samples were collected and analyzed for soil nutrients (check out more about soil testing with these resources). But the most interesting results have come from the wheat yield and soil moisture.

High rates = high impact

In both crop rotations, wheat yields have been stable, but the highest rate of compost (20 tons/acre) has shown the highest yields in most years since 2016.

One thing to keep in mind is that none of the areas treated with compost received external fertilizers—which means that the compost is supplying sufficient essential crop nutrients, resulting in the highest yields.

With the large amounts of compost being applied, you might expect to see increases in soil organic matter. One of the tricky things about soil organic matter is that it takes time to increase. The microbes in the soil, while working diligently, can only stabilize the components in the compost that will eventually make up soil organic matter so quickly. There is an upward trend with the highest application rate (20 tons/acre), but it is not statistically significant (data not shown). That is the beauty of a long-term trial—we can see what it might look like in a few years!

Storing more water for future use

The piece of this work that is most encouraging to dryland farmers revolves around soil moisture. The farmers in the region typically use a fallow, or a time when a field is not growing a crop, to help preserve soil moisture for the subsequent cash crop.

This study showed that seven years after the compost applications, soil moisture levels are higher where the compost was applied, especially at the highest application rate (20 tons/acre). We think this could be due to the mulching effect of the compost.

If the soil is bare, water escapes through the surface and returns to the atmosphere. This isn’t great, especially when every little bit of water in these dryland cropping systems is important.

Mulching can create a barrier between the soil surface and the atmosphere, reducing the amount of water that moves upward through capillary action. The compost in this trial may increase the water-holding capacity of the soil by slowly increasing soil organic matter while simultaneously reducing water loss to the atmosphere through the mulching effect.

We aren’t the first to see results like this; we found another study from Utah State University, which also reported that a one-time application of compost showed its effectiveness after 16 years of application (Reeve et al., 2012). Looking at the soil and yield benefits from a single application of compost, we think it will have practical and economic appeal to the farmers. Therefore, our results indicate that a high rate of compost application, near 20 tons/ acre has the potential to be used as a soil amendment for dryland wheat production systems.

If you are looking for ways to make compost applications work for you, check out these resources:

• WSDA Compost Reimbursement Program
• Soil Health Financial Resources webpage ‘search for compost’

References

Doran, J.W. (1996) Soil Health and Global Sustainability. Soil Quality Is in the Hands of the Land Manager.

Nigussie, A., Kuyper, T.W. and de Neergaard, A. (2015) Agricultural Waste Utilisation Strategies and Demand for Urban Waste Compost: Evidence from Smallholder Farmers in Ethiopia. Waste Management, 44, 82-93.

Reeve, J. R., Endelman, J. B., Miller, B. E., & Hole, D. J. (2012). Residual effects of compost on soil quality and dryland wheat yield sixteen years after compost application. Soil Science Society of America Journal, 76(1), 278-285.

About the Author

Shikha is is a Research Assistant Professor at the Washington State University Lind Dryland Research Station where her research focuses on soil health and soil management.

This article was originally published by the Washington Soil Health Initiative. For more information and resources on soil health, visit the WaSHI website and subscribe to the WaSHI newsletter.