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Municipal Compost Use in Agriculture: A Question of Cost and Value

Posted by Karen Hills | September 23, 2020

This is part of a series highlighting work by Washington State University (WSU) researchers through the Waste to Fuels Technology Partnership between the Department of Ecology and WSU during the 2017-2019 biennium. This partnership advances targeted applied research and extension on emerging technologies for managing residual organic matter.

Composting rather than landfilling organic waste, such as food waste and yard trimmings, has several benefits from a climate perspective. A recent study in Washington concluded that composting organic waste likely decreases greenhouse gas emissions from organic waste compared to landfilling (Jobson and Khosravi, 2019). Other benefits of composting organic waste include saving space in landfills, and producing a valuable organic product that can improve soil properties when applied to the landscape.

Dump truck finishes unloading organic waste onto compost pile.
Figure 1. Composting organic waste diverts this material from landfills and yields a product that improves soil properties. Photo: DVO, Inc.

The expansion of municipal composting programs has led to an increased supply of compost in many areas, including around Seattle, Washington. Agriculture could provide an outlet for large volumes of this compost. However, despite the increased supply of municipal compost, the interest from farmers in using it seems to have lagged. I was part of a project team at Washington State University that drilled into this question further, particularly the potential value of compost in agriculture.

The simplest explanation of this disconnect in supply and demand is that the price at which municipal compost is offered is greater than its value to farmers. Two issues seem to dominate this disconnect: 1) plastics contamination from municipal compost, which is important to resolve to expand markets, and 2) the value that compost can provide in crop production. Compost quality, particularly plastic contamination, has been a major barrier to interest from farmers in using municipal compost. Plastics and other contaminants get into the compost stream and are difficult and expensive to remove. Composters are actively working to reduce plastics contamination through a variety of methods.

Our team, on the other hand, delved into the second issue in the disconnect between supply and demand, and estimated both the cost and potential value of municipal compost for five crop scenarios. These estimates of cost and value were developed for western Washington, as that is where the supply of municipal compost has greatly increased.

Estimating the cost of compost was relatively straightforward. We received price quotes from a large regional compost supplier in the Puget Sound (Cedar Grove Compost) for a specific compost product, delivered. We selected our delivery locations by identifying agricultural regions that are relatively close to the Seattle metro area, since transportation is a large fraction of costs for a bulk amendment such as compost. This allowed us to estimate the per cubic yard cost for compost, including delivery and spreading, at around $27.

The more complicated task was estimating the value of compost, since compost’s value to a particular user depends upon both agronomic factors (crops and soil conditions) and economic factors (cost of inputs and crop market price). We used the available regional literature to estimate possible “good case scenario” first year yield increases due to compost application in both low value (wheat) and high value (raspberries, blueberries, direct-market mixed vegetables) crops. Information on prices received for these crops helped us translate yield increases to increased revenue per cubic yard of compost, which varied from $0.85 to $63.80, depending on the scenario (Figure 2).

Bar graph showing potential value of compost to five crop classes.
Figure 2. Comparison of cost and potential value of compost for a number of crop type scenarios. Enterprise budgets for direct market mixed vegetable scenarios were from British Columbia (BC) and Colorado (CO). Figure from data in Hills et al. 2019.

For a lower value crop, such as soft white winter wheat, the cost of compost greatly exceeded the potential value ($0.85 vs. $27.05; Figure 2). For direct market mixed vegetables, which command a high net revenue per acre, the opposite was true: the potential value of compost more than doubled its cost ($58.38 or $63.80 vs. $27.05). Compost’s value in blueberry ($19.93) was not enough to justify application while for raspberry, the potential value ($38.43) did exceed cost in this scenario.

Yes, these numbers are an estimate and do not account for the vast array of cropping systems, soil types, and soil management histories present on farms in western Washington, nor do they account for multiple-year benefits of compost application for crop production, nor the reduced greenhouse gas emissions relative to landfilling the organic waste. However, our results do put dollar amounts to something that has previously been quite nebulous—the value of compost.

Our results demonstrate two important points: 1) Compost can have a wide range of values, depending on the type of crop and soil condition to which it is applied; and 2) The value of compost appears to exceed cost for some higher value crops grown in western Washington. Our analysis also suggests that consideration of strategies to improve compost value by reducing contamination may be worth additional attention. In tandem with ongoing field trials to elucidate the farm situations where compost is likely to provide the greatest benefit, reducing contamination could bridge the disconnect between supply and demand of compost in higher value crops close to large urban areas, providing a climate benefit while enhancing food system sustainability and reducing the use of non-renewable agricultural inputs.

For more detail on this work, see the project summary (5 pages; Chapter 3  in Hills et al. 2019a) or the in-depth technical report (17 pages; Hills et al. 2019b).

References

This article is also posted on AgClimate.net.

6 thoughts on "Municipal Compost Use in Agriculture: A Question of Cost and Value"

  1. Eric Brennan says:

    Thanks for the interesting blog post. I have a question. Wouldn’t the potential value of the compost in a given cropping system depend somewhat on the application rate? (Perhaps this was covered in your more detailed report). I’m particularly interested in this from a vegetable production perspective. I’ve been doing research on compost and cover crops in Salinas, California for several years. Here’s a video showing some of the results
    -Lessons from long-term cover crop research in the “Salad Bowl of the World” YouTube. https://www.youtube.com/watch?v=JurC4pJ7Lb4

    1. Karen Hills says:

      Hi Eric,
      Thanks for the question and for the link showing the work in Salinas. To answer your question, yes, we would expect application rate to impact potential value of the compost in a given cropping system. In this work, we assumed application rates based on values found in the literature on the cropping systems we were considering (raspberries, blueberries, wheat, mixed vegetables). Since we were using that literature as a basis for yield increase assumptions, we used the application rates used in those studies. Assumptions about application rate and yield increases can really impact value so it’s important to note that these were hypothetical scenarios aimed at providing some actual dollar values to work with. I would expect lots of variation in different settings depending on soil type and management history. Hope that helps and thanks for reading!

  2. Leilani Wood says:

    Shouldn’t there be concern about other contaminates in the compost (residual chemicals and fertilizer from lawn clippings and unknown sources?) I would expect this fact to reduce the usefulness to food production. I never recommend to apply municipal compost to vegetable production. It is a hazard waiting to happen.
    Leilani Wood
    Horiculturist

    1. Karen Hills says:

      Hi Leilani,
      Thank you for your interest in the article. Commercial composters do regular testing for a variety of contaminants, but certainly we would encourage anyone applying compost to understand the testing protocols and sources of feedstock used by the company from whom they are purchasing compost.

  3. Neel says:

    Thanks for this post. Really interesting implications for closing loops in the food and agriculture economy.

    What were the major categories within the agronomic and economic aspects of compost value? Reduction in input costs (due to increased fertility)? Reduction in irrigation costs (due to increased water retention)?

    And did any of those categories have an outsized effect on the overall value calculation? Meaning, did you find that a particular aspect of the value of compost was really driving the overall value?

    1. Karen Hills says:

      Hi Neel,
      Thank you for your interest in this study. Compost value was calculated simply from assumed yield increases (based on values in the literature). Reductions in fertilizer costs and irrigation costs were not accounted for, in part because we didn’t have supporting information particular to these crops and regions. We took a fairly simplistic (and likely conservative) view of the value of compost. This work could certainly be expanded upon in the future, but our goal was to demonstrate the differences that exist between crop types, even without taking those additional sources of value into consideration.

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