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.
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).
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).