Commercial Compost Application on Western Washington Farms

by Doug Collins, Hallie Harness and Andy Bary, WSU

Reprinted with permission in BioCycle March/April 2016, Vol. 57, No. 3, p. 63; visit for more articles about compost utilization.

Over the past several years, the Snohomish Conservation District has been partnering with Washington State University Snohomish County Extension on the Compost Outreach Project. 

The District has participated in the compost trials by: 

  • generating landowner interest 
  • participating in trial set-ups 
  • helping at harvest
  • contributing at stakeholder meetings

Demonstrating the benefits of compost to local producers has the potential to close local waste stream loops while building the tilth of cultivated soils. 

Combining diverse skills on this project has created relationships and opportunities that will allow all institutions and landowners involved to continue to work towards these goals even after the grant has ended. It has also contributed greatly to our knowledge of the effects of compost in commercial agricultural operations

                - Carrie Brausieck, Resource Planner

With at least 13 commercial composting facilities, and more than 900,000 tons of food scraps and yard trimmings composted annually at these facilities, Western Washington is at the forefront of organic materials recovery. Although compost is available on a large scale, agricultural markets make up less than five percent of the total compost market in Washington State. The Washington State University (WSU) Compost Outreach Project is working to evaluate the benefits of compost on local crops and address the challenges faced when using compost.

Since 2011, WSU Cooperative Extension in Snohomish County has collaborated with local compost producers, county offices and local conservation districts to promote and evaluate use of commercial food scraps and yard trimmings compost on farms in Snohomish and northern King County (WA) through compost use trials. 

While farmers are consistently seeking sources of organic matter, in 2015, 81 percent of farmer respondents (35 out of 43 WSU Compost Trials Participants) had not used food scraps and yard trimmings compost prior to participating in the trials. Local compost producers, Bailey Compost, Cedar Grove Composting and Lenz Enterprises, have donated over 4,500 tons of compost to the project since 2011, with the goal of expanding its use in agriculture.

The research trials and on-farm demonstrations conducted as part of the Compost Outreach Project are described in this article. Scientific research trials validate use of compost on local crops, while demonstration trials provide the opportunity for farmers to get firsthand experience using commercial compost and test it out in their operations.

Correspondence and focus groups with farmers in Snohomish and King counties have revealed several challenges to using compost. The most significant barriers to using more compost in agriculture are compost price, compost spreading (time and equipment), and lack of information.

Pre-2015 Research Trials

Research trials conducted through the program prior to 2015 compared two treatments: A growers’ Business As Usual (BAU) chemical fertilizer application vs. BAU + Compost. Trials took place on several farms in Snohomish County. At Carleton Farm, trials evaluated the effect of cumulative multiyear compost applications. In 2012, two years of compost application (approximately 20 dry ton/acre) increased pumpkin yield by 28 percent. In 2013, with three years of compost application (2013 application rate was approximately 15 dry tons/acre), sweet corn ear weight increased by 24 percent. In 2014 at Carleton Farm, no additional compost was applied and the three previous years of compost application resulted in a 35 percent increase in cucumber yield. 

In 2014 at Darrell Hagerty Farms, a light application rate (6.5 dry tons/acre) of registered organic compost increased organic green bean yield by 19 percent. Beet seed at Williams Farm showed a 21 percent increase in yield with a 20 dry ton/acre application. Each of these results was statistically significant and used commercial food scraps and yard trimmings compost. 

On-Farm Demonstrations in 2015

There were 49 demonstration trials in 2015, which involved qualitative observation of crop growth with compost applied next to a no-compost treatment. 

Crops included sweet corn, hay, mixed vegetables, berries, tomatoes, pumpkins, Christmas trees, salad greens, cut flowers, hazelnuts, brassicas, and more. 

Farmer feedback was collected through the Compost Outreach Project’s annual survey (conducted since 2012). Farmer collection of yield and/or soil testing data is optional in the demonstration program. While the drought in 2015 posed significant challenges, farmers reported that compost improved crop production in 68 percent of the trials (out of 47 trial crops). Fifty-five percent of farmers found compost increased soil water retention.

Christmas tree farmers have observed improved tree growth and health, and hope to sell the trees mulched with compost earlier than anticipated. This translates to potential increased profit for these growers. A farmer using compost on sweet peppers reported larger and more productive plants. Blueberry plants have thrived in rows mulched with compost, and compost consistently has shown positive crop yield and health results on pumpkins. Several participants reported the compost did nothing. There was no obvious observable effect of the compost on their crops.

2015 Research Trials

Experimental Design

Research trials in 2015 were designed to evaluate the nitrogen contribution from compost as well as changes to soil physical properties on two separate farms with sweet corn as a crop. The design was a replicated strip-plot experiment where compost was either applied or not applied in strips and nitrogen fertilizer (urea) was broadcast preplanting at four different rates within the strip, including a zero-N application. The authors hypothesized that compost would compensate for some nitrogen deficiency through mineralization of the organic nitrogen in compost to plant-available nitrogen. 

A different high rate of nitrogen fertilizer was chosen at each farm based on preseason soil testing and estimated nitrogen contribution from organic matter. In addition to the high rate, three other rates were applied for a total of four, where “X” is the full rate: 1.0X, 0.75X, 0.5X, and 0.0X. Corn ear weight, plant biomass, soil nitrate concentration, and bulk density were evaluated. Soil nitrate concentrations are an indication of nitrogen availability for plant uptake.

The two collaborating farms (A and B) have been involved in the Compost Outreach Project since 2011. At Farm A, compost (from Cedar Grove) was applied at a rate of 7.8 dry tons/acre and at Farm B, compost (from Bailey) was applied at a rate of 8.6 dry tons/acre. The difference in application rates was due to differences between manure spreaders used at each farm. At Farm A, the 1X rate of nitrogen was 196 lbs/acre and at Farm B the 1X rate was 100 lbs/acre. Other pre-experiment soil properties are shown in Table 1.


Neither corn ear weight or plant biomass were significantly affected by fertilizer or compost. Mid-season soil nitrate concentrations were not affected by compost, but were significantly affected by fertilizer nitrogen application (Figure 1). 

The mid-season soil nitrate test was meant to be taken around the same time farmers would test their soil to decide if a sidedress application of nitrogen is necessary. This test, also known as the pre-sidedress nitrate test (PSNT), can be used to guide mid-season nitrogen applications. 

Fertilizer rates should be made based on soil nitrate levels when sweet corn is at the five or six leaf stage. If soil nitrate levels are less than 10 ppm, then as much as 145 lbs N/acre are recommended. If mid-season nitrate levels are greater than 40 ppm, then perhaps no fertilizer nitrate is necessary (Hart, 2010). 

Mid-season nitrate levels were nearly 100 and 175 ppm at the zero nitrate fertilizer rate, much greater than what would suggest that crops would likely be deficient in nitrogen. There was likely no compost or fertilizer effect on crop yield because of naturally high levels of available nitrogen from previous management. Fields with a history of application of manure or other organic amendments are not likely to result in a yield increase from compost. In previous experiments on different fields, compost resulted in a 20 percent or larger increase in yield on several specialty crops. 

Bulk density was decreased by compost applications at both farms, though the effect was only significant at Farm B where there was a 6 percent decrease (Figure 2). Bulk density (weight/volume) is a measure of soil compaction. Practices that improve soil structure (cover cropping, reduced tillage, or organic matter application) can reduce soil bulk density. The 2015 research study designed to evaluate the effects of fertilizer and compost use will be repeated again in 2016.

Additional Project Activities

Farmers have continually pinpointed compost price, spreading (equipment and time), compost delivery, plastic contamination of compost, and lack of information as challenges to using compost. Educational workshops and presentations have increased farmer knowledge of when and how to use compost. An ongoing dialog with composters and farmers is shaping a mutually beneficial relationship. 

Conservation districts continue to enhance their focus on compost education, targeting farmers and landowners. Snohomish and King County Solid Waste Divisions, with support from Waste Management, continue to develop and expand the agricultural end use market to ensure the success of the local composting industry and the continued availability of compost for use on local farms.

Nitrogen fertilizer was applied at different rates in the 2015 trials based on preseason soil testing and estimated nitrogen contribution from compost. 

The Compost Outreach Project has achieved notable success, working with 73 farmers since 2011. In 2015, 62 percent (23 out of 37 participating farmers) reported they are motivated to continue using compost and nine farmers purchased loads of compost outside of the program in 2014 and 2015. 

The Compost Outreach Project continues to leverage diverse funding sources and partners to break down barriers to increased farmer use of compost. Financial support comes from Snohomish County, a Washington State Department of Agriculture Specialty Crop Block Grant, and King County. Additional partners include Snohomish and King conservation districts, compost producers, and Waste Management.           

Doug Collins is Extension Specialist at Washington State University. Until recently, Hallie Harness was the Program Coordinator of the Compost Outreach Project at WSU Snohomish County Extension. Andy Bary is Soil Scientist at Washington State University Puyallup. Photos courtesy WSU Extension


Hart, J.M., D.M. Sullivan, J.R. Myers, and R.E. Peachey. 2010. Sweet Corn, Western Oregon. Oregon State University Extension, EM 9010-E.