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Most of the land-applied biosolids in the US are used as an agricultural soil amendment, and this is true here in King County. About three-quarters of the Loop we produce are used to grow crops around Washington State. Farmers value biosolids because they increase crop yields--in some cases outperforming conventional fertilizers. They also improve soil tilth, increase water retention, and reduce erosion. King County has several long-time partners who use Loop to grow a diverse array of crops: Boulder Park Inc. and Natural Selection Farms Inc.

Boulder Park Project

Boulder Park Inc., located in eastern Washington’s Douglas County, is the largest farmer-owned biosolids cooperative in the nation. Formed in 1992 with just three partners, the program has grown to include more than 130 landowners and encompasses more than 75,000 acres of farmland. There is even a waiting list of neighboring farmers hoping to reap the benefits of biosolids for their lands. King County and more than 25 other biosolids producers from around the state deliver biosolids to this project.

Biosolids have shown numerous benefits at Boulder Park, helping create excellent harvests for farmers and sustain a reliable market for biosolids. Wheat germination and survival rates are both increased with biosolids, ultimately improving overall crop yields. High plant density helps retain soil moisture and inhibit weed growth, reducing water loss and herbicide demand. Biosolids also reduce soil erosion, allowing farmers to recover more straw from their fields while still leaving ample stubble to protect the soil from wind erosion.

BP_spreading2 After being delivered to the farm, biosolids are loaded into a spreader vehicle and applied to crop fields


Once the crops ripen, they can be harvested and brought to market.


Biosolids are applied at agronomic rates calculated to meet the nutrient needs of crops while protecting water quality

Boulder Park has also served as a living laboratory for long-term biosolids research. Studies conducted there by King County and our WSU soil scientists have confirmed that fields amended with biosolids produce crop yields equal to or better than those treated with conventional inorganic fertilizer. Repeated biosolids applications produced significant long-term increases in soil carbon, soil nitrogen, and plant available nitrogen.

Natural Selection Farms

Home to orchards and fields of hops, corn, and biofuel canola, Natural Selection Farms, Inc. have used biosolids to enrich farmland in the Yakima Valley since 1991. Over 35,000 acres are available for biosolids application with product coming from King County and other Washington biosolids producers. Natural Selection has been a leader for environmentally sustainable farming in their community. They believe holistic farm management practices should be used to promote crop production and maintain harmony with nature. Using biosolids supports tis approach to farming.


Biosolids are tilled into the soil in preparation for planting hops


which are later harvested


Natural Selection Farms also has vineyards (right), orchards, and other crops.

Biofuel research

In 2003, Ted Durfey of Natural Selection Farms partnered with researchers from the University of Washington to investigate whether biosolids would be an effective fertilizer for growing canola to be used for biofuel. A relative of mustard, canola is known for producing more oil per-acre than most other crops, making it a likely candidate for biofuels. The oil-rich seeds are crushed and processed to produce biodiesel, which has lower sulfur and greenhouse gas emissions than conventional petroleum-based fuels. The remaining plant parts can be used for cattle feed, while the biofuel production process creates glycerine for soaps and cosmetics. Overall, canola is a highly efficient crop with numerous benefits. The study results showed that canola grown with biosolids and without irrigation produced better yields of oil per-acre than any other treatment, demonstrating the effectiveness of biosolids for this application.


Researchers examine biofuel canola crops