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Decades of research in western Washington forests have shown that biosolids make an excellent soil amendment and source of nutrients for trees. Trees grow faster and wood quality is similar to other fertilized trees or trees grown on fertile soils.

Since 1987, Loop has been used to fertilize forest plantations in east King County. King County's forestry projects are part of a unique program to protect and enhance forests and wildlife habitat along the scenic I-90 corridor east of Seattle. The nonprofit Mountains to Sound Greenway Trust  initiated this program, which now includes several public and private partners: Washington Department of Natural Resources, King County, Hancock Timber Resources, and the University of Washington.

Why Fertilize Forests?

Because forests grow slowly on the nutrient poor soil left by the glaciers 12,000 years ago, forest managers often fertilize trees to increase growth. Plantation forests are typically fertilized with urea, a commercial fertilizer that provides only nitrogen, while Loop provides nitrogen and all other essential plant nutrients, including phosphorus, potassium, zinc, boron, magnesium and calcium. Decades of forest research in western Washington forests and elsewhere in the U.S. have demonstrated that biosolids make an excellent forest fertilizer.

Biosolids provide many benefits when applied to forest sites:

  • Trees and understory: Nutrients provided by biosolids stimulate healthy plant growth
  • Soil: Organic matter improves soil structure and helps soil hold water, reducing runoff and erosion
  • Soil Biota: Soil organisms benefit from nutrients in biosolids and the healthy soil it creates
  • Wildlife: Lush, healthy growth in the understory provides more food and hiding cover for animals

In addition to fertilizing trees, biosolids produce healthy understory vegetation, providing better forage for herbivores and higher quality habitat. In fact, research by the University of Washington has shown that wildlife prefer areas treated with biosolids and produce more offspring than animals living in unfertilized areas because of the higher nutrient content of the vegetation (Anderson, 1983 & 1985).

Snoqualmie forestKing County has vast expanses of working forests

tree_ringsThis tree grew much faster after biosolids fertilization, resulting in larger yearly growth rings

Applying Loop in the Forest

King County uses a rigorous process to ensure that our forestry operations maximize the benefits of biosolids while protecting the environmental quality. The process begins with site selection, which is performed in consultation with landowners and scientific advisors from the University of Washington. Good candidate sites should have gentle terrain, well-drained gravelly or sandy soil, adequate road access, few streams or drainages, and be well-stocked with trees.

Once a site is chosen, soil samples are collected to determine existing soil nitrogen levels. This information is used to calculate the proper amount of biosolids to be applied to each site to meet the nitrogen needs of the trees and understory plants. The goal is to add enough biosolids to stimulate growth but not allow buildup of excess nutrients that could leach to nearby water bodies. Typically, rates between 3-7 dry tons per-acre are appropriate, with young, fast-growing stands receiving higher rates than more mature areas. Loop biosolids become part of the soil as they gradually decompose and enrich the upper layers of soil, where plant roots take up nutrients.

Environmental Safeguards

Forest sites are managed in accordance with state biosolids management guidelines, including careful site selection and layout, correct nitrogen application rates, and monitoring of soils and streams. These best management practices were based on years of research, monitoring, and operational experience.

Additional precautions are taken to protect water quality near forest application sites. Before biosolids are applied, thorough surveys are conducted to identify water bodies and establish buffer zones to protect streams and wetlands. No biosolids are applied in these buffer areas, which range in size from 50 feet to 200 feet, depending on site-specific conditions. 

The county performs regular stream monitoring to verify that surface water quality is protected. Stream sampling sites are located near application areas. When possible, upstream samples are also collected for comparison. Monitoring is performed three times per year to capture seasonal fluctuations in ambient water quality. Sampling is also performed during storm events to verify that site buffers are adequate to prevent runoff. Two decades of monitoring forest streams near biosolids sites has shown no changes to surface water quality from biosolids fertilization.

Since the 1970's, UW researchers and others have conducted studies on the effects of biosolids applications on forest wildlife. From soil microbes to larger animals, biosolids benefit the forest ecosystem. Biosolids create healthier soil by providing food for soil microbes and plants. After application, forest vegetation grows thick and lush, and the activity of soil organisms mixes biosolids into the soil. Because of this increased plant growth, herbivores such as deer and small mammals have a more abundant and nutritious food supply, more protective cover, and produce more offspring.

forest_spreaderBiosolids being applied in the forest.

 SoilSample600Sampling a soil pit in the forest

stream testingStream water sampling

forest wildlifeDeer, elk, and small mammals benefit from enhanced growth of forest vegetation.


Public Involvement

Neighbors and interested parties are welcome to visit or comment on these projects. Contact King County biosolids to submit comments or arrange for a tour. If you are visiting one of our forest sites and see signage indicating restricted access due to a recent biosolids application, please be sure to heed the instructions on the sign.


For more information about Loop biosolids in the forest, contact Brian Vrablick, Forestry Project Manager.