Biosolids and forestry

Biosolids make an excellent soil amendment and source of nutrients for trees, as has been shown by decades of research in western Washington forests and elsewhere in the U.S. Biosolids were first used in 1987 for fertilizing plantations on the Snoqualmie Tree Farm in east King County. The Mountains to Sound Greenway Biosolids Forestry agreement expanded the program in 1995 to include state forests in the 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(external link) initiated this program which now includes several public and private partners: the state Department of Natural Resources, King County, Hancock Timber Resources and the University of Washington.

• Trees and understory: Nutrients provided by biosolids act as a fertilizer, stimulating healthy plant growth.
• Soil: Organic matter in biosolids improve soil structure, which reduces runoff and erosion.
• Soil Biota: Soil organisms benefit from nutrients in biosolids and the healthy soils it creates.
• Wildlife: Lush, healthy growth in the understory provides more food and hiding cover for animals.

Tree growth and biosolids

By counting the growth rings on a cross-section of a tree you can tell its age. This Douglas-fir tree was cut when it was about 30 years old. When it was 20 years old, the tree was fertilized with biosolids, resulting in the wider growth rings.

Why fertilize forests?

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

Growth response of trees

Using biosolids can lead to impressive growth response, because it supplies additional nitrogen to native forest soils. Biosolids increases growth rates to match those of trees found on more fertile soils. Water is another factor limiting tree growth during Northwest summers. The organic matter in biosolids helps improve the ability of soils to hold water and keep it available for trees.

University of Washington researchers have measured the growth of biosolids-fertilized trees for more than 30 years. Increases in growth are greatest in sites with coarse, nutrient deficient soils. Gains in diameter are larger than increases in height. Wood quality studies have shown that the grade and usability of lumber from biosolids fertilized trees is the same as other fertilized or fast-growing trees.