Research in soil improvement of forest stands with municipal sewage sludge has been carried out by the University of Washington College of Forest Resources during the last 13 years. In 1973 the College entered into an intensive research program, funded by the Municipality of Metropolitan Seattle, investigating the concept that municipal sewage sludge could improve forest soils and enhance tree growth. This program was and continues to be conducted at the University's Charles Lathrop Pack Demonstration Forest near Eatonville, WA.
Municipal sewage sludge contains a relatively high concentration of nitrogen (4-6%) in both available and organic forms. Virtually every other nutrient required for plant growth is also provided by sludge. Additionally, due to the organics and fine particles in sludge, it acts as an excellent soil
conditioner.
Experiments in sludge applications at Pack Forest for the most part have been conducted on soils of low productivity, which have an advantage for test plots. These soils are typically nutrient deficient and coarse textured, and thus positive growth results are quickly evident. Additionally, these soils are well to excessively drained, and constituents which are not utilized or stored in the system are quick to show up in leachate and drainage waters. A variety of tree species have been grown in sludge amended soil in test plots in these studies.
This presentation will include some of the 13-year history of the Pack Forest Program which has led to development of appropriate application techniques and rates and expected growth response of trees of the Pacific Northwest, and has also aided in the development of national, state and local guidelines.