Compaction results from loss of soil pore space, often due to traffic from heavy equipment, particularly when the soil is wet. The ideal amount of pore space in soil is 50% of the volume. Compaction limits root growth, as well as water storage and drainage. Controlled traffic and improved aggregate stability can help to reduce compaction.
Resources
The Science of Soil Health: Compaction 4 1/2 min video produced by NRCS (general information, not specific to WA).
Impacts of green manures WSU Extension, Grant and Adams Counties.
Soil health in orchards Tianna DuPont, David Granatstein, and Bernardita Sallato, WSU Extension. October, 2018.