World Soil Day
Types of soil are categorized by a variety of features, including the amount of organic material, how readily water is absorbed, and how easily the soil can be worked. As soil scientists developed a classification scheme for the soils of the world during the late 20th century, they ultimately recognized twelve soil orders (the coarsest tier in the classification scheme). There were soils typical of grasslands (mollisols), deserts (aridisols), bogs (histosols) and so on. One of the last soil orders to be added to the list, in 1989, were soils in which volcanic ash was a major component. These are the andisols which comprise only around 1% of ice-free land surface in the world. In the continental United States, these soils are almost entirely within the Pacific Northwest. Many soils at Mount Rainier are andisols.
What does all of this mean for the plants and landscapes of Mount Rainier? In general, the glassy minerals in volcanic ash tend to result in soils with high water and nutrient holding capacities. Similar to how rivers deposit nutrient-rich sediments after flooding, episodic volcanic eruptions (whether from Mount Rainier, Mount St. Helens, or Mount Mazama) deposit new nutrients in the form of volcanic ash. The downside of these volcanic ash-rich soils at Mount Rainier is that the resultant glassy sand tends not to have much internal cohesion (as opposed to clay). This makes the soils easier to erode. Often the only protections these volcanic ash-rich soils have from erosion are plant cover and, sometimes, a cryptobiotic soil crust formed from algae, fungi, or lichen. All of these are vulnerable to disturbance and destruction by foot traffic. Help protect the unique soils of Mount Rainier by staying on trails!
NPS Photo of mountain heather mats holding together andisol soil in the high subalpine terrain along Burroughs Mountain Trail. ~kl
