What are “boutique” soils?

An old adage goes “the apple doesn’t fall far from the tree” and the same goes for soils formed by weathering of bedrock. If you are a regular reader of Soils Matter, you will recall that soil is formed thought a process called CLORPT. (If not, read here!) Bedrock is one of the materials that can be the “Parent” material. And, when bedrock goes through the CLORPT process, it can become some unusual – and beautiful –soils!

yellow flowers on a field of lava rock

Soils that formed over this lava flow area can support a limited amount of plant life. Credit: Hannah Kang @hannahandplants IG

Bedrock is the solid rock underlying soils and human structures. It becomes soil through hundreds to hundreds of thousands of years of being exposed to the elements, rain, snow, roots, and microorganisms. Although you might expect thousands of years would leave soil completely unrecognizable from its parent material, it still resembles its original bedrock in many ways.

A soil’s physical appearance and its chemistry still largely mirrors the rocks it came from. For most soils, they inherit a wealth of inorganic nutrients from their parent rocks. This includes elements like potassium, calcium, magnesium, and iron. They also inherit their earthy colors of browns and reds from the weathering of their primary minerals, particularly iron.

Most soils are able to support plants and healthy ecosystems. They can provide structure for roots and retain water and minerals to feed the plants. But not all bedrocks give rise to these types of soils. There are “boutique bedrocks” found across the world that would challenge your idea of what a soil looks like and what it can do.

greyish rocks and soils

Serpentine soils come from the aging (CLORPT) of serpentinite, a rare type of rock. Credit: Hannah Kang @hannahandplants IG

A great example of a boutique bedrock and its effect on soil is serpentinite and serpentine soils. Serpentinite is an ultramafic rock. This means its mineral content is low in calcium and potassium, but high in of magnesium and iron. Serpentinite is quite rare. It is formed by minerals being exposed to high temperatures and pressures in the presence of water. These types of geologic processes generally only happen near volcanoes and subduction zones.

Because serpentinite has very little calcium and potassium relative to other bedrocks, like granite, trees struggle to grow in serpentine soils. Calcium is needed for healthy root and shoot growth. So most serpentine soils are mostly home to tough grasses and stunted trees. Otherwise, they lie barren.

soil layers showing blue bottom layer

The author found this soil with an unusual blue color deep within a soil profile. Its parent material was a phyllite-limestone rock. The layers above the Cr layer came from glacial action. Credit: Justin Richardson

Another aspect of a boutique soil is inheriting an interesting physical trait. During my research, I stumbled upon a blue soil color I did not expect! It was not in the Munsell color system that soil scientists use. This is part of the color identification system first created by artist Alfred Munsell. The Natural Resource Conservation Service, NRCS, started to use the Munsell system to identify soil color over 55 years ago.1

In the area where I was digging, I found a light blue layer of the soil. It had formed from the weathering of phyllite-limestone metamorphic rock (the Parent material). Soil scientists call the various layers found in soil “horizons.” This horizon is designated as Cr because it still strongly resembles the bedrock, but was soft and could be dug using a shovel. There were many plant roots and earthworm burrows working to turn the rock into soil. The brown A and B horizons above the light blue Cr horizons are not related. The A and B horizons had been developed from material left behind by glaciers. As the light blue Cr horizon ages and is exposed to oxygen (rust) it will turn a more earthy brown.

Answered by Justin Richardson, University of Massachusetts-Amherst

  1. http://munsell.com/color-products/color-communications-products/environmental-color-communication/munsell-soil-color-charts/

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