Soils

Nature Notes by Dr. Frank Lang

Soil is that and living terrestrial layer of mineral particles, pore space, moisture, organisms that covers most of the surface of earth. Without soil, life as we know it would not exist. Dirt, on the other hand, is what is you find under your fingernails or what you sweep under the rug.

The erosive action of mechanical and chemical agents reduces bedrock, known in the soil trade as parent material, to soil particles of various sizes. Mechanical agents include freezing and thawing, heating and cooling, and the forces of wind and water. Chemical agents include the mixture of carbon dioxide from the atmosphere and water that forms weak carbonic acid or organic acids produced by lichens growing on rock surfaces that may hasten the breakdown of some rocks.

Erosion not only forms soil particles but transports them as well. Water transports alluvial soils. Their particles are characteristically smooth, and rounded by the constant bumping and grinding action of the water. For le, the presence of smooth rounded pebbles is evidence that water once flowed across the surface of Upper Lower Table Rocks in Jackson County, Oregon.

There are many different kinds of soils with different characteristics caused by a complex interaction of different materials, climates, and living organisms. As precipitation falls on the soil’s surface, the water moves through the soil profile dissolving and redepositing mineral components as it goes. In dry climates the water evaporate before reaching the water table, leaving behind a hardpan layer of calcium carbonate.

The resulting layers, called horizons, have different characteristics, different colors, different textures. Horizons, particularly the “0” or organic horizon, have a host of tiny and not so tiny arthropod inhabitants. Some credible adaptation to their soil homes. One such beast is the ant lion, the larval stage of the lace wing. Adults bear a general resemblance to dragonflies with short-bobbed antennae.

Ant lions make pits in sandy soil about the diameter of a silver dollar where the 0 horizon is missing. Perhaps you have seen them in your yard. Mine are underneath the strawberry tree on the south side of the house. Ant lions lie with fearsome pinchers just showing in the sandy bottom of the pit waiting patiently for an ant to stumble in. Ant lions are sensitive to vibrations and make quick work of foolish ants. Ants are grasped by the pinchers, immobilized by a secretion, predigested, then sucked dry. In case you are worried about your child or pet, ant lions are measured in millimeters.

Not only is parent material eroded to form soil, but wind and water erode soil. Unfortunately, soil is disappearing faster than it is being formed. The loss of top soil is an ecological and economic disaster of major proportions. Long forgotten are the lessons of the 1930s Dust Bowl of the American midwest, too much money to be made.

Try the Atlas on the Biology of Soil Arthropods by Eisenbeis and Wichard at the Southern Oregon University library for more information and great SEMicrographs. If you want to learn more about soil erosion try Worldwatch Paper 60, available from the Worldwatch Institute.

And don’t let me ever hear you say dirt when you mean soil.

— Dr. Frank Lang  

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