The Soil Under Our Feet

The beauty of Mt. Lebanon lies with its trees, flower gardens, and lawns. These plants depend on the soil for their water and nutrients, yet how many of us really understand what soil is? Contrary to our casual reference to the material, soil is not ‘dirt.’ Soil is a valuable resource that is made up of mineral particles, water, air, and organic matter (humus). By definition, soil supports plant growth.

Soil forms from the breakdown of rocks by weathering processes over a very long period of time. For example, a typical natural soil in southwestern Pennsylvania may have formed over a period of 20,000 to 40,000 years. In Mt. Lebanon, the parent material for the soil is the sandstones, shales, and limestones that form the bedrock. These rocks have broken down through time into a soil profile that is generally 30 to 60 inches deep. Based on soil mapping from the U.S. Dept. of Agriculture, the Mt. Lebanon area is underlain by three strongly weathered alfisol soil types referred to as the Culleoka, Dormont, and Guernsey Series. Each of these are a variety of silt loam to clay loam. The term loam refers to a soil that is considered the best mixture of sand (~40%), silt (~40%), and clay (~20%) for plant growth. However, these natural soils exist in a relatively undisturbed state in only a very few areas in Mt. Lebanon such as Bird Park, Twin Hills Park, the golf course, and the cemetery. Elsewhere, the soils are considered ‘Urban’ as they have been significantly altered by earth moving equipment, removed, or covered with fill. I’m sure many of you have found fragments of building materials (bricks, gravel, etc.) and other debris in your garden.

A typical natural soil has several layers or horizons (see figure). The topmost layer is called the ‘O’ horizon. It is made up of decomposed organic material such as leaves and twigs. The next layer down is called the ‘A’ horizon and it is what many people would refer to as topsoil. It is 8 to 10 inches thick and is dark colored because it is made up of both mineral and decomposed organic matter. Below this is the ‘E’ horizon. This horizon is 5 to 6 inches thick and is composed of light-colored materials resulting from the leaching of clay, calcium, iron, and magnesium to lower horizons. The yellow-brown colored ‘B’ horizon is the thickest, up to 35 inches thick. It is composed of clay, iron oxides, carbonate and other material leached from the overlying horizons. The ‘B’ horizon is commonly referred to as the subsoil. The 5-15 inch thick bottom layer is the ‘C’ horizon that is made up of weathered bedrock material. The entire soil profile sits above unaltered bedrock (R). The soil layers are in a very fragile chemical balance of organic decay and the transfer of chemical ions. When natural soils are disturbed by urban development, the chemical system is destroyed. It will take thousands of years for the soil to reestablish the natural layer sequence.

Soil is an ecosystem teeming with life that we can and cannot see. For example, in one tablespoonful of soil, there are more bacteria than there are people on the entire planet. A quarter of a million of them could sit on the period at the end of this sentence. They can live in air, water, extremes of heat and cold, and are able to function without sunlight. Most bacteria break-down dead organic matter into components that can be used directly by plants. Other kinds of bacteria can take nitrogen from the air in the soil and convert it into nitrates that are also needed by higher plants for growth. Fungi are also essential to the breakdown of woody organic matter. Soil is teeming with unseen microscopic animals – from simple-celled amoebae and protozoa living their lives in the soil moisture, through nematodes that can damage the roots of the plants we want to grow. Of course, we cannot forget earthworms. These creatures play a huge part in mixing organic matter from the surface into the lower depths of the soil, and in doing so, they provide the source of food for countless numbers of other organisms who feed on the organic matter. Their burrowing also leaves huge (by comparison) aeration channels and fissures in the soil, along which air can diffuse and water drain. It is estimated that there are somewhere between 200 and 500 worms in every cubic yard of your garden.

Our natural soils are ideally suited to support the indigenous Appalachian Oak Forest community here in western Pennsylvania. This forest contained various oaks, hickory, tulip poplar, and white pine.

Natural soils are a dwindling resource with the continued urban sprawl. Because we cannot ‘make’ natural soils, we should strive to protect areas where they still exist.

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