Forest Preserve District of Cook County, Illinois
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Topsoil and Subsoil
Nature Bulletin No. 372-A   March 7, 1070
Forest Preserve District of Cook County
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation

The primeval rocks, weathered and broken down by air, sunlight and water, produced soil materials which were distributed by water and wind. Then appeared the algae, fungi, lichens, mosses, herbaceous plants, and woody plants such as trees and shrubs -- in that order -- also animals, and finally man. With out soil, land plants could not grow; plant-eating animals could not live, and the meat-eating animals would perish. The soil-making process is still going on but it may take 1000 years for nature to make one inch of topsoil.

In agriculture, topography and climate and the type of soil determine what kind of crops can be grown in any area and, therefore, what livestock can be raised. Plants take food from the soil -- elements dissolved by rain water -- and using energy from sunlight, manufacture our food and that of our animals: fats, proteins, starches, sugars, minerals and vitamins. The ten most essential elements are oxygen, hydrogen, nitrogen, carbon, phosphorous, potassium, calcium, sulfur, magnesium and iron, but plants differ widely in how much of each they require for lusty growth and soils vary in how much they contain.

We eat plants, or plant products such as milk and eggs. The growth and health of animals, and our own, depend on the presence of sufficient amounts of these essential elements in a soil and how much is stored in the plants grown there. Also, tiny amounts of certain "trace elements" -- copper, boron, zinc, manganese, cobalt, chlorine and iodine -- should be present, and domestic animals are seriously affected by the absence of some of these from their diet. So are we.

Soil is more than decomposed rock, or rock ground to powder by glacier (such as clay), enriched by decayed plant and animal matter. It is extremely complex and there are many types resulting from differences in climate, underlying rocks, native vegetation, surface conditions such as slope of the land, and age. In general, soil lies in layers called "horizons". The upper layer or topsoil is the "A" horizon. In forests and prairies, this is covered with humus -- decayed and decaying remains of plants and animals. The topsoil, which may be a scant few inches or several feet deep, merges with the mineral subsoil, the "B" horizon, such as clay, or loess, or muck. This contains some of the elements needed by plants, in forms which must be changed before they can be used. Beneath is the "C" horizon -- "parent material" such as glacial drift or rock weathered by exposure ages ago but still unchanged by soil-building processes -- resting on the unweathered and unchanged parent material.

It is a mistake to think of soil as a dead inert substance. Imperceptibly, as the centuries pass, it changes and usually becomes more fertile if undisturbed. It is penetrated and made porous by the roots of grasses, herbs, shrubs and trees that live and die. Bacteria, minute plant and animal life, fungi, molds, insects, centipedes and millipedes, worms, and burrowing mammals live in the soil -- making it more porous, mixing it, and enriching it. Most of these are in the humus and the upper inches of topsoil. There may be as many as 60 million bacteria in a single crumb of surface loam; three thousand humus-eating mites and tiny spiders in the top 3 inches of a square foot of ground; hundreds of thousands of ants or a million earthworms in an acre.

Due to wasteful farming and forestry methods, we have lost over 50 million acres of our best croplands. Over 100 million acres more are badly damaged. City folks, as well as farmers, are vitally concerned with soil conservation .

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