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

****:TOPSOIL AND SUBSOIL

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 .