Fundamental Soil Mechanics - Mathematics and Statistics for the Environment - page 307

But, given enough time—perhaps a millennium—the scars heal over, and a new, virgin layer of
soil forms where only bare rock once existed. The series of events taking place in this restoration
process is known as bare rock succession. It is indeed a true “succession”—with identifiable stages.
Each stage in the pattern dooms the existing community as it succeeds the state that existed before.
Bare rock, however it is laid open to view, is exposed to the atmosphere. The geologic processes
that cause weathering begin breaking down the surface into smaller and smaller fragments. Many
forms of weathering exist, and all effectively reduce the bare rock surface to smaller particles or
chemicals in solution. Lichens appear to cover the bare rock first. These hardy plants grow on the
rock itself. They produce weak acids that assist in the slow weathering of the rock surface. The
lichens also trap wind-carried soil particles, which eventually produce a very thin soil layer—a
change in environmental conditions that gives rise to the next stage in bare rock succession.
Mosses replace lichens, growing in the meager soil the lichens and weathering provide. They
produce a larger growing area and trap even more soil particles, providing a more moist bare rock
surface. The combination of more soil and moisture establishes abiotic conditions that favor the next
succession stage. Now the seeds of herbaceous plants invade what was once bare rock. Grasses and
other flowering plants take hold. Organic matter provided by the dead plant tissue is added to the
thin soil, while the rock still weathers from below. More and more organisms join the community
as it becomes larger and more complex.
By this time, the plant and animal community is fairly complicated. The next major invasion is
by weedy shrubs that can survive in the amount of soil and moisture present. As time passes, the
process of building soil speeds up as more and more plants and animals invade the area. Soon trees
take root and forest succession is evident. Many years are required, of course, before a climax forest
will grow here, but the scene is set for that to occur (Tomera, 1989).
Today, only the remnants of the former, incomparable pyramid-shaped peak are left in the form
of soil—soil packed full of organic humus, soil that looks like mud when wet and that, when dry,
most people would think was just a handful of dirt.
12.2 SOIL: WHAT IS IT?
In any discussion about soil (after air and water, the third environmental medium), we must initially
define exactly what soil is and explain why soil is so important to us. Having said the obvious, we
must also clear up a major misconception about soil. As the chapter's introduction indicates, people
often confuse soil with dirt. Soil is not dirt. Dirt is misplaced soil—soil where we don't want it,
contaminating our hands or clothes, tracked in on the floor. Dirt we try to clean up and keep out of
our environment.
But soil is special—mysterious, critical to our survival, and, whether we realize it or not, essen-
tial to our existence. We have relegated soil to an ignoble position. We commonly degrade it—we
consider only feces to be a worse substance, but soil deserves better. Before moving on, let's take
another look at that handful of “dirt” that our modern man is holding after the mountain peak was
crafted into soil by the sure hand of Nature over millions and millions of years.
What is soil, really? Perhaps no word causes more confusion in communications among various
groups of laypersons and professionals—environmental scientists, environmental engineers, spe-
cialized groups of earth scientists, and engineers in general—than the word “soil.” Why? From the
professional's perspective, the problem lies in the reasons why different groups study soils.
Pedologists (soil scientists) are interested in soils as a medium for plant growth. Representing a
corresponding branch of engineering soils specialists, soil engineers look at soil as a medium that
can be excavated with tools. A geologist 's view of soil falls somewhere between that of pedologists
and soil engineers—they are interested in soils and the weathering processes as past indicators of
climatic conditions and in relation to the geologic formation of useful materials ranging from clay
deposits to metallic ores.