What is this stuff we call soil? How do we systematically describe or compare
soils? What are the components of soil? These questions and others are briefly
answered in this lesson. Why briefly answered? The answers are brief because
short answers to lots of questions at this point in your studies will allow
you to put more detailed answers into context as those answers emerge later
in the course.
- One might ask, does soil spawn life? What is this material we call
soil, that when found in the presence of sunlight and water it seems to generate
living things. Do not get too hung up on semantics. Lots of people have written
definitions of soil and you can write one too if you like. No one's definition
is necessarily right or wrong. So, what asked does soil contain life-giving
powers, the answer depends on whose definition of soil you choose. The USDA
definition (see page 2 in textbook) requires that soil contain living matter.
An engineer's definition would be quite different. We encounter difficulties
as we impose human systems over natural phenomena. When you view a forest,
we humans would tend to say, "up there are the trees and down there is
the soil," separating the plants from the soil. However, we normally
consider the microorganisms living on the same soil to be an integral part
of the soil. Again, learn concepts and facts but do not get too hung up on
- Soils exist in and on the earth's crust. The crust tends to be about
3 to 20 miles deep (see Figure 1-1 in the textbook). The crust is highly dynamic.
Imagine the processes and forces at work resulting in the presence of a previous
surface accumulation of sedimentary materials buried miles below the surface.
The unconsolidated materials (that one would call soil) are sands, silts and
- Soil is three-dimensional. Consider how much water you could you
add to a quart jar filled with dry soil. Probably about one pint, or half
the quart volume of water could be added. Why? Because about half of a soil's
volume is solids, the other half is pore space. The top surface of soil is
what we normally see. Looking at the top of a soil, how might one tell a "good"
soil from a "bad" soil? That answer leads to another question, "good"
for what purpose? If one wants a good soil on which to raise horses, one might
considered such soil surface features as slope, color, vegetation, texture,
etc. The soil is more than the surface, it also has a vertical dimension.
A vertical cross-section of soil is called a soil profile. Horizontal layers
in the profile are called horizons (see photos after page 160 in the textbook).
The uppermost horizon or topsoil is the "A" horizon, rich in organic
material, with soluble material leached or washed out. This horizon is often
about 1 foot deep. The next horizon typically is a subsoil horizon called
the "B" horizon. Soluble and mobile materials wash into this horizon;
it is often high in clay and salt. B horizons are often about 1 to 2 feet
thick. A and B horizons collectively are called solum. Below the B horizon
is material that is more or less unaffected by surface phenomena. This material
is called parent material, or "C" horizon.
- The pore space within a soil volume is air and water. The water is
called the soil solution. This solution contains soluble salts, organic solutes,
and some suspended colloids (solids that suspend in a fluid). The behavior
of soil water is controlled to a great extend by pore size. Small pores have
a great affinity for water and hold it very tightly. Larger pores allow water
to escape easily, such as into a plant root or into the atmosphere by evaporation.
Soil air has more CO2 but less O2 than the atmosphere.
This is because of the time lag in diffusing gases into and out of the soil.
Respiring organisms in the soil consume O2 and produce CO2.
Because of this concentration gradient between the soil and the atmosphere,
one can think in term of these soil gases constantly seeking, but never achieving,
equilibrium with the atmosphere. For similar reasons, the soil air always
has a relative humidity near 100%. Respiration releases water which evaporates
much more slowly in the soil than on or above the soil.
- The solids in soil are minerals and organic matter. Minerals can
be either primary or secondary. The primary minerals on the earth cooled from
a molten mass, and have not changed chemically since the day they came into
existence. Secondary minerals form by precipitation or recrystallization of
elements that were released by the weathering of primary minerals. Quartz
and feldspar are primary minerals. The clays are secondary minerals. Rocks
are mixtures of minerals. Igneous rock formed from molten magma (see Figure
1-2 in the textbook). Sedimentary rocks are cemented accumulations of mineral
materials. Common sedimentary rocks include limestones, sandstones, quartzite,
and shale. Metamorphic rocks include slate (hardened shale) and marble (hardened
limestone). The various rocks and minerals decompose and weather to form soil
minerals. These soil minerals may be small versions of primary minerals (sand
is usually small quartz rocks) or may be secondary minerals (as is the case
with most clays). Soil minerals are mostly O, Si, and Al (see Detail 1-1 in
- Organic matter includes living organisms and decomposing flora and fauna.
Microorganisms include bacteria, fungi, algae and others. Living macro
organisms are usually not considered part of the soil per se. Organic material
can be actively decomposing, releasing nutrients. Within a year or so, organic
materials such as crop residues stabilize and then the remaining residues
decay very slowly. Stable organic material, very slowly decomposing, is called
humus. Organic matter affects soil tilth, nutrition, and water relations.
Decomposing organic matter provides plant nutrients. Stable organic matter
does not, but increases the soil's ability to hold nutrients and water. An
organic soil is dominated by organic matter, rather than minerals. Such soils
are found in wetlands and in cool regions where production of organic materials
exceeds rates of organic matter decomposition.
- Soils is habitat for higher plants. Plants need support, O2,
water and nutrients, all of which come from the soil. Soils differ greatly
in their ability to provide these necessities. Proportions of coarse material
(sand) and fines (clays) influence availability of O2 and water.
Plants also need at least 13 mineral nutrients plus carbon and oxygen from
CO2 and hydrogen from water (see Detail 1-2 in the textbook).
Students are encouraged to look up the following vocabulary words in the textbook
glossary and to browse the following web sites.
Soil Science Society of America Homepage. This is the professional society to which
most soils researchers and professors belong.
Digital video of soil microbiology.