Lecture 2

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Chapter 1

Soil Components


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.


  1. 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 semantics.

  2. 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 clays.

  3. 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.

  4. 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.

  5. 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 the textbook).

  6. 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.

  7. 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.



igneous rock
metamorphic rock
sedimentary rock


Web sites

Soil Science Society of America Homepage. This is the professional society to which most soils researchers and professors belong.
URL: www.soils.org

Digital video of soil microbiology.
URL: www.agron.iastate.edu/~loynachan/mov/


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