University of St. Thomas Landscapes: Physical Systems (Geog 112)
What are Soils?
Soils in the Environment
Soil Components
Soils have five basic components:
Air 
- Soil Air refers to the soil atmosphere, the gaseous phase of the soil that is neither solid nor liquid.
Water 
- Soil Water or Soil Solution is the liquid phase of the soil containing dissolved salts and chemicals (ions) that are free-floating and not attached to any solid particles (mineral surfaces).
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Mineral Particles: The Mineral Side of Soil 
- The mineral part of soil is composed of varying amounts of sand, silt, and clay. With a few exceptions that we will cover, these particles are not derived from materials that were once living, meaning that minerals are inorganic. Their characteristics greatly influence soil behavior and management needs.
Soil Particle Size
The behavior of water in the ground is influenced by the type of soil present. Soils are classified according to their particle size as follows:
There are several different systems in place that denote the particle sizes. The values given above pertain to the USDA Soil Taxonomy system. You probably have a good idea of what gravel and sand particles look like, but maybe not silt or clay. Silt particles are about as big as the thickness of your hair, and clay particles are much smaller than that!
Generally, soils consist of a mixture of different particle types, such as "sandy clay", or a "silty sand".
Sand
What Is It?
- Sand is mainly small rock fragments and hard minerals such as quartz (silicon dioxide). It contains few plant nutrients.
What Are Some of Its Characteristics?
- Of the three types of soil particles, sand is the largest in size. If you laid about 125 sand particles side-by-side, it would equal about one inch. If a sand grain were enlarged 1000 times, it would be about as big around as a man is tall.
Why Does It Matter?
- Moderate amounts of sand in the soil improve drainage, aeration, and tilth.
Silt
What Is It?
- Silt consists of ground up sand (quartz) and rock minerals. Silt contains few nutrients by itself, but it can have nutrients clinging to its surface.
What Are Some of Its Characteristics?
- Silt is between sand and clay in terms of size. If a silt particle were enlarged 1000 times, it would be about the size of a basketball.
Clay
What Is It?
- Unlike sand and silt, clays are aluminum-silicate minerals that also have varying amounts of plant nutrients such as potassium, calcium, magnesium, iron, etc. A good part of a soil's native fertility can come from its clay portion.
What Are Some of Its Characteristics?
- Clay particles are the smallest of the three. If you laid about10,000 particles side-by-side, it would equal about one inch. If you enlarged a clay particle 1000 times, it would be about the size of a marble.
- Clays have a negative charge (-) that makes them act like tiny magnets to attract and hold plant nutrients that contain a positive charge (+): potassium (K+), calcium (Ca++), magnesium (Mg++), and the ammonium form of nitrogen(NH4+). This helps to keep nutrients from being carried downward beyond the root zone by rainfall or irrigation. This type of loss is referred to as "leaching."
- Each clay particle is really a laminated structure consisting of tiny plates. This lattice arrangement plus small particle size gives clays an amazing amount of surface area for attracting and holding positively-charged nutrients. For example, one cubic inch of clay particles contains about 300 square feet of surface area!
Why Does It Matter?
- High clay content usually makes for harder plowing, more compaction, and poorer drainage, but it does assure good water holding capacity.
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Organic Matter, A Soil's Best Friend 
- Organic Matter in the soil includes plant and animal residues at various stages of decomposition, cells and tissues of soil organisms, and substances synthesized by plant roots and soil microorganisms. Most cultivated topsoils contain about 2-4% organic matter by weight. Despite its small proportion, organic matter has a remarkable beneficial effect on soil behavior and crop yields. Organic matter in the soil is frequently in the form of humus, partially decomposed organic matter that has become dark and crumbly and continues decomposing at a slow rate. Humus benefits the soil in many ways:
- It can improve overall physical condition (tilth), especially in clayey soils.
- It can help reduce soil erosion by wind and water because it acts as a "glue" to bind soil particles together into "crumbs," called aggregates, that improve water intake rates and lessen runoff. Aggregates are resistant to being moved by wind or flowing water.
- It stores and supplies nutrients, especially nitrogen, phosphorus, and sulfur. These are slowly released for use by plant roots as organic matter decomposes. It is estimated that for each one percent of organic matter in the topsoil, over 500 pounds per acre of maize can be produced without additional fertilizer.
- It increases the water-holding capacity of sandy soils.
- Its high negative charge helps prevent positively-charged nutrients from leaching. Per equal weight, humus has 30-40 times the negative charge of many clays and can account for the major part of a soil's nutrient-holding ability. In addition, negative charge improves a soil's buffering capacity, or its ability to resist changes in pH.
- It can reduce the incidence of some soil-borne diseases and stimulate growth of beneficial soil bacteria, fungi, and earthworms.
Biology: An Underground Zoo (Microorganisms!) 
This is an important, living component of the soil. We'll cover this separately and in detail in the next section, Soil Forming Factors. 
Soil Value:
The price we give a commodity is a measure of how much we value that commodity. We can compare the costs of soils used for different purposes to gain some understanding of how much we value our soils.
Compare the value given to the seven different soil uses below.
|
1 acre of soil |
Cost |
|
City |
$10,000,000 |
|
Suburbs |
$150,000 |
|
Horticultural Land |
$20,000 |
|
Land for Growing Wheat |
$1,000 |
|
Pastoral Land for Sheep/Cattle |
$300 |
|
Native Bushland (not for building) |
$10,000 |
|
Desert |
$10 |
How do we value our soils?
How have we decided to value land?
Do you think this is a fair valuation?
What are we not valuing in our land and soils that maybe we should be placing a value on?
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