SOIL TEXTURE AND COMPOSITION
The inorganic portion of soil is made of many different size particles, and these different size particles are present in different proportions. The combination of these two factors determines some of the properties of the soil.
Figure 1. A loam field.
- A permeable soil allows water to flow through it easily because the spaces between the inorganic particles are large and well connected. Sandy or silty soils are considered ‘light’ soils because they are permeable, water-draining types of soils.
- Soils that have lots of very small spaces are water-holding soils. For example, when clay is present in a soil, the soil is heavier, holds together more tightly, and holds water.
- When a soil contains a mixture of grain sizes, the soil is called a loam (figure 1).
When soil scientists want to precisely determine soil type, they measure the percentage of sand, silt, and clay. They plot this information on a triangular diagram, with each size particle at one corner (figure 2). The soil type can then be determined from the location on the diagram. At the top, a soil would be clay; at the left corner, it would be sand, and at the right corner it would be silt. Soils in the lower middle with less than 50% clay are loams.
Figure 2. Soil types by particle size.
Using the chart as a guide, what is the composition of a sandy clay loam? If you would like to determine soil type by feel, here’s a chart from the USDA to help you.
SOIL HORIZONS AND PROFILES
A residual soil forms over many years, as mechanical and chemical weathering slowly change solid rock into soil. The development of a residual soil may go something like this.
- The bedrock fractures because of weathering from ice wedging or another physical process.
- Water, oxygen, and carbon dioxide seep into the cracks to cause chemical weathering.
- Plants, such as lichens or grasses, become established and produce biological weathering.
- Weathered material collects until there is soil.
- The soil develops soil horizons, as each layer becomes progressively altered. The greatest degree of weathering is in the top layer. Each successive, lower layer is altered just a little bit less. This is because the first place where water and air come in contact with the soil is at the top.
Figure 3. Soil is an important resource. Each soil horizon is distinctly visible in this photograph.
A cut in the side of a hillside shows each of the different layers of soil. All together, these are called a soil profile (figure 3).
The simplest soils have three horizons.
Called the A horizon, the topsoil is usually the darkest layer of the soil because it has the highest proportion of organic material. The topsoil is the region of most intense biological activity: insects, worms, and other animals burrow through it and plants stretch their roots down into it. Plant roots help to hold this layer of soil in place. In the topsoil, minerals may dissolve in the fresh water that moves through it to be carried to lower layers of the soil. Very small particles, such as clay, may also get carried to lower layers as water seeps down into the ground.
The B horizon or subsoil is where soluble minerals and clays accumulate. This layer is lighter brown and holds more water than the topsoil because of the presence of iron and clay minerals. There is less organic material. Look at figure 4.
Figure 4. A soil profile is the complete set of soil layers. Each layer is called a horizon.
The C horizon is a layer of partially altered bedrock. There is some evidence of weathering in this layer, but pieces of the original rock are seen and can be identified.
Not all climate regions develop soils, and not all regions develop the same horizons. Some areas develop as many as five or six distinct layers, while others develop only very thin soils or perhaps no soils at all.
TYPES OF SOILS
Although soil scientists recognize thousands of types of soil—each with its own specific characteristics and name—let’s consider just three soil types. This will help you to understand some of the basic ideas about how climate produces a certain type of soil, but there are many exceptions to what we will learn right now (figure 5).
Figure 5. Just some of the thousands of soil types.
Deciduous trees, the trees that lose their leaves each winter, need at least 65 cm of rain per year. These forests produce soils called pedalfers, which are common in many areas of the temperate, eastern part of the United States (figure 6). The word pedalfer comes from some of the elements that are commonly found in the soil. The Al in pedalfer is the chemical symbol of the element aluminum, and the Fe in pedalfer is the chemical symbol for iron. Pedalfers are usually a very fertile, dark brown or black soil. Not surprising, they are rich in aluminum clays and iron oxides. Because a great deal of rainfall is common in this climate, most of the soluble minerals dissolve and are carried away, leaving the less soluble clays and iron oxides behind.
Figure 6. A pedalfer is the dark, fertile type of soil that will form in a forested region.
Pedocal soils form in drier, temperate areas where grasslands and brush are the usual types of vegetation (figure 7). The climates that form pedocals have less than 65 cm rainfall per year, so compared to pedalfers, there is less chemical weathering and less water to dissolve away soluble minerals so more soluble minerals are present and fewer clay minerals are produced. It is a drier region with less vegetation, so the soils have lower amounts of organic material and are less fertile.
Figure 7. A pedocal is the alkaline type of soil that forms in grassland regions.
A pedocal is named for the calcite enriched layer that forms. Water begins to move down through the soil layers, but before it gets very far, it begins to evaporate. Soluble minerals, like calcium carbonate, concentrate in a layer that marks the lowest place that water was able to reach. This layer is called caliche.
In tropical rainforests where it rains literally every day, laterite soils form (figure 8). In these hot, wet, tropical regions, intense chemical weathering strips the soils of their nutrients. There is practically no humus. All soluble minerals are removed from the soil and all plant nutrients are carried away. All that is left behind are the least soluble materials, like aluminum and iron oxides. These soils are often red in color from the iron oxides. Laterite soils bake as hard as a brick if they are exposed to the sun.
Figure 8. A laterite is the type of thick, nutrient poor soil that forms in the rainforest.
Many climates types have not been mentioned here. Each produces a distinctive soil type that forms in the particular circumstances found there. Where there is less weathering, soils are thinner but soluble minerals may be present. Where there is intense weathering, soils may be thick but nutrient poor. Soil development takes a very long time, it may take hundreds or even thousands of years for a good fertile topsoil to form. Soil scientists estimate that in the very best soil-forming conditions, soil forms at a rate of about 1mm/year. In poor conditions, soil formation may take thousands of years!
Soil is only a renewable resource if it is carefully managed. Drought, insect plagues, or outbreaks of disease are natural cycles of events that can negatively impact ecosystems and the soil, but there are also many ways in which humans neglect or abuse this important resource.
Figure 9. Organic material can be added to soil to help increase its fertility.
One harmful practice is removing the vegetation that helps to hold soil in place. Sometimes just walking or riding your bike over the same place will kill the grass that normally grows there. Land is also deliberately cleared or deforested for wood. The loose soils then may be carried away by wind or running water. In many areas of the world, the rate of soil erosion is many times greater than the rate at which it is forming. Soils can also be contaminated if too much salt accumulates in the soil or where pollutants sink into the ground. There are many practices that can protect and preserve soil resources. Adding organic material to the soil in the form of plant or animal waste, such as compost or manure, increases the fertility of the soil and improves its ability to hold onto water and nutrients (figure 9). Inorganic fertilizer can also temporarily increase the fertility of a soil and may be less expensive or time consuming, but it does not provide the same long-term improvements as organic materials.
Figure 10. Steep slopes can be terraced to make level planting areas and decrease surface water runoff and erosion.
Agricultural practices such as rotating crops, alternating the types of crops planted in each row, and planting nutrient rich cover crops all help to keep soil more fertile as it is used season after season. Planting trees as windbreaks, plowing along contours of the field, or building terraces into steeper slopes will all help to hold soil in place (figure 10). No-till or low-tillage farming helps to keep soil in place by disturbing the ground as little as possible when planting.
- Soil texture and composition, plus the amount of organic material in a soil, determine a soil’s qualities and fertility.
- Given enough time, rock is weathered to produce a layered soil, called a soil profile.
- Each type of climate can ultimately produce a unique type of soil.
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