Reading: 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 1).

Several transparent bowls, each holding a different kind of soil.

Figure 1. Just some of the thousands of soil types.


A forest of tall, thin trees

Figure 2. A pedalfer is the dark, fertile type of soil that will form in a forested region.

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 2).

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.


A grassy field with a few wildflowers

Figure 3. A pedocal is the alkaline type of soil that forms in grassland regions.

Pedocal soils form in drier, temperate areas where grasslands and brush are the usual types of vegetation (figure 3). 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.

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.



Figure 4. A laterite is the type of thick, nutrient poor soil that forms in the rainforest.

In tropical rainforests where it rains literally every day, laterite soils form (figure 4). 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.

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!


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