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Geology LibreTexts

11.3: Soils

Soils cover the majority of the earth's land surface. Soil has an enormous effect on the ability of land to sustain vegetation (or crops for human consumption), and therefore is primarily responsible for the carrying capacity of land. Carrying capacity is the maximum number of organisms that an area can support without experiencing degradation. In other words, the quality of soil in an area determines the type and quality of agriculture. The better the soil in a region, the more people it can feed. It is therefore very important to understand the components that make up 'good' soil, and those which make up 'bad' soil, and we can use this knowledge to better understand the distribution and economic development of human populations.

The science of studying soils is called pedology. Soil scientists study soils in units called pedons. A pedon is a hexagonal column measuring 1 to 10 square meters in top surface area. Many pedons together make up a polypedon. A pedon is the basic sampling unit in a soil survey and a polypedon is the soil unit used in preparing soil maps.

The next time you walk by a construction site where the crews have dug a deep pit to lay a buildings foundation, take a look at the profile of the soil that has been exposed. Soils are made up of layers called horizons, which are roughly parallel to the surface of the ground. The top layer of the soil profile is labeled as O or organic. The O layer of soil is made up of humus, a complex mixture of decomposed organic material, microorganisms, and sediment. The bottom of a soil profile is called the R or rock horizon, made up of bedrock.

The soil horizons between the O and the R horizon are labeled alphabetically: A, B, C. Sometimes, after the A horizon, there is a layer of coarse sand, silt and minerals resistant to dissolving by water. This process, by which water percolating through the soil removes fine particles and minerals leaving behind more coarse material, is called eluviation. Therefore, this horizon directly after the A horizon is termed the E horizon, for the eluviation that takes place. Typically, plant roots interact with the O, A and E horizons.

The B horizon is below the A and E horizons, and the fine particles and minerals that are dissolved from the E horizon are generally deposited in the B horizon. The B horizon is typified by reddish or yellowish hues because of the minerals and organic oxides that are deposited. The C horizon is comprised of weathered bedrock or regoliths. The C horizon is outside of the zone of biological influence and therefore has little organic matter included in it. This deposition process is termed illuviation. So a typical soil profile might be labeled as: O, A, E, B, C, R.

Soils are described by their color, structure, texture, porosity, moisture and chemistry. We will discuss the most important of these factors from an agricultural standpoint: texture and structure. There are five key factors in soil formation:

  • Type of parent material
  • Climate
  • Overlying vegetation
  • Topography or slope
  • Time

Type of parent material influences the soil pH, structure, color, etc., in a profound way. High-rainfall climates tend to have less-fertile soils, due to rainwater's effect in leaching nutrients down to lower levels of the soil profile, and have more acidic soils. Low-rainfall climates tend to accumulate salts near the surface and have generally higher soil pH. Soils that form under coniferous forests tend to be more acidic than those under deciduous forests, and root action is also critical in soil formation. Soils generally have a harder time forming on steep slopes, due to runoff of soil particles during rain events. The longer a soil has to form, the deeper its profile is going to be.

Now that you have the basics of soil science under your belt, check out your state soil (you probably didn't know that you had a state soil!). Compare the state soils of several different regions of the USA.