soil formation


The steeper the slope, the less likely material will be able to stay in place to form soil. Material on a steep slope is likely to go downhill. Materials will accumulate and soil will form where land areas are flat or gently undulating.

rock type

The original rock is the source of the inorganic portion of the soil. The minerals that are present in the rock determine the composition of the material that is available to make soil. Soils may form in place or from material that has been moved. Residual soils form in place. The underlying rock breaks down to form the layers of soil that reside above it. Only about one-third of the soils in the United States are residual. Transported soils have been transported in from somewhere else. Sediments can be transported into an area by glaciers, wind, water, or gravity. Soils form from the loose particles that have been transported to a new location and deposited.

biological activity

The partial decay of plant material and animal remains produces the organic material and nutrients in soil. In soil, decomposing organisms breakdown the complex organic molecules of plant matter and animal remains to form simpler inorganic molecules that are soluble in water. Decomposing organisms also create organic acids that increase the rate of weathering and soil formation. Bacteria in the soil change atmospheric nitrogen into nitrates. The decayed remains of plant and animal life are called humus, which is an extremely important part of the soil. Humus coats the mineral grains. It binds them together into clumps that then hold the soil together, creating its structure. Humus increases the soils porosity and water-holding capacity and helps to buffer rapid changes in soil acidity. Humus also helps the soil to hold its nutrients, increasing its fertility. Fertile soils are rich in nitrogen, contain a high percentage of organic materials, and are usually black or dark brown in color. Soils that are nitrogen poor and low in organic material might be gray or yellow or even red in color. Fertile soils are more easily cultivated. Click image to the left or use the URL below. URL:


Soils thicken as the amount of time available for weathering increases. The longer the amount of time that soil remains in a particular area, the greater the degree of alteration.


Scientists know that climate is the most important factor determining soil type because, given enough time, different rock types in a given climate will produce a similar soil (Figure 1.1). Even the same rock type in different climates will not produce the same type of soil. This is true because most rocks on Earth are made of the same eight elements and when the rock breaks down to become soil, those elements dominate. The same factors that lead to increased weathering also lead to greater soil formation. More rain equals more chemical reactions to weather minerals and rocks. Those reactions are most efficient in the top layers of the soil, where the water is fresh and has not yet reacted with other materials. Increased rainfall increases the amount of rock that is dissolved as well as the amount of material that is carried away by moving water. As materials are carried away, new surfaces are exposed, which also increases the rate of weathering. Climate is the most important factor in determining the type of soil that will form in a particular area. Increased temperature increases the rate of chemical reactions, which also increases soil formation. In warmer regions, plants and bacteria grow faster, which helps to weather material and produce soils. In tropical regions, where temperature and precipitation are consistently high, thick soils form. Arid regions have thin soils. Soil type also influences the type of vegetation that can grow in the region. We can identify climate types by the types of plants that grow there.


soil formation

How well soil forms and what type of soil forms depends on several different factors, which are described below.

instructional diagrams

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what type of climate produces the thickest soils?

a) continental

b) arid

-->  c) tropical

d) subarctic

in the united states, residual soils are more common than transported soils.

a) true

-->  b) false

increased rainfall increases

a) weathering rate.

b) the amount of rock that is dissolved.

c) the rate that soil is formed.

-->  d) all of the above

a soils parent rock determines

-->  a) the composition of the material that can become soil.

b) whether the soil will be residual or transported.

c) the thickness of the soil that can form.

d) both a & b

humus increases a soils

a) porosity

b) ability to hold water

c) fertility

-->  d) all of the above

the longer the amount of time the soil remains in an area, the greater the degree of alteration.

-->  a) true

b) false

steep slopes tend to develop rich soils.

c) true

d) false

decomposing organisms speed up soil formation because they

a) create organic acids that increase weathering.

b) break down complex organic molecules into simpler ones.

c) use up nitrogen, which speeds up soil-forming processes.

-->  d) a & b

soils that are fertile contain a lot of

a) oxygen

-->  b) organic materials

c) clay

d) sand

diagram questions

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