Soil Formation
How well soil forms and what type of soil forms depends on several different factors. Some
of these factors are the climate, the original rock the soil formed from, the slope, the amount
of time and biological activity. Climate is ultimately the most important factor and will
determine the type of soil that forms in a particular region. The climate of a region is
principally determined by temperature and the amount of precipitation. This also influences
the type of vegetation that grows in the region. We can identify different climates by the
types of plants that grow there. Depending on how closely you look, we can divide land
areas into many different climate regions (Figure9.6).
Figure 9.6: Climate is the most important factor in determining the type of soil that will
form in a particular area. ( 13 )
Given enough time, even different rock types will produce a similar climate related soil.
Climate is such an important factor that even the same type of rock in different climates
will produce a different climate related soil. This is true because the rocks on Earth are
predominantly composed of eight elements and as rock breaks down, there will mostly be
just these eight elements. Surely, if an element is not present in the original rock, then it
will also not be present in the soil that forms from it. The amount of precipitation in an
area is important because it influences the rate of weathering. The more it rains, the more
rainwater passes through the soil and the more it reacts chemically with the particles. Those
reactions are most efficient in the top layers of the soil and become less effective as the water
continues to percolate through lower layers of soil. The top layers of soil in contact with the
freshest water react most. Increased rainfall in a region increases the amount of rock that is
dissolved as well as the amount of material carried away by moving water. As materials are
carried away, new surfaces are exposed and this also increases the rate of weathering.
The temperature for a region is important too. The rate of chemical reactions increases with
higher temperatures. For every 10oC increase in temperature, the rate of chemical reactions