doubles. Warmer regions also have more vegetation because plants and bacteria grow and
multiply faster. This means that in tropical regions, where temperatures and amounts of
rainfall are consistently high, thick soils form with no unstable minerals and therefore no
nutrients. Conversely, arid regions produce thin soils, rich in unstable minerals. The rate of
soil formation increases with greater amounts of time. The longer the amount of time that
soil remains in a particular area, the greater the degree of alteration.
The original rock is the source of the inorganic portion of the soil. Chemical reactions from
weathering break down the rock’s original minerals into sand, silt and clays. A soil is called
aresidual soilwhen it forms in place, with the underlying rock breaking down to form the
layers of soil that reside above it. Only about one third of the soils in the United States
form this way. The rest of the soils form from materials that have been transported in from
somewhere else. These soils are calledtransported soils.Glaciers bring bits of rock from
far away, depositing the materials they carried as the ice of the glacier melts. Wind and
rivers also transport materials from their places of origin. These soils form from the loose
particles that have been transported in to a new location and deposited. For transported
materials, the rate of soil formation is faster because the transported materials have already
been weathered. The closer the materials are to their place of origin, the greater the influence
of the original materials. The further those materials move from their origin, the greater the
degree of weathering and the influence of the original materials becomes obscured.
Soils thicken as the amount of time available for weathering increases. The warmer the
temperatures, the more rainfall and greater amount of time, the thicker the soils will become.
Biological activity produces the organic material and nutrients in soil. The partial decay of
plant material and animal remains also forms organic acids which in turn increase the rate
of soil formation and the rate of weathering. The organic material increases the ability of
the soil to hold water, create a soil’s structure and enhance its fertility and ability to be
cultivated.
The decayed remains of plant and animal life are calledhumus. Humus is an extremely
important part of the soil. It coats the mineral grains, binding them together into clumps
that then hold the soil together. The humus in soil also increases the porosity and water
holding capacity of a soil. Humus helps to buffer rapid changes in soil acidity and helps the
soil to hold its nutrients. Decomposing organisms in the soil breakdown the complex organic
molecules of plant matter and animal remains to form simpler inorganic molecules that are
soluble in water. Bacteria in the soil change atmospheric nitrogen into nitrates.
One indicator of a soil’s fertility is its color. Soils that are rich in nitrogen and contain a
high percentage of organic materials 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.