Soil - Agroecology: The Ecology of Sustainable Food Systems - page 100

Depending on local conditions and geological history,
the regolith can be recently formed, lightly weathered, and
made up of mostly primary minerals, or it may have been
subjected to intensive weathering and be made up of more
resistant materials such as quartz.

C HEMICAL W EATHERING

While the regolith is forming and living organisms begin
to have their impacts on it, chemical weathering is occurring
as well. Chemical weathering includes natural chemical
processes that aid in the breakdown of parent materials,
the conversion of materials from one form to another in
the soil, and the movement of materials within the soil.
Four different chemical processes are of primary impor-
tance in soil formation and development: hydration,
hydrolysis, solution, and oxidation.
Hydration is the addition of water molecules to a min-
eral's chemical structure. It is an important cause of crystal
swelling and fracturing. Hydrolysis occurs when various
cations of the original crystalline structure of silicate min-
erals are replaced by hydrogen ions, causing decomposi-
tion. In regolith with low pH, the greater concentration of
H + accelerates hydrolysis. The release of organic acids as
a byproduct of the metabolic activities of living organisms,
or from decomposition of dead organic matter, can add to
this process as well. Solution occurs when parent materials
with a high concentration of easily soluble minerals (such
as nitrates or chlorides) go into solution in water. Lime-
stone is particularly susceptible to solution in the presence
of water, high in carbonic acid; in extreme cases the solu-
tion of limestone leads to the formation of limestone caves
in areas of underground water flow. Finally, oxidation is
the conversion of elements such as iron from their original
reduced form into an oxidized form in the presence of
water or air. Softening of the crystalline structure usually
accompanies this process.
Once minerals are released from the consolidated
parent material, another chemical process that is of great
importance is the formation of secondary minerals — the
most important being clay minerals. Clay mineralogy is a
very complex field of study, but it is important to under-
stand some basic aspects of clay formation, since they have
such dramatic impacts on plant growth and development.
Clay minerals are very small particles in the soil, but
they affect everything from water retention to nutrient
availability, as will be discussed elsewhere. They are
formed by complex processes in which silicate minerals
are chemically modified and reorganized. Depending on
the combination of climatic conditions and parent mate-
rial, the secondary minerals that are formed are of two
basic types: silicate clays that are predominantly made up
of microscopic aluminum silicate plates with different
arrangements and the presence or absence of other ele-
ments such as iron and magnesium; and hydroxide clays
that lack a definite crystalline structure and are made up
of hydrated iron and aluminum oxides in which many of
the silicon ions have been replaced.
Eventually, the clays found in any soil will be a
mixture of many subtypes of these two basic types of
secondary clay minerals, although one or a few subtypes

Transport

As rock is broken down into smaller and looser materials,
it can remain in place and eventually form residual soils,
but a more likely fate is for it to be carried some distance
and deposited. The forces of wind, water movement, grav-
ity, and glacial ice movement can all transport weathered
soil particles. Transported soils have different classifica-
tions depending on the manner in which their particles
were transported. Soil is called

• Colluvium, where it has been transported by

gravity

• Alluvium, where it has been transported by the

movement of water

• Glacial soil, where it has been transported by

the movement of glaciers

• Eolian soil, where it has been transported

by wind

B IOTIC P ROCESSES

Sooner or later, depending on the consistency of the
regolith, plants establish themselves on the weathered
material. They send roots down that draw nutrients from
mineral matter, store them for a while in plant matter, but
eventually return them to the soil surface. Deep roots fur-
ther break down the regolith, capture nutrients that have
leached from the upper surface, and add them to the soil
surface in an organic form. Plant residue then serves as an
important source of energy for the bacteria, fungi, earth-
worms, and other soil organisms that establish in the area.
Organic matter is broken down into simpler forms
through decomposition and mineralization. Macroorgan-
isms in the soil — centipedes, millipedes, earthworms,
mites, grasshoppers, and others — consume freshly depos-
ited plant debris and convert it to partially decomposed
material, either in the form of excreta or their own dead
bodies. This material is then further decomposed by
microorganisms, mostly bacteria and fungi, into an array
of compounds such as carbohydrates, lignins, fats, resins,
waxes, and proteins. Mineralization eventually breaks
these complex compounds down into simple products such
as carbon dioxide, water, salts, and minerals.
The fraction of organic matter left in the soil as a result
of decomposition and mineralization is called humus . It
has a certain lifetime in the soil, after which it is broken
down itself. New humus, however, is constantly replacing
old humus, and the equilibrium point between the two is
an important factor in soil management.