Agriculture Reference
In-Depth Information
by freezing and thawing, wetting and drying, and heating and cooling. In addition
chemical weathering such as oxidation, hydration, dissolution, and being attacked by
chemicals given off by roots occurs and also results in the breakdown of rock. Clay
can be released from rock during its decomposition and undergo chemical changes
in the soil environment to form other clays. Clay can also be synthesized in the soil
as rocks disintegrate and components dissolve and recombine.
The material resulting from these activities is called the soil parent material, and it
must undergo soil formation to have a soil with horizons. The soil factors involved in
soil forming are time, climate, biota, parent material, and topography, which interact
over time to produce a fully developed soil. A fully developed soil would have many
horizons and be perhaps 150 cm deep. To develop this soil would take on the order
of 1000 years. This is why protecting soil from accelerated erosion is so important.
Should a person be able to discern the characteristics of a soil simply by looking at
the composition of the rock from which it is forming? Unfortunately, the answer is no.
This is never the case. To cover all the reasons why this is so would require a topic of
some significant length. Here we will discuss only some of the more important reasons
why soil is different from its underlying rock. The three chief reasons are leaching, by
which components are lost from the forming soil, deposition of soil or soil parent
material from water or air, and the loss of soil from its surface due to erosion.
Soil and soil components are constantly in motion, being carried by air, water, ice,
and gravity. This means that a soil in one area is always losing material while other soils
are gaining. Thus material is constantly being lost and gained. In the case of deposition,
soil has deposited on it material from other areas, which is not like the rock from which
the soil is forming. This means that the composition, particularly of plant nutrients, will
be different than would be expected in soil forming only from one type of rock.
Because soil is different from underlying rock, it is not possible to know about the
fertility of a soil, an area, or landscape without actually doing a chemical analysis of the
soils or the area in general.
Of the inorganic components in soil the clays are the most important because
of two important characteristics. Clays have a large surface area and thus have high
adsorptive and absorptive capacities. Because it is often hard to distinguish between
absorptive and adsorptive processes in soil, the process is often called sorption.
Both inorganic and organic components can be sorbed by soil clays.
A second important characteristic of clay is cation exchange capacity (CEC). Clays
carry negative charges at edges where bonds are broken. Many clays, including those
commonly found in soil, also have isomorphic substitution that gives rise to additional
negative charges. Both of these sources of negative charge result in positively charged
species, mostly metal cations, being attracted to the surfaces of clays. This attraction is
called the cation exchange capacity. The word exchange is included because these
cations can readily exchange with cations in the soil solution. One of the main plant
nutrients is potassium that occurs as a positive cation, K
þ
, in soil and on the CEC.
Because this cation is easily exchangeable, it is readily available to plants.
Other important cations, such as calcium Ca
2þ
and magnesium Mg
2þ
, are also
attracted to the exchange sites and are thus also readily available to plants. Calcium
and magnesium are essential nutrients for plants but are in high concentration in
