Agriculture Reference
In-Depth Information
CHAPTER
1
SOIL BASICS I
M
ACROSCALE
F
EATURES
Soil is essential to life. All life supporting components derive, either directly
or indirectly, from the soil. Plants growing in soil are directly used for food or
are fed to animals, which are then used for food. These same plants take in
carbon dioxide produced by animals and give off oxygen. Soil and the plants
it supports moderate the amount of liquid and gaseous water in the environ-
ment by serving as a reservoir controlling its movement. Elements essential
to life, even life in water, are released from soil solids and recycled by soil
chemical and biologically mediated reactions.
Thus, although, as will be seen, soil is extremely complex, an understanding
of its physical characteristics and the chemistry occurring in it are important
in its analysis and the instruments used in this analysis.
Soil is vastly more complex than simply ground-up rock. It contains solid
inorganic and organic components in various stages of decomposition and dis-
integration, an aqueous solution of elements, inorganic and organic ions and
molecules, and a gaseous phase containing nitrogen, oxygen, carbon dioxide,
water vapor, argon, and methane plus other gases. In addition, it contains a
large and varied population of macro-, meso-, and microscale animals, plants,
and microorganisms. If any of these components is missing, it is not soil!
The solid portion of soil is composed of inorganic sand, silt, clay, and organic
matter, which interact to produce the large soil features
1
(i.e., peds, profiles,
pedons, landscapes). These features, not considering rock, are discussed in this
chapter. In Chapter 2, components smaller than sand, which soil scientists
define as those inorganic particles smaller than 2.00 mm in diameter, are dis-
cussed. Geologic features and gravel, stones, rock, and other substances are
not discussed.
Large soil components consisting of sand, silt, clay, and organic matter are
peds, profiles, pedons, and landscapes. Peds are formed by the aggregation of
sand, silt, and clay particles to form larger (secondary) soil structures that
result from the action of the soil forming factors (see Figures 1.1-1.5). Profiles
develop in the loose material on the earth's surface and are composed of
1
Many soils contain gravel, stones, and rock; however, these components, because of their low
reactivity, will not be considered in this topic.
