Geology Reference
In-Depth Information
Soil is formed not just by the weathering of minerals derived from rocks,
but also by the input of organic matter from the decomposition of plants
and, to a lesser extent, the decomposition and waste products of animals
and microorganisms. The relative amounts of the mineral and organic
components can vary widely, from almost all mineral materials, such as in
desert sand, to highly organic soil such as peat. It is the combination of the
mineral and organic components that gives soil its characteristics and
makes it a valuable resource, which can be used to grow crops for a number
of uses, such as food, fibre, and energy. These two aspects - soil develop-
ment and soil use - are reflected in two definitions of soil given by the Soil
Science Society of America (http://www.soils.org/sssagloss/):
The unconsolidated mineral or organic material on the immediate surface of
the earth that serves as a natural medium for the growth of land plants.
The unconsolidated mineral or organic material on the surface of the earth that
has been subjected to and shows the effects of genetic and environmental factors
of:
climate
(including water and temperature effects), and
macro- and micro-
organisms
, conditioned by
relief
, acting on
parent material
over a period of
time
. A product - soil differs from the material from which it is derived in many
physical, chemical, biological, and morphological properties and characteristics.
The five points underlined in the second definition are known as the soil-
forming factors, which interact to produce a soil of particular characteristics
in any given place. The parent material is acted upon by a series of
weathering processes, outlined in Figure 1. Initially these are mainly
physical processes, which bring about a decrease in particle size, and hence
an increase in surface area. Weathering is strongly influenced by climate,
and, once living organisms colonize the developing soil, by biological
processes. Climate and relief interact to influence the movement of water
and weathering products through the soil. Most of these physical, chemical,
andbiologicalprocessesproceedslowly,andsosoilisevolvingwithtime.
Soil is a combination of all of the major components of the surface
environment: the atmosphere, hydrosphere, lithosphere, and biosphere
(Figure 2). It is the mix and interaction of these four components that
result in the particular properties of a specific soil. The solid components
(mineral and organic matter) make up approximately 50% of soil by
volume. Air and water occupy the pore space between the solid phase,
and their relative amounts can vary considerably, resulting in important
effects on the chemical and biological processes in a soil. Various
measurements can be made to express the air-water balance in a soil
(Table 2).
