Agriculture Reference
In-Depth Information
CHAPTER I
INTERNAL ENVIRONMENT,
MICROCLIMATE AND RESOURCES
I.1
THE SOIL ENVIRONMENT
1.1
Soil components and structures - the major groupings
Soils form through the conjoint influences of lithological, topographic, climatic and
biological factors that interact over time to condition the natures, directions and rates of
soil forming processes. The varied and complex expression of these soil forming factors
over the surface of the earth has lead to the development of an enormous diversity
of soils with a correspondingly-large range of soil environments. Over time periods
ranging up to the geological, soils develop from their parent materials (often but not
necessarily rocks) and undergo a series of progressive changes to eventually become
depleted in nutrient elements and, in the biological sense, exhausted.
On a global scale, the climatic regime is the dominant soil forming factor. In contrast,
topography is regionally important in controlling the ways in which soils vary from
site to site while, at a single site, considerable local spatial variability and vertical
differentiation are features of the soil system. Finally, at the lowest level of pedologica
organisation, the primary particles of the soil become associated to form aggregate
which determine the micro-environments that occur within and between them.
At all spatial and temporal scales, the major influences defining the environments that
exist within soils are the physical, chemical and surface properties of their components
and their spatial dispositions. Further determinants include the soil microclimate,
the nutritional resources available to the dominant biota and the presence of inhibiting
compounds. An appreciation of these influences and the interactions that occur between
them is fundamental to an understanding of the conditions under which the inorganic soil
components, the biota and its by-products interact to mediate the unceasing changes
that occur during soil development.
Soil is a complex and intimate mixture of materials distributed among the solid,
liquid and gaseous phases and occurs in a wide variety of physical and chemical forms.
The properties and functioning of the whole soil, both in natural and in applied
situations, will largely be determined by the properties of its individual components
and their relative proportions. The volume of the soil that may be occupied by the liquid
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