Agriculture Reference
In-Depth Information
In the liquid state‚ water expends part of its potential energy in the physical transfor-
mation of parent materials and transport of solid particles‚ either at the surface (erosion)
or within the soil profile (translocation). Because of its polarity‚ water adheres to solid
particles‚ and this is of especial importance in the small pores and capillaries where solid‚
liquid and gaseous phases interact. As stated above‚ water is also a good conductor and
therefore has an important role in heat transmission. As a reactant‚ water promotes
hydrolysis and dissolution‚ two important processes in the weathering and transforma-
tion of both organic and inorganic components during soil formation (Chapter II).
Also‚ as considered later in this chapter (Section I.2.2.2)‚ variation in soil water content
exerts an important control over the nature and activity of the biota present and on the
directions and rates of many soil reactions. Water-logging that lasts for more than short
periods results in substantially-reduced soil oxygen concentrations‚ reducing conditions
and the production of compounds inimical to the growth of most plants. The plants
that inhabit wetlands normally possess special structures that facilitate gas exchange
with the atmosphere‚ or physiological adaptations that allow them to cope with these
conditions (Section I.2.2.3).
As the major component of living entities‚ water may be considered as a resource.
Its availability in time and space‚ the competition that occurs for it between organisms
and the energy necessary for plants and other organisms to absorb it from soil pores
are characteristics of paramount importance in controlling the activities of the soil biota.
The energy relationships of soil water are given particular emphasis in this section.
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