Agriculture Reference
In-Depth Information
I.2
THE SOIL MICROCLIMATE
The microclimate of the soil is defined in terms of its internal temperature and
hydrological regimes and is therefore generally determined by the external climate.
Since this varies broadly with latitude‚ elevation‚ rainfall distribution and‚ to a lesser
degree‚ with vegetation type and cover‚ aspect and a range of other factors‚ it is clear
that soil microclimates are likely to differ almost as widely as those of the surface‚
albeit increasingly buffered from rapid change with greater depth in the profile.
The soil properties that determine water entry and movement within the soil are
clearly important in defining the microclimate. Soil temperature regimes and the factors
governing their variation in time and space form the remaining pan of its definition.
2.1
Soil temperatures
The temperature regimes that pertain in soils influence many processes that occur
therein and play a part in controlling the rates and processes of soil development and
the composition and activities of the biota. Agricultural productivity is frequently
limited by either low or excessively high temperatures‚ although both effects are often
related to moisture status. In the pedogenetic sense‚ water is a major agent of physical
weathering through expansion: contraction processes and particularly the frequency
with which the 0 °C boundary is crossed (Figure I.21a). The rates of many biological and
chemical weathering processes are also temperature dependent
All species have required minimum and maximum temperatures for growth and
survival and‚ in the arthropods‚ these may differ between developmental stages.
However‚ most species usually have a somewhat narrower preferred range.
The species present in extreme environments usually possess specific adaptations to
permit their survival therein. However‚ a minimum requirement for the persistence of
most species is that temperature and moisture regimes be regularly within a favourable
range for a sufficient period to permit successful reproduction and development.
2.1.1
SURFACE ENERGY BALANCE
Soil temperatures are directly related to the energy balance at the surface. This may be
stated as the following equality (Bonneau‚ 1979):
where a is the albedo‚ is the incident solar radiation (direct + diffuse)‚ is the long
wave atmospheric radiation‚ C is the energy gained by the condensation of water‚ E is
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