Environmental Engineering Reference
In-Depth Information
amount of water is applied, virtually all of it remains 'suspended' in the
sandy layer by capillary forces and remains available for growing turf-
grass plants. If a large amount of water is applied, for example by heavy
rain, its weight will overcome capillary forces in the upper layer and
force water into the lower, coarse level, where the large pore spaces allow
for quick, unrestricted water drainage. Downward flow will continue
until capillary forces once again balance gravitational ones, and then
infiltration will stop. In such cleverly modified soils, the very low
water-holding capacity of the upper sandy layer can be increased sub-
stantially, but very rigorous specifications must be followed. Small dif-
ferences in texture or thickness of the top layer may substantially change
its hydraulic properties. For example, a surface layer that is too shallow
may not drain properly; one that is too deep may drain well but result in
an excessively dry surface layer.
Except in these very specific situations, where soil layering can be
used to our advantage, strongly contrasting soil layers should be
avoided. They substantially change hydraulic properties of soil and
may consequently negatively influence not only water movement but
also soil aeration, microbial activity and numerous other chemical and
ecological processes in the soil.
The Soil Air
As mentioned in Chapter 1, turf-grass roots and most soil organisms
require oxygen for respiration. During the respiration process, roots
and microorganisms release mostly carbon dioxide, but also some other
gases, including toxic ones that may have negative effects on turf and
other soil organisms. Carbon dioxide and toxic gases will build up in the
soil unless exchange takes place between the air in the soil and the above-
ground atmosphere. This process of gas exchange, called
soil aeration,
occurs primarily by diffusion through the soil pores (Fig. 3.10).
The total space not occupied by soil particles in a bulk volume of soil is
called the
pore space
and can range from30% to 70%of the total soil volume.
The size distribution of pores in the soil also varies greatly depending on
the soil texture and structure. Coarse-textured soils (sands) contain a
higher percentage of large pores than do fine textured soils (clays),
which in turn contain large numbers of small pores. Large pores help
aeration, but small pores are needed for water retention. The ratio of
large pores to small pores is therefore considered far more important
than total pore space. Numerous studies indicate that the 1:1 ratio that
typically exists in sandy loams is the most favourable. Some species
tolerate deviation from this ideal ratio better than others, so selection of
the most suitable turf-grass species for a given soil is important.
