Geoscience Reference
In-Depth Information
Fig. 10.9
The decrease of
total water content in
millimeters due to
evaporation from the top
180 cm of soil in rainless
summer is reduced by a thin
layer of plant residues left on
the soil surface
the coarse pores between the aggregates promptly lose water very early in the fi rst
stage of evaporation. With continuous paths for liquid water fl ow being broken, the
fi nal segment for water moving upward to the soil surface is realized by diffusion of
water vapor. With diffusion causing a substantially smaller fl ux than that of liquid
water, the evaporation rate decreases sharply.
A structureless soil surface is characterized by small clods that quickly disinte-
grate as they are pelted by a downpour of raindrops. With the surface changed into
mud during each rainfall event, portions of it always proceed deeper into the soil
profi le. It is not at all unusual that a dry crust containing only small pores has been
formed on a structureless soil before it rains again. The upshot of a continuity of
small pores is their high conductivity compared with vapor diffusivity between
aggregates in structured soil. Continuous liquid fl ow up to the surface in a structureless
soil results in a higher fl ux than the fl ux in a structured soil, especially during the
second stage of evaporation.
Soil tillage causes a short-term increase in porosity, but this positive effect is
accompanied by a long-term decrease of aggregation that eventually ends in a struc-
tureless soil. This unfavorable situation has been solved by relatively frequent fal-
lowing or by introducing clover and many other leguminous plants into crop
rotations. Additionally, classical plowing is either restricted or replaced by a sort of
disking using various types of chisel and moldboard plows. Such disk plowing
keeps substantial quantities of crop residues on the soil surface that restrict the loss
of water from the soil profi le. Because a soil layer without a continuous water fl ux
allows only vapor diffusion, evaporation is lowered. We see in Fig.
10.9
that the
total content of water in a soil profi le does not sink as sharply during the summer
month of July as it does in the same soil without this protective layer of residues. In
spite of the many recommended treatments and management techniques, the com-
plete return of a structureless soil to its original structure with stable aggregates is a
diffi cult task.
