Agriculture Reference
In-Depth Information
during the wetting up, there is convective movement of solutes, so that the disper-
sion equation (with boundary conditions imposed by conditions in the macropores)
must be used to describe the solute movement. When the velocity becomes very
small, the solute movement then approximates to a diffusion process and can be cal-
culated from Fick's law. Thus, the salts which is transporting toward the centers of
the aggregates (micropore regions), redistribute eventually to a uniform equilibrium
condition throughout the aggregates. When the macropores are empty, the aggre-
gates become almost isolated so that redistribution of water and solutes occurs only
within the aggregates.
The bimodal soil structure of aggregated soils has a profound effect on bulk
soil hydraulic conductivity and on soil hydrology. Such structure occurs when
cracks and fissures are produced during natural soil shrinkage on drying or dur-
ing mechanical drainage operations, and also as a result of tillage. The presence of
interconnecting macropores assists drainage and promotes rapid leaching of solutes
when the macropores are full of water, but inhibits these processes when empty.
7.4.2 Basic Solute Transport Processes
Solutes are transported through three basic processes:
i. Convection
ii. Diffusion
iii. Dispersion
Solutes can exist in all three phases of liquid, solid, and gaseous. The decay
and production processes can be different in each phase. Solutes are transported by
convection and dispersion in the liquid phase, and by diffusion in the gaseous phase.
7.4.2.1 Convective Solute Transport
Convective solute transport refers to the movement of dissolved solutes through the
soil with the flowing water. In the absence of sorption and dispersion, the solutes
move at the same average velocity of water. Under such a condition, the total mass
flow of solute is related to the law that governs the movement of water through the
soil. Thus, the convective flux of solute passing through a unit area of soil can be
expressed as
q
c
=
qC
(7.1)
where
convective flux density of solute (kg/m
2
/s)
q
c
=
Darcian water flux
or
volumetric flux density (m
3
of water/m
2
/s)
q
=
concentration of solute in water (kg/m
3
of water)
C
=
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