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Drainage
is a problem in many
soils, especially in those that contain a
lot of clay. Clay is comprised of extremely small particles
surrounded by very small channels called
micro-pores. That is
why these soils drain slowly and hold only small amounts of
oxygen needed for plant roots.
 Larger
soil particles such as
sand and gravel have plenty of large
spaces called macro-pores. Therefore, it would seem logical
that drainage could be improved by mixing quantities of these
materials into clay or placing a layer of them beneath the
clay. When the water moved through the fine clay and hit the
coarse textured sand or gravel, it should rapidly drain away.
Sounds logical. Unfortunately, it does not
work that way.
Water moves through the soil from large
particles to small, not the other way around. At the
conference, we saw a film of water moving through a fine
textured soil toward a layer of gravel beneath. When the water
reached the gravel, instead of moving more rapidly, it stopped
and backed up. The water did not move into the gravel until
the entire layer above it was totally saturated. Then it moved
into the gravel.
French drains which are basically trenches
filled with stones or rocks only work if the stones extend all
the way to the surface. If a layer of fine soil is placed on
top of them, drainage is slowed considerably since this layer
must become totally saturated before the water moves into the
stones.
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