Water and Soil Types
Name: Homer H.
Date: January 2001
How do soil types affect the speed that water is absorbed?
The speed that water passes through soil is related to the size and chemical
characteristics of the soil particles and how tightly they are packed. In
general, the smaller the soil particles are, the more energy the water will
have to expend getting through the pores, resulting in slower passage.
However, in actual soils, the individual particles are rarely uniformly
sized, but consist of a range of particle sizes.
So, the uniformity of the particle sizes also affects how water flows
through soil. In soils that engineers refer to as "well-graded," the soil
contains enough smaller particles to pack around the big ones and fill the
voids, resulting in low water flow. (Just to make it confusing, what
engineers refer to as well-graded geologists call poorly-graded and
vice-versa.) Additionally, the more tightly packed the soil is, the less
permeability it has. So, the rate that water infiltrates into soil can be
reduced by packing it down with rollers and other compaction equipment.
Finally, clay particles in soil have a chemical attraction to water. So,
they will grab onto water molecules as the water passes by. Since clay
particles can be so tiny that an electron microscope is required to see
them, these trapped water molecules basically get jammed in between the clay
particles and shut off any further flow. This is why landfills and
hazardous waste containment areas are often lined with pure clay. The clay
is so impermeable that anything flowing out of the landfill is stopped by
the clay barrier. Some clays, particularly some found in Texas and other
parts of the West, actually can hold so much water that they swell up when
they get wet. This attraction is so strong that these swelling clays can
actually lift and break foot-thick concrete slabs just through water
This water-absorbing characteristic of clay has many other positive uses.
For example, the anti-diarrhea medication Kaopectate contains purified
kaolin, which is a type of clay with the largest particle size. Because kao
lin has relatively large particles, it absorbs water, but, thankfully,
doesn't swell much. So, when it gets into the intestines, it absorbs excess
water and slows things down.
Soil type is very important in not only
how fast water is absorbed into soil, but
also in water retention (how much is held
in the soil as opposed to how fast it drains out).
Some soils take up water very slowly, such as clay soils,
and also hold it well (they do not drain well).
Some soils absorb water well, such as peat and sandy
soils. Peat holds the water well (doesn't drain
very fast because of the high organic content) whereas
sandy soils do not hold water well, allowing it to
drain out fairly quickly.
David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory
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