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Name: Homer H.
Status: Student
Age: 13
Location: N/A
Country: N/A
Date: January 2001


Question:
How do soil types affect the speed that water is absorbed?



Replies:
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 absorption.

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.

Andy Johnson


Homer,

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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