Soil and Water Retention
How do different soils types hold water?
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The spaces that exist between soil particles, called pores, provide for the
passage and/or retention of gasses and moisture within the soil profile. The
soil's ability to retain water is strongly related to particle size; water
molecules hold more tightly to the fine particles of a clay soil than to
coarser particles of a sandy soil, so clays generally retain more water
(Leeper and Uren, 1993). Conversely, sands provide easier passage or
transmission of water through the profile. Clay type, organic content and
soil structure also influence soil water retention
For example, sand has a lot of space between its grains and water can
collect in those spaces
Whereas soil compacted really hard such that it becomes metamorphic igneous
rock doesn't have any room to store water.
The ability of soil to hold water depends on several factors,
but particularly on it's composition and the amount of air
space (porosity) within it.
Soil can be composed of mineral and organic parts, often
characterized as clay, silt, and sand. Clay particles are
generally very small, sand particles are generally very
large, and silt particles are medium size.
A very sandy soil is not capable of holding water well,
whereas soil with a high clay content is very good at holding
water. The difference between them is not so much the amount
of air space between the soil particles as it is the size of the
air spaces in the soil. In high clay soils, the air spaces are
more numerous and much smaller than in sandy soils, making it hard
for water to drain out. Water can more easily drain through
high sand content soil than through high clay content soil.
Therefore, high clay content soil retains water much more
easily than doe high sand content soil.
David R. Cook
Climate Research Section
Environmental Science Division
Argonne National Laboratory
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Update: June 2012