Michigan State University Extension
Ornamental Plants plus Version 3.0 - 00001812
11/12/99

Soil Water


A good garden soil contains approximately 25 percent
water. It takes approximately 300 pounds of water to
produce 1 pound of plant dry matter. Because clays and
silts have more exposed surface in a given amount of soil,
they have a greater water-holding capacity than sandy
soils. Because water will rise higher in clays and silts,
these soils will support plants better than sandy soils in
dry weather.

Water exists in the soil in three forms: capillary,
hydroscopic and gravitational.

Capillary Water

Capillary water adheres to a soil particle in the same way
a film of water adheres to any object. This film of water
moves, by "capillary attraction," from one soil particle
to another. The smaller particles, such as clay, have
greater exposed capillary surfaces. As a result, water
will rise higher in a 1-inch tube containing clay than in
a 1-inch tube containing sand.

Hydroscopic Water

This is a very thin film of moisture that "sticks" to each
soil particle. Even in very dry soil some hydroscopic
water is present. The only way to remove all of the
hydroscopic water from a soil sample is to bake the sample
in an oven for a long time. Hydroscopic water is so
tightly bound to soil that roots cannot absorb it.

Gravitational Water

Gravitational water is pulled out of large pores by
gravity after rain or irrigation. As the water is pulled
out, it pushes out toxic gases and a new oxygen supply
moves into the soil.


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