If
the mix is primarily clay, we call it a clay soil. If the soil is a
relatively equal mix of all three, containing no more than 35
percent clay, we call it a loam.
Available Moisture (inches of water per foot of soil)
Soil Texture Average Amount
Very coarse sand 0.5
Coarse sand 0.7
Sandy 1.0
Sandy loam 1.4
Loam 2.0
Clay loam 2.3
Silty clay 2.5
Clay 2.7
Source: _Fundamentals of Soil Science_.
Adhering water films can vary greatly in thickness. But if the water
molecules adhering to a soil particle become too thick, the force of
adhesion becomes too weak to resist the force of gravity, and some
water flows deeper into the soil. When water films are relatively
thick the soil feels wet and plant roots can easily absorb moisture.
"Field capacity" is the term describing soil particles holding all
the water they can against the force of gravity.
At the other extreme, the thinner the water films become, the more
tightly they adhere and the drier the earth feels. At some degree of
desiccation, roots are no longer forceful enough to draw on soil
moisture as fast as the plants are transpiring. This condition is
called the "wilting point." The term "available moisture" refers to
the difference between field capacity and the amount of moisture
left after the plants have died.
Clayey soil can provide plants with three times as much available
water as sand, six times as much as a very coarse sandy soil.
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