This direct relationship between particle size, surface area, and
water-holding capacity is so essential to understanding plant growth
that the surface areas presented by various sizes of soil particles
have been calculated. Soils are not composed of a single size of
particle. If the mix is primarily sand, we call it a sandy soil. 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