THE ROOT 187 
Water is drawn up in loam more rapidly than in sand or clay. 
The attraction of a soil for water increases with the total surface 
of the particles; consequently loam draws water with greater 
force than does sand. According to this reasoning we should 
expect clay to draw water more rapidly than loam; but clay is 
composed of such small particles, and the spaces between the 
particles are so small, that water is held with great force and 
there is less tendency for the water to percolate or be drawn up 
than in the case of loam. 
Loam allows sufficient water to percolate through it, and 
then draws it up again with considerable rapidity. For this 
reason loam is better for agricultural purposes than is either 
sandy soil or clay. Moreover, the air spaces are large enough 
to allow for sufficient aération, which is not likely to be the 
case with clay. 
The upward movement of water takes place in the films 
around the particles, and so any interference with the conti- 
nuity of the films tends to retard the movement. Water can 
be conserved by working the soil near the surface into a mulch, 
or loose layer, as by this means the continuity of the films is 
interrupted, so that water is not drawn to the surface, where it 
would be lost by evaporation. That water is drawn to the sur- 
face to a greater extent in compact than in loose soil is shown 
in the case of footprints in a cultivated field. The soil under 
the footprints is more compact than the surrounding soil and 
has a darker color due to the greater amount of water that it 
contains. 
Soil structure. The term sod structure is used to denote the 
manner in which the particles are arranged in a soil. When the 
fine particles are aggregated in granules, the soil is said to have 
a granular, or crumb, structure. Such a soil is loose and friable 
and is considered to be in good physical condition. If a clay 
soil is worked when it is wet, the granular structure is destroyed 
and the soil becomes compact and poorly aérated. The soil is 
then in poor physical condition. The wet condition is very 
persistent, and at the same time the soil is impervious to the 
