186 A TEXTBOOK OF GENERAL BOTANY 
SOIL 
Texture of soil. Soils are composed of particles which are 
classified according to their size as gravel, sand, silt, and clay. 
These divisions are arbitrary, and the size limits assigned to the 
various classes by different authorities are not always the same. 
Particles over 1 millimeter in diameter may be regarded as gravel 
or rock ; those between 0.05 millimeter and 1 millimeter, as sand ; 
those between 0.005 millimeter and 0.05 millimeter, as silt; and 
those less than 0.005 millimeter, as clay. A soil containing a 
large proportion of sand is called a sandy soil, one with much 
clay a clay soil, while soils that are intermediate are loams. 
The amount of water held by a soil varies with the total sur- 
face of the particles, and so clay, being more finely divided than 
sand, holds more water than sand, which dries out rather quickly. 
Movement of water in soil. During a rain, water enters the 
spaces in the soil and expels the air. The action of gravity tends 
to carry the water down into the soil, but films are left around 
the soil particles. The downward movement of the water is 
known as percolation. After the rain has ceased, air is drawn 
into the soil as the water moves downward. Percolation con- 
tinues until a region (the water table) is reached in which all the 
spaces are occupied by water. The percolation of water is most. 
rapid in coarse soils and slowest in fine soils. In clay soil, perco- 
lation may be so slow that after the upper layers have become 
saturated much water runs off the surface, while the underlying 
layers remain relatively dry. 
The percolation of water is very important, as by this means 
a reserve supply is carried to lower levels. The soil loses water 
by evaporation from the surface, with the result that water is 
drawn up from the lower levels in much the same way as oil 
moves up a wick. Also, when a plant absorbs water from the 
soil particles in contact with the root hairs, water is drawn from 
more moist particles to those from which the plant has taken 
water. It is owing to this movement of soil water that a soil 
dries out rather uniformly. 
