WATER, AIR, AND HUMUS OF THE SOIL 153 
and air which the roots and micro-organisms of the soil must have. 
Although not tightly packed, the particles adhere to each other 
when moist, and this feature and the weight of the soil enable 
roots to obtain a firm anchorage. 
/)) oe Benes 
Nil 
Fic. 136. — Diagram of two root hairs and the soil around them. The 
soil particles are shaded and the light area around each soil particle repre- 
sents a film of water. The large light areas among the soil particles are air 
spaces. Modified from J. G. Coulter. 
Water, Air, and Humus of the Soil. — To the plant the water 
of the soil has two important functions. First, it is the reservoir 
upon which the plant depends for water. Second, water is the 
solvent in which the soil substances become dissolved before 
entering the plant. 
The amount of soil water varies for different kinds of soils, and 
for the same soil at different times. Thus garden soil rich in 
humus or a very heavy clay soil will hold two or three times as 
much water as a sandy soil. Just after a heavy rain soils are 
saturated with water, that is, all of the spaces are filled. But 
much of this water, known as free water, gradually sinks away 
toward the center of the earth in response to its own weight, leav- 
ing the pores partially empty. The water then remaining in the 
pores consists chiefly of capillary water, which is held in the pores 
by the force of capillarity. In addition to the capillary water, 
which does not respond to the influence of gravity, there is also 
the hygroscopic water, which remains, after the capillary water is 
removed, as a thin film around each particle and so firmly held 
