WATER, AIR, AND HUMUS OP 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. 



Fig. 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 

 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 fiUed. 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 



