THE SOIL AND IT,S IMPORTANCE TO PLANTS 29 



inent tends to contimic unl-il tlu^ water films are of (hjuuI thickness 

 throughout the entire soil mass, causing it to be uniformly moist. 

 When water is removed at any particular point, as by surface 

 evaporation or root absorption, it is therefore drawn thither 

 from all other points until equilibrium is restored. 



Fig. 17. — A foot-pnnt in loose soil. A vertical slice through the soil under 

 and near a foot-pnnt, showing how the particles which were under the foot have 

 been pressed together. This establishes a better capillary connection with the 

 lower soil layers and causes a more rapid movement of water to the surface, thus 

 often making the foot-print moist while the soil surface around it is dry. 



In soils which have lost all their capillary water by evaporation, 

 there still remains around each particle an exceedingly thin 

 film of hjgroscopic water, which clings so tenaciously that it may 

 be driven off only by subjecting the soil to a high temperature. 

 When the air is very dry, this water is present in minimum 

 amount, but when humidity rises, more water may be taken up 

 directly from the air, or hygroscopically. This type of water is 

 removed with such difficulty from the soil particles, however, 

 that the plant is able to obtain little or none of it. 



Air. — Since oxygen is essential for the healthy growth of ordi- 

 nary plant roots, the presence in the soil of a plentiful supply of 

 air is a matter of vital importance. If the spaces between the soil 

 particles become filled with water, most of the air is necessarily 

 driven out, and when this condition of saturation is long main- 

 tained, ordinary plants suffer. We have seen, however, that such 

 excess of water normally passes downward by percolation, and as 

 it does so the soil spaces fill again with air. In most cultivated 

 soils, from 20 to 35 per cent of the volume consists of air spaces. 

 The composition of the air which fills these is often somewhat 

 different from that of the atmosphere, the proportion of carbon 



