CHAPTER V. 



THE BIOLOGICAL CONDITIONS IN THE SOIL. 



IT is now necessary to study the soil conditions that determine the 

 growth not only of plants but also of the micro-organisms that, as we 

 have seen, make new plant food out of old plant residues, and render 

 possible the continuation of vegetable life on the earth. These con- 

 ditions are water supply, air supply, temperature, food supply, and 

 absence of injurious factors. 



The Water-Supply of the Soil. 



The rain-water falling on the soil immediately begins to soak in, 

 but during its passage a certain amount is retained on the surface of 

 the particles and never drains away ; it forms a series of continu- 

 ous films exhibiting, as Briggs (55) has shown, all the special pro- 

 perties associated with the surfaces of liquids. Thus, the water re- 

 mains on the particles against the force of gravity. Further, it tends 

 to distribute itself evenly throughout a uniform mass of soil by mov- 

 ing from places where the curvature of the films is flat to places where 

 the curvature is sharp. How far this tendency is an important factor 

 in the distribution of water in the soil is not known ; Leather's re- 

 sults at Pusa (167) indicate that it may not be very marked. There 

 are at least three factors that complicate the problem : 



1. Evaporation is continually reducing the thickness of the films, 

 and finally breaks them altogether, so that the soil becomes dry. It 

 then remains dry till more rain falls, because the waxy material on 

 the particles, like grease on a surface of glass, prevents the spread of 

 the films, and may even make wetting by the rain a matter of some 

 difficulty. 



2. The pores of the soil are so small that there is considerable 

 friction, retarding very much the speed of the water movements. In 

 clay soils this retardation is particularly marked, and not uncommonly 

 causes soils to crack with drought even within two or three feet of a 

 stream. Not only are the film movements affected but the downward 

 percolation also, so that free water is retained near the surface for a 



