CHAPTER X 
THE BIOLOGY OF THE SOIL 
WE are now able to form an idea of the true nature of the 
soil in which plants live. In the preceding chapter we 
looked upon it as a mixture of rock-fragments supplied 
with a certain amount of water, air, and solar warmth. 
It is really much more than this. Upon its surface rests 
a covering of green, and within it dwells a living world 
of animals and plants which, by their activity, fit it to 
become an abode for the green vegetation that rises from 
it into the air and light. 
A broad view of the soil will therefore include the follow- 
ing considerations : 
1. The earth or soil itself, its physical and chemical 
properties. 
. The water contained in it. 
. The air contained in it. 
. The warmth received by it from the sun. 
The humus contained in it. 
. Its living populations : 
> i> oO bo 
(a) Its surface covering of green. 
(6) Its underground inhabitants. 
(i.) Plants—e.g., fungi and bacteria. 
(ii.) Animals—e.g., earthworms. 
Osmosis and Selective Absorption.—In the last chapter 
we described the mode of occurrence of water in the soil. 
The plant absorbs water through its root-hairs. These 
are in contact with the soil-particles, and consequently 
with the water-films surrounding them. When water 
diffuses from these films into the root-hairs, water is 
drawn along all the adjacent films until equilibrium is 
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