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 reaUy much more than this. Upon its surface rests 

 a covering of green, and within it dwells a hving 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. 



2. The water contained in it. 



3. The air contained in it. 



4. The warmth receivied by it from the sun. 



5. The humus contained in it. 



6. Its living populations : • 



(a) Its surface covering of green. 

 (6) Its underground inhabitants. 



(i.) Plants — e.g., fimgi 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 soU. 

 The plant absorbs water through its root-hairs. These 

 are in contact with the soU-particles, and consequently 

 with the water-films surrounding them. When water 

 diffuses from these films into the root-hairs, water is 

 drawn along aU the adjacent films until equilibrium is 



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