THE BOOTS 37 



Irregular branching lines or rougliened areas will 

 show where the roots have touched the polished 

 surface, and the hairs have etched away the mineral. 



41. The fate of the particles of the soil. — The 

 particles of soil may be roughly classed as mineral 

 and organic. The latter comprise the remains of 

 plants and animals as described above, and constitute 

 what is known as humus. The humus is constantly 

 being broken down by the action of the soil bacteria 

 and other minute organisms. This action of the 

 bacteria is to obtain the material necessary for their 

 own food, and in the process great quantities of sub- 

 stance are formed which the higher plants use. The 

 deep layer of decajdng leaves and twigs in a forest 

 that has not been burned, or laid bare by grazing 

 animals, is a good example of humus formation. 



The mineral particles may not be said to decay, 

 but they are constantly being broken up. The water 

 in the soil always contains carbonic acid set free by 

 living plants and by the action of bacteria on the 

 humus, as well as by the decomposition of certain 

 minerals themselves. Other acids are found in the 

 air and are washed down into the soil by rains. The 

 thin film of water which surrounds each particle con- 



