100 



THE SOIL AND ITS IMPROVEMENT 



b. Decomposition is a chemical process ; its work is 

 done by changing the composition of the particles. Rocks 

 rugt^ as iron does. That is, their surface particles com- 

 bine with oxygen from the air. Then the powdery rust 

 particles are easily removed. Rocks are also acted upon 

 by certain acids which the decay of organic matter forms 

 in the soil. In this way parts of the rocks are dissolved ; 

 and then they are carried away by running water. De- 

 composition and disintegration fre- 

 quently work together, in this way, on 

 the same rock, each aiding the other 

 in forming soil. 



58. Humus. Humus is partly de- 

 cayed vegetable and animal matter in 

 the soil. It is found in a pure state 

 around rotten stumps. The greater 

 part of humus comes from plants that 

 have decayed in the presence of mois- 

 ture and of the oxygen in the air. 

 Humus is a very important part of the 

 soil. It has the following uses : 



(1) It makes the soil lighter and 

 more porous, thus improving its con- 

 dition. 



(2) It helps the soil to hold heat 

 and moisture. 



SOIL AUGER. 



Used for examining 

 soils at different depths. 



(3) It supplies food for growing plants. 



(4) It promotes the growth of useful bacteria. 

 Humus is obtained from roots, stubbles, and manures, 



which are plowed under and left to decay in the soil. 

 Usually it makes the surface soil darker than the subsoil, 

 or what lies below the reach of the plow. In fact the 

 color of the soil is often taken as an indication of the 

 amount of humus present. 



