The Environment of Plants 



143 



FIG. 81. The results of deep tillage and shallow tillage. 



material, which gives the brown and black colors to rich 

 agricultural land, is composed of the partially decayed re- 

 mains of plants. Leaves and other plant organs that fall 

 to the ground are slowly changed and broken up by bacteria 

 and other agencies until only the brown, powdery humus 

 remains. 



Humus favors plant growth by increasing the water-hold- 

 ing capacity of the soil and so rendering the water supply 

 more uniform throughout the growing season. It improves 

 the physical properties of the soil by making it mellow. Hu- 

 mus also makes it possible for bacteria and other organisms 

 that increase fertility to live within the soil. 



In agricultural literature the importance of the soil factors 

 is emphasized. Plowing, harrowing, disking, and cultivating 

 are methods of keeping the soil in good physical condition. 

 Adding manures to increase the humus, and adding nitrates, 

 phosphates, and other salts as fertilizers, are methods of im- 



