THE SOIL 65 



dead bodies of animals and plants had simply remained 

 upon the surface of the earth and had not decayed into 

 an unrecognizable, powdery mass in the soil ? The layer 

 of trash would soon become so deep that seedlings upon 

 the ground would find it impossible to push their way 

 up into the sunlight ; there would be no room for animal 

 life; the earth would become uninhabitable. The lower 

 organisms perform a great service in reducing the re- 

 mains of plant and animal life to the black residue in the 

 soil which .we call "humus". The humus represents but 

 a small portion of the great bulk, the greater part being 

 released into the air in the form of gases and water 

 vapor, through the action of our infinitely small friends. 

 But this does not tell the whole story. We have shown 

 that plants require food as well as animals, and that this 

 food is derived from the soil, air and water. The ma- 

 terials from these sources would have long since be 

 come exhausted if it were not possible to use them over 

 and over again. Thus we speak of the "cycles" of the 

 plant-food elements, and in these the lower organisms 

 play the leading part. In the decay of plant and animal 

 matter, carbon dioxide gas is given off, and this carbon- 

 dioxide can again be used by living plants as before ex- 

 plained. In a similar manner nitrogen may be used 

 over and over again, and likewise phosphorus, though 

 instead of a gas these substances are converted into 

 nitrates and phosphates, in which form they become 

 available to plants. The cycle is not always so simple. 

 It is the business of the microorganisms to prepare the 

 food for the higher plants, and the plants and animals 

 in turn furnish the food and energy through which 

 microorganisms can do their work. The one depends 

 upon the other. 



