38 LIFE ON THE FARM. 



built into plant tissue, and passed on to animals. 

 In the process of organic decay, carbon again 

 unites with oxygen and passes back to the air. It 

 also occurs as carbonates with some of the metals, 

 such as calcium and magnesium. 



Iron occurs both in a free state and combined 

 with some of the other elements, especially oxygen 

 and sulphur. It is found widely distributed 

 through all soils in such quantities that it is rarely 

 necessary to apply it as a fertilizer. It does not 

 enter to any great extent into the composition of 

 plant tissue, but is thought by ?ome to aid in the 

 processes of growth. 



The other elements are found in the earth as 

 compounds, and enter into plant structure. When 

 they are lacking to any great extent it is necessary 

 to supply them to the soil in some form o'r other. 



RELATION OF SOME SMALL ANIMAL? TO THE SOIL. 



Besides giving up their bodies in decay to add to 

 its richness, some animals do not a little in improv- 

 ing the texture of the soil, and making it more 

 open for the free circulation of air anI water. 

 Such animals are those that live in the ground 

 permanently, or those that burrow into it foi shel- 

 ter. 



The crayfish, in many places, has the habvt of 

 digging deep holes in the ground, and bringing; up 

 the earth from a depth of several feet. This 

 earth, or mud, is generally built into a sort of 



