CHAPTER III 



THE AIR IN THE SOIL 



Tillable land is made up of soil particles, each of which 

 is inclosed in a film of moisture while the spaces between 

 them are filled with air. A square rod of ordinary soil 

 may contain from sixty to seventy-five cubic feet of air 

 in the upper fourteen inches. 



15. Why Air is Necessary in the Soil. — Air, which 

 consists chiefly of oxygen and nitrogen, plays a very 

 important part both in soil-making and in plant growth. 

 We have already learned that plants, decayed through 

 the action of fungi and bacteria, help to form soil. These 

 organisms, you will remember, break down the dead 

 plants into simple compounds which the living roots can 

 absorb again. (See Section 5.) 



The fungi and bacteria which cause decay can not live 

 without oxygen, nor can the roots of plants live without 

 it. Moreover, a germinating seed takes in oxygen and 

 throws off carbon dioxide. In fact, as we shall later 

 learn, it is impossible for any seed to germinate without 

 oxygen. 



In addition to oxygen and nitrogen, air also contains 

 carbon dioxide. This gas, which is given off by germinat- 

 ing seeds and decaying vegetable matter, helps to dissolve 

 the plant food in the soil. 



16. Why we seek to control the Air Space in the Soil. 



— Since the air in the soil is so essential to plant growth 



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