THE AIR IN THE SOIL 29 



Harrowing, discing and cultivating all help to increase 

 the amount of air in the soil. Rolling and subsurface 

 packing, on the other hand, diminish the amount of air in 

 the soil. These latter operations are performed largely 

 in the dry farming regions. 



How much more water did the soil containing humus 

 take up than the same soil lacking humus in Exercise 10? 

 Remember that it was there shown that the air space in a 

 soil increases with the amount of humus, or organic matter, 

 which it contains. By adding manure to the soil, we 

 increase the amount of air in it. 



In Exercise 10, we expelled the air from the soil by add- 

 ing water. When we remove water from the soil by tile 

 drains, we allow air to take the place of the water. More- 

 over, a generous supply of air enters these tiles and moves 

 upward through the soil. 



Many plants, such as alfalfa and sweet clover, send 

 their roots deep into the soil. When these crops are 

 plowed up, the roots decay, allowing the air to fill the 

 space which they had occupied. This is one of the bene- 

 fits of crop rotation. 



QUESTIONS 



1. What are the uses of air in the soil? 



2. How many cubic feet of air may the upper fourteen inches 

 of a square rod of soil contain ? 



3. What influence has plowing and discing upon the volume 

 of air in the soil? 



4. Why is it that farmers in the dry farming regions work 

 to reduce the volume of air in their soils? 



5. What influence do clods have upon the soil air? 



6. Name two ways in which tile drains help to get air into 

 the soil. 



7. What influence has manuring upon the amount of air 

 in the soil? 



8. How does the rotation of crops influence soil air? 



