26 SOILS AND PLANT LIFE 



and soil-making, it follows that any method we may em- 

 ploy to get more air into the soil will ordinarily increase 

 its fertility. In regions, however, where the rainfall is not 

 sufficient to settle the soil well or to allow the decom- 

 position of the organic matter, the amount of air in the 

 soil may be too great. In this case, the upward move- 

 ment of water, such as was shown in Exercise 8, will be 

 interfered with, and not enough moisture will reach the 

 roots or germinating seeds. The aim of the farmer in 

 these dry regions is to increase the moisture-holding ca- 

 pacity of the soils without making them so light that the 

 movement of the air will dry them out. 



The amount of air a soil contains depends (1) upon the 

 character of the soil, (2) upon the amount of organic 

 matter it contains and (3) upon the amount of moisture 

 present. 



EXERCISE 10 



Object. — To determine the amount of air space in the 

 soil. 



Procedure. — Secure as many ordinary tin cans as you 

 have samples of fertile and infertile, air-dried and sifted 

 soils. Fill each can with a soil and tap lightly on the table 

 to settle it. The space between the soil particles is now 

 occupied by air. Add water slowly to each can, keeping 

 a careful record of the amount. 



When the water has displaced all the air and stands at the 

 surface of the soil, you will be able to determine the amount 

 of air in the sample by the amount of water that has been 

 required to displace it. 



Conclusion. — State which soils contain the greatest 

 amount of air space. Does the amount of organic matter, 

 or humus, make any difference in the amount of air the 

 soil contains? Why is it that a soil needs air? 



