444 



THE PRINCIPLES OF SOIL MANAGEMENT 



may be disastrous if the soil is not kept compact by 

 careful tillage. On the other hand, in a humid region and 

 in clay soil, there is likely to be too small a supply of 

 oxygen for the use of crops and lower plant life unless 

 the soil is well stirred. 



310. Tillage. The ordinary operations of tillage 

 influence greatly the ventilation of the soil. When a soil 

 is plowed, the soil at the bottom of the furrow is exposed 

 directly to the air at the surface, and, by the separation 



Fig. 119. The Acme harrow. An efficient pulverizer on clean soil, 



free from stones. 



of adhering particles and aggregates of particles, air 

 is brought in contact with particles that may previously 

 have been completely shut off from the air. It is largely 

 because of its effect upon soil ventilation that plowing 

 is beneficial, and the necessity for its practice is greater 

 in a humid region and upon a heavy soil than in a region 

 of small rainfall and on a light soil. The practice of list- 

 ing corn, by which the soil is sometimes left unplowed 

 for a number of years, although in the semi-arid region, 

 productive of crops of sufficient yield to make them 



