466 



THE PRINCIPLES OF SOIL MANAGEMENT 



soil than its direct effect. This complex effect is well 

 illustrated by drainage, which not only removes excess 

 water and admits air, but it thereby affects the soil 

 temperature, growth of organisms and the elaboration 

 of plant-food. Similarly, tillage, first of all, is designed 

 to alter the structure of the soil, and through this alter- 

 ation in structure, the retention of moisture, aeration 

 and root-penetration, not to mention many other 



Fig. 127. Bottom view of a modern plow, showing the parts. 1, share; 2, 

 moldboard; 3, landside; 4, frog; 5, brace; 6, beam; 7, clevis; 8, handle. 



relations, are changed. In fact, every practice which 

 may be applied to the soil influences in some degree 

 every phase of the soil mechanism. The relative promi- 

 nence of these different effects depends on the character 

 and condition of the soil. 



The application of these various practices has been 

 indicated in the foregoing pages, in connection with 

 the principles discussed. 



II. TILLAGE 



Tillage, or the manipulation of the soil by means of 

 implements, is so general in its application and so 



