40 PRINCIPLES OF AGRICULTURE 



means: (a) by apply mechanical force, as in all 

 the operations of tilling; (b) by setting at work 

 various physical forces, as weathering (fall-plow- 

 ing is a typical example), and the results follow- 

 ing under-draining; (c) by applying some ma- 

 terial that acts chemically on the particles. (The 

 first caption, a, is illustrated in paragraphs 26, 

 26a, 26&, 27, 28; and it is further explained in 

 Chapter iv.) 



57. (b) Under-drainage has two general uses, 

 it removes superfluous water, and improves 



the physical condition of the soil. The latter 

 use is often the more important. The improve- 

 ment of the texture is the result, chiefly, of 

 preventing water-soaking and of admitting air. 

 Under- drained soils become "deeper." The water- 

 table is lowered, since the depth at which water 

 stands tends to approach nearer and nearer to 

 the depth of the drains and thereby the plant 

 roots are enabled to penetrate more deeply. 



58. (c) Some substances have the power to 

 break down or to pulverize hard soils, or to bind 

 together loose ones, or otherwise to modify the 

 structure. Such materials which are applied 

 for their remote or secondary chemical effects 

 are called amendments. Lime is a typical ex- 

 ample. Quick-lime is known to make clay lands 

 mellow, and it is supposed to cement or bind 

 together the particles of sands or gravels. Most 



