182 THE PRINCIPLES OF SOIL MANAGEMENT 



it is reduced to near the minimum capillary content, 

 or wilting point. At this stage the movement is exceed- 

 ingly slow, because of the excessive friction. This funda- 

 mental principle is made use of in soil mulches and de- 

 termines their usefulness, and should direct their man- 

 agement. (See page 203.) 



86. Structure. Structure affects capillary movement. 

 It has been shown how capillary movement is largely 

 due to the size of the individual spaces in the soil. 

 The size of the spaces is due, (1) To the size of the par- 

 ticles. (2) To their arrangement. (See page 203.) 

 The smaller the particles, and therefore the smaller the 

 pores, the greater the capillary power and the slower 

 the movement. In so far as the arrangement of the 

 particles or structure effects a change in the effective 

 size of the pores, it affects the capillary movement. 

 In a puddled structure the movement is much more 

 slow than in a soil having a granular or crumb structure. 

 Any tillage operation which alters the structure, in 

 either one direction or the other, thereby alters the 

 capillary power and the rate of movement. Compact- 

 ing a soil is well known as a process which seems to 

 draw moisture into the compacted zone; while culti- 

 vation, or loosening the soil structure, has the opposite 

 effect. Upon this fact v are based many important 

 tillage operations, such as rolling after seeding small 

 grains. 



87. Surface tension. Surface tension affects capillary 

 movement in the same way that it affects the capillary 

 retention of water. It represents the cohesive properties 

 of the liquid, and corresponds to an elastic membrane. 



