The Soil 53 



dered quartz, one coarse, the other very fine, will hold when 

 saturated, more than 40 per cent of their volume as water. But 

 when drained, the coarse sand will retain about 7 per cent while 

 the fine quartz holds 44.6 per cent of water. The latter will 

 loose, in fact, no water by drainage. 



Gravels and coarse sand retain the least water when drained. 

 As the particles become smaller, the retention of water increases. 

 Colloids, jelly-like bodies, as clay and humus, increase the power 

 of retaining water, as such bodies swell up when wetted and hold 

 the water in jelly-like substances; The addition of humus to 

 soils is one of the best ways of increasing their water retaining 

 capacity. 



Water from below may supply a surface soil if a saturated sub- 

 soil exists at a moderate distance. Such water is said to be 

 raised by capillary action, which simply means that the surfaces 

 of the soil particles exert an attraction for water. The finer the 

 particles and the closer they are packed, the greater the height to 

 which water will be carried by capillary action. When the dis- 

 tance it has to travel increases, the quantity reaching the surface 

 diminishes. When the fineness of the particles exceeds a certain 

 point the quantity of water raised also diminishes. It is not 

 always the soil with the finest particles that brings most water 

 to the surface. There is a certain degree of fineness of soil par- 

 ticles that acts most effectively. Capillary action is seldom able 

 to maintain a sufficient water supply at the surface. In Wiscon- 

 sin every few years crops suffer from drought, although a per- 

 manent water supply exists several feet below the surface. Cap- 

 illary action is most effective in the case of silty soils ; such soils 

 were deposited from running water and consist of very uniform 

 particles, but without any true clay. Some western soils, which 

 are capable of growing wheat with a winter rainfall of 10 to 12 

 inches and a continuous summer drought of three months' dura- 

 tion, are deep, fine grained, and uniform, with practically no 

 particles of the fineness of clay to check the upward lift of 

 capillarity. 



