THE SOIL 105 



This water is retained by "capillary attraction? 

 which is the power that causes water to flow into any 

 very small cavities, and is commonly well exhibited in 

 sugar, blotting-paper, and similar porous substances. 

 If one of these is gently brought into contact with a 

 drop of water, the water enters the small pores or 

 cavities and spreads over a large area, where it is 

 retained, and from which it will not drain away again. 

 By means of this power, soils retain a sufficiency of 

 water for the use of plants, the small spaces being filled 

 with water while the larger spaces contain air. The 

 soil is thus provided with both of these requisites for 



plant growth. 



Clay. 



It has already been said that clay is formed from 

 felspar by the action of water and carbon dioxide. 

 Pure clay consists of extremely minute particles, but 

 soils are never pure clay, there being always a 

 certain amount of sand present. The fine particles of 

 clay have a tendency to collect together in groups or 

 masses. If this were not the case all the small openings 

 and passages in the soil would be choked, and drainage 

 rendered impossible. Clay also has the power of 

 absorbing water and becoming plastic ; that is to say, it 

 can be kneaded and moulded by the hand, a property 

 which is taken advantage of in making bricks and 

 pottery. When strongly heated, clay loses this property. 



The operations of tillage are partly directed toward 

 breaking up the masses of clay, admitting air into the 

 soil, and increasing the size of the capillary spaces. 

 They also increase the tendency which the fine particles 



