i8 



PRINCIPLES OF FARM PRACTICE 



water draw films of water around them; these films extend 

 to the next layer of particles, and so on until the surface is 

 reached. If the lamp chimney is filled with clay, the water 

 will pass upward very slowly, requiring perhaps several hours 

 to reach the top. The rate of capillary rise evidently de- 

 pends upon the size 

 of the soil particles; 

 the smaller the parti- 

 cles, the slower the 

 rate. 



The effect of the 

 different sized parti- 

 cles on capillary ac- 

 tion is important in 

 another way. If, in- 

 stead of short lamp 

 chimneys, long glass 

 tubes filled with sand 

 and clay are used, it 

 will be seen that when 

 water reaches a cer- 

 tain height in the 

 sand it will rise but 

 little higher, whereas in the clay it will continue to rise 

 slowly for a distance of several feet. In other words, the 

 lifting power of sand through capillarity is much less than 

 that of clay. Clay soils are, therefore, able to draw water 

 from greater depths than sandy soils, and consequently are 

 less affected by dry weather than sandy soils, provided there 

 is a supply of free water below. The great power of clay 

 soils of lifting water by a capillary pull is somewhat offset 

 by the slowness of its action. The same means suggested 



Diagram showing the relations of soil particles 

 and water film to a root-hair. 



A. Root-hair. B. Soil particle. 



C. Film of water thickened at angles. 



D. Air space. 



Arrows show direction that water takes. 



