62 The Plant World. 



content, the capillary pressure will be higher in the dryer spots 

 and less in the wetter. There will be, therefore, a tendency to 

 move water from the wetter spots to the dryer and hence to 

 make the water content uniform throughout. This uniformity 

 is actually attained when sufficient time is allowed and when no 

 other forces are acting. Usually, however, there are other and 

 disturbing forces. The case of capillary rise against gravity 

 has already been mentioned. Here the water content (when 

 rise is complete) will vary continuously from bottom to top. 

 The capillary pressure at any point in the column depends upon 

 the total of the tensions in the film-systems above that point. 

 It decreases with increasing height. On the other hand, the 

 gravitational force acting at any point depends upon the weight 

 of the suspended water below that point, and therefore increases 

 with height. The balance of these capillary and gravitational 

 forces results in the condition stated — a continuous (though 

 not necessarily uniform) decrease of water content from bottom 

 to top. * 



Of course the capillary pressure is very intimately related 

 to the general physical nature of the soil and especially to its 

 mechanical composition. When finer particles compose the 

 soil mass not only are there more interspaces and hence more 

 water films, but the tensions of these films are relatively stronger, 

 since their curvature is (in general) greater. It follows , therefore, 

 that the total capillary pressure of a soil at any given water 

 content will be greater the finer it is in mechanical composition. 

 The finer soil will draw water from the coarser, and when two 

 soils of difTering mechanical composition are so juxtaposed that 

 their water film systems are in contact and united the establish- 

 ment of equilibrium will produce a higher water content in the 

 finer soil than in the coarser. A clay soil can extract water (by 

 capillary movement) from a sand which is far less moist than it 

 (the clay) is. When clay and sand are in "capillary equiU- 

 brium"t the former will have many times the water content 

 of the latter. Similarly the height of capillary rise in a clay soil 

 may reach five or six feet while a pure sand will show one foot 



*The contimiitv of the oirve representing water content against height is apparently lost 

 at the top of the column. The decrease in water content is there more abrupt than it 

 should be. This will be discussed below. 



JThe condition in which the capillary pressures of the two soil are equal, or, dynamically 

 defined, the condition in which capillary forces will produce no movement of water 

 from one soil to another. 



