MOVEMENTS OF DISSOLVED SUBSTANCES. 63 



to take up a form with the minimum surface. Con- 

 sequently if a portion of soil be only just moist the 

 water films on its particles will be very thin and have 

 relatively a large surface, while if another portion of 

 the soil be very wet the water films in it will be thick 

 and have relatively a small surface, and thus if the 

 two portions of soil be placed in contact, the surface 

 tension, which takes the form of a definite inward 

 pressure per unit of surface, causes the water to move 

 from the very wet to the less wet particles. This mo- 

 tion takes place in any direction, and may be either 

 helped or hindered by gravitation. If, however, the 

 interstices of a soil be completely filled with water, 

 surface tension within the soil becomes zero and 

 gravitation alone acts on the water. Thus when the 

 surface soil becomes dried by evaporation, water from 

 below is brought up by this passage from particle to 

 particle, the impelling force being surface tension. 

 Whitney has shown" that nearly all dissolved mineral 

 substances increase the surface tension, while or- 

 ganic bodies and ammonia diminish it. 



(b) Gravitation, This of course always acts in the same 

 direction, and, as has just been shown, is the only 

 force acting on the water in a fully saturated soil. It 

 greatly affects distribution of dissolved substances in 

 soil, though its tendency to cause the liquid to sink 

 in the soil is in many cases overcome by the surface 

 tension phenomenon above described. 



As a rule the motions of the water in a soil caused by the 

 two agencies just described overpower the diffusive tendencies 

 of the substances in solution. Thus in dry weather the water 

 from the subsoil is brought up to the soil by the surface ten- 

 sion effect. Evaporation and consequent concentration of the 

 solution takes place and the diffusive powers of the dissolved 

 substances would tend to move them downwards to the weaker 

 solution in the subsoil. This they undoubtedly do to some 

 extent, but not as rapidly as the liquid moves upwards. The 

 net result thus is that in dry weather the solution of plant food 



* Weather Bureau, U.S.A., Bulletin No. 4, 13. 



