58 



RISE OF WATEE IN SOILS 



below which all the interstices of the soil are filled with water, is near 

 the bottom of the diagram, and the thickness of the water films sur- 

 rounding the soil particles gradually diminishes towards the actual 

 surface of the soil. 



The lessening of loss of water by stirring the upper layers of soil, as 

 in " mulching," is achieved, not, as is so often stated, by " breaking the 

 capillary tubes," which we have seen do not play an important part, 

 but by disturbing the continuity of contact between moistened par- 

 ticles. 



When some of the dry particles from the surface are turned under, 

 the upward motion of the soil- water is interrupted, since, before the 

 concave surfaces of liquid at the points of contact of the soil particles 



FIG. 4. Diagrammatic section of soil. 



can be established, the dry particles have to receive a liquid film. The 

 wetting of a dry particle can only be effected by a slow, creeping move- 

 ment of water, and is not aided by the effect of the concave surfaces 

 as described. 



While stirring the surface of a soil in dry weather increases for a 

 time the rate of evaporation, the total loss of water may be greatly 

 diminished owing to dry particles of soil impeding the upward flow 

 of water by abolishing, for a time, the concave surfaces of the water 

 films at the points of contact of the soil particles which are so power- 

 ful in producing the movement. 



The rise of water in a soil from below is usually attended with 

 beneficial results the maintenance of sufficient moisture for the needs 

 of plants in the surface soil and the concentration of the very dilute 

 solution of plant food but, in some cases, it produces disastrous effects. 



