THE FILM WATER 



INDIGO CARMINE IN PENN. LOAM SOIL (/ = 21 C.). 

 Solution contained 2 grains dye per liter. 



It has also been shown repeatedly by experiment that the 

 movement of moisture is relatively rapid when the moisture con- 

 tent of the soil is above the optimum, but that the movement is 

 exceedingly slow when the soil has a lower water content than 

 the optimum; that is, the point at which the water is entirely in 

 the form of film water. For instance, if a moderately wet sample 

 of soil be brought into intimate contact with an air-dry sample 

 of the same soil, there will, at first, be a relatively rapid move- 

 ment of the moisture, but as soon as the wetted portion has been 

 brought to the "optimum" condition, no further movement can 

 be detected, although the experiment has been tried of leaving 

 such samples together for months and with a difference of water 

 content amounting, in the case of clay soils, to 15 or 20 per cent. 

 Since the drought limit, or the soil moisture content at which 

 plants wilt, is, for most soils, considerably below the optimum 

 water content, the movement of film water is obviously a problem 

 of the first importance from a practical point of view as well as 

 of the highest theoretical interest. 



The movement of water vapor, or its distillation from place 

 to place in the soil, is another problem often confused with the 

 above. Its importance is not yet clear, although according to 



