162 NATURE AND PROPERTIES OF SOILS 



tational water has had time to drain away. This represents 

 the hydroscopic plus the capillary water. A determination 

 of the hygroscopic coefficient on another sample yields a figure 

 which, when subtracted from the total water, will give the 

 capillary water present in the soil. The capillary water at 

 various points in a soil column may be obtained by subtracting 

 the hygroscopic coefficient from the various percentages of 

 moisture present, since the hygroscopic moisture is little in- 

 fluenced by height of column or ordinary structural condi- 

 tions. 



The determination cited above may or may not give the 

 maximum water-holding capacity of a soil. To fill such a need 

 a laboratory method has been devised by Hilgard, 1 which 

 attempts to show the maximum retentive power of a soil for 

 water. 



A small perforated brass cup is used, having a diameter 

 of about 5 centimeters and capable of containing a soil column 

 1 centimeter in height. A short column is used, since it is 

 only under such conditions that a soil may retain against 

 gravity the greatest amount of water. Also the soil is able 

 to expand or contract, as the case may be, on the assumption 

 of water until an equilibrium is reached. A filter-paper disc 

 is often placed in the metal cup, and the soil is poured in, 

 gently jarred down, and stroked off level with the top of the 

 cup. The cup is then set in water and the soil is allowed to 

 take up its maximum moisture. After draining, the weight 

 of the wet soil plus the cup, together with the weights pre- 

 viously obtained, will allow a calculation of the total water 

 retained based on the absolutely dry soil. If the maximum 

 capillary water is desired, the hygroscopic coefficient may be 

 subtracted from the maximum water retained. 



Since this method is a laboratory procedure and the soil 

 used is not in its normal structural state, the results cannot 

 be accurately applied to field conditions. While the figures 



1 Hilgard, E. H., Soils, p. 209, New York, 1911. 



