THE FORMS OF SOIL- WATER 155 



is evident, therefore, that not only must any method be more 

 or less arbitrary but that its value can only be comparative. 



In the actual procedure, 1 the sample of soil may be air- 

 dried or dried at 100° or 110° C. If the former method is 

 followed, the sample after exposure is heated for four or five 

 hours at 100° or 110° C, the loss being considered as hygro- 

 scopic water. If oven-dried soil is utilized, the gain in weight 

 due to the exposure to the moist air is the hygroscopic mois- 

 ture. If a saturated air is made use of, the gain is maximum 

 hygroscopicity, from which can be calculated the percentage 

 of hygroscopic water based on dry soil, called the hygroscopic 

 coefficient. If a partially saturated air is utilized, a sample 

 of stock soil, the hygroscopic coefficient of which is known, is 

 exposed at the same time. The determination on the known 

 sample shows what proportion of possible hygroscopic water 

 has been taken up. From this the hygroscopic coefficient of 

 the unknown soil sample can be calculated. 2 



88. Hygroscopic capacity of soils. — Since hygroscopic- 

 ity depends almost directly on the colloidal nature of the soil, 

 it is evident that texture, external factors being under con- 

 trol, will be an important factor in determining the' hygro- 

 scopic coefficient. When the organic matter of soils is more 

 or less the same in amount, the inorganic colloids seem to con- 



1 Hilgard, E. W., Soils; pp. 196-201, New York, 1911. This method 

 is practically the same as that used for the comparative estimation of 

 the colloidal content of the soil, the hygroscopic coefficient being the 

 comparative figure obtained. See note to paragraph 74 of this text. 



Bouyoueos determines the hygroscopic coefficient in an approximate 

 way by means of the dilatometer method. The dilatometer is an 

 apparatus which measures the expansion of water on freezing. If a given 

 amount of soil and water is reduced below zero, the expansion attained 

 will reveal the amount of water remaining unfrozen, due to its soil 

 relationships. Bouyoueos finds that the amount of moisture unfrozen 

 after supercooling to —4° C. (slightly more freezes at -78° C.) correlates 

 fairly well with the hygroscopic coefficient. Bouyoueos, G. J., A New 

 Classification of Soil Moisture; Soil Sci., Vol. XI, No. 1, pp. 33-47, 

 Jan., 1921. 



2 Alway, F. J., and Clarke, V. L., Use of Two Indirect Methods for 

 the Determination of the Hygroscopic Coefficients of Soils; Jour. Agr. 

 Ees., Vol. VII, No. 8, pp. 345-351, Nov., 1916. 



