THE FOEMS OF SOIL-WATEE 



157 



Apparently, the finer the soil, the higher the hygroscopic 

 coefficient. This is due to the fact that most of the inorganic 

 colloidal matter is carried by the finer separates. In consid- 

 ering the hygroscopicity, however, the influence of the organic 

 matter must not be forgotten. Organic colloidal matter has 

 a very marked influence on absorption, and as the organic 

 matter of the soil increases, the hygroscopicity rises rapidly. 

 The following data from Beaumont^ is interesting in this 

 respect : 



Table XXX 



THE HYGROSCOPIC COEFFICIENT^ COMPARED TO CERTAIN OTHER 



SOIL FACTORS 



Soil 



Clay 

 % 



Igni- 

 tion 

 % 



■ — - 



Humus 

 % 



Htgeo- 

 scopic 

 Coeffi- 

 cient 



% 



Dunkirk silty clay loam, surface 

 Dunkirk silty clay loam, subsoil 



Clyde clay loam, surface 



Vergennes clay, subsoil 



12.9 

 20.0 



20.1 

 74.5 



5.08 

 3.05 



14.54 

 5.79 



1.26 

 .20 



4.34 

 .49 



3.80 

 5.77 



18.90 

 17.40 



In comparing the two Dunkirk soils it is apparent that the 

 colloidal clay is the dominant factor in determining the mag- 

 preferable in that the basis for calculation is not a changeable one as is 

 the weight of moist soil. The dry basis is practically always used in 

 soil work. 



Where two soils of different volume weight are compared, the per- 

 centage relationship does not give a true idea of the relative amounts 

 of water present. A volume expression should then be used. If a cubic 

 foot of soil, weighing 100 pounds, contains 10 pounds of water it would 

 be carrying (10 X 27.6) or 276 cubic inches of water. This would 

 equal (276 — 1728) X 100 ot 15.9 per cent, by volume or (10 -^ 5.2) = 

 1.92 surface inches. 



* Beaumont, A. B., Studies in the Beversilility of the Colloidal Condi- 

 tion of Soils; Cornell Agr. Exp. Sta., Memoir 21, pp. 501-504, April, 



= Moisture content in this text unless otherwise indicated will always 

 be expressed on the dry soil basis. 



