68 RELATIONS OF SOIL TO WATER 



Looking first at the volume of water in a given volume of 

 soil, we see in each case that the proportion is lowest in a 

 sandy soil, rises somewhat when the soil is calcareous, or is 

 a loam or clay, and reaches its highest point when humus is 

 present. The figure obtained by Schwarz for peat is doubtless 

 below the truth ; as his figures were obtained by calculation 

 from specific gravities, the result of the swelling of the peat 

 on wetting is thus excluded. 



Hilgard and Loughridge (Rep. Agri. Exp. Stat. California, 

 1892-3-4, 80) found that the California soils were usually 

 saturated by 44-60 per cent, their volume of water ; a very 

 coarse sand was saturated with 37*5 per cent, its volume. 

 The soils which distinctly swelled or shrank when wetted are 

 excluded from this statement, as their final volume was not 

 accurately known. The proportion by weight varied from 

 23-68 of water per 100 of air- dried soil. 



Hilgard's method for determining the water capacity of a 

 soil is to use a circular brass box having a sieve bottom, 10 mm. 

 in depth, and having a capacity of 25 cc. ; the weight of 

 the box is known. This box is filled with the air-dried soil 

 in powder, the soil settled by tapping the box on the table, 

 the surface struck level with a thread of silk, and the box 

 then weighed. The box is then supported on a triangle in 

 water, so that the bottom of the box is just beneath the 

 surface of the water. The box is left thus for an hour or 

 more, till the soil is fully saturated ; the box is then rapidly 

 wiped and weighed. The result can be calculated both by 

 volume and weight. 



In analyses of soil the proportions of water, and other con- 

 stituents, are usually stated by weight ; this mode of state- 

 ment may, however, lead to a serious misinterpretation of the 



