128 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



ture and the degree of saturation of the atmosphere. The 

 capacity of the soil to hold hygroscopic water can be determined 

 by placing a thin layer of soil in a vessel, the air of which is 

 saturated with* water ; the soil will take up a certain amount of 

 water, which can be determined by the gain in weight of the soil. 

 The temperature of the containing vessel must be uniform, for 

 variations in temperature in a saturated atmosphere will be liable 

 to form dew on the soil. The layer of soil must be very thin, not 

 over one millimeter thick. This method gives the maximum 

 hygroscopic capacity ; if the air is not fully saturated, lower 

 results will be obtained. 



The amount of hygroscopic water taken up depends very 

 largely on the character of the soil. According to Hilgard and 

 Loughridge, 1 soils absorb the following amounts of hygroscopic 

 moisture : 



Hygroscopic moisture 

 Per cent. 



Sandy soils (less than 5 per cent, clay) 3 



Sandy loams 3-5 



Loams 5 



Clay loams 5-7 



Clay 7-10 



The amount of hygroscopic water taken up by a given sub- 

 stance depends partly upon its surface area. A mass of quartz 

 will absorb much more moisture when in fine powder than when 

 in large fragments. It is also greatly influenced by the amount 

 and character of the colloid constituents in the soil, such as 

 hydrated ferric oxide, alumina, gelatinous silica, hydrated silicates, 

 and especially humus. Pure clay has a somewhat lower absorp- 

 tive power than these. 



Value of Hygroscopic Moisture. Plants are not able to utilize 

 the hygroscopic moisture of soils. At least, they wilt before the 

 moisture in the soil is withdrawn to the amount held by 

 hygroscopic power. 



Heinrich 2 grew plants in very small boxes until well developed 



1 Rep. Cal. Exp. Sta., 1897-8. 



2 Jahresber, f. Agr. Chem., 1875-6, p. 368. 



