98 BULLETIN 1059, IT. S. DEPARTMENT OF AGRICULTURE. 



The discovery that within certain limits the moisture of the soil 

 follows the laws of osmosis, or more precisely speaking, the laws of 

 dilute solutions with respect to its freezing point, has naturally led 

 to the idea that the soil solution might also be considered as having 

 a definite vapor pressure at a definite osmotic concentration. If this 

 were true, then a soil placed in a moist atmosphere should give off 

 or absorb vapor, according to whether its original solution repre- 

 sented a lower or higher osmotic pressure than that represented by 

 the atmosphere of vapor in which it was placed. Furthermore, if 

 this vapor pressure manifested itself properly and in accordance 

 with the laws of solutions, then, through vapor transfers, one soil or 

 a hundred soils simultaneously might be brought into vapor-pressure 

 equilibrium, and thereby into osmotic equilibrium, with a solution 

 whose osmotic pressure is readily determined ; and the moisture con- 

 tents corresponding to such osmotic pressure might then be readily 

 measured for one or all of the soils. This plan was conceived as a 

 possible means of avoiding some of the difficulties of the freezing- 

 point method of osmotic determinations, which are especially bother- 

 some in treating coarse soils. That the theory is correct may hardly 

 be questioned now, and full discussion of the available data will be 

 given later. This subject has been mentioned here because of its 

 possible bearing on the hygroscopic coefficient determinations. It is 

 rather readily seen that, if the laws of solutions prevailed under all 

 conditions of soil moisture, a soil exposed to completely saturated 

 water vapor should go on absorbing moisture indefinitely, because 

 the dilute solution of the soil would always stand for some osmotic 

 pressure, while saturated water vapor would stand for none at all. 



Whether this does not occur in the hygroscopicity tests because of 

 the failure to create a completely saturated atmosphere, or because 

 there is a sharp line between the behavior of water vapor in the soil 

 and liquid water, is for the future to decide. That it probably has 

 no practical bearing on the hygroscopic coefficient under the empiric 

 conditions set for that test, is perhaps enough in itself. It will 

 help to clarify the matter if it is remembered, first, that Bouyoucos 

 (109) has shown that at about the moisture content at which wilting 

 occurs, the water of the soil ceases to behave as a liquid and refuses 

 to freeze; and secondly, that Briggs and Shantz (114) have shown 

 that the hygroscopic coefficient falls considerably below the wilting 

 coefficient, the former being usualty about 0.7 of the magnitude of the 

 latter. 



Since the determination of the hygroscopic coefficient begins with 

 air-dry soil, it does not deal with liquid water in the soil, but more 

 probably with water molecules more or less separated, like individual 

 vapor molecules. 



