68 BULLETIN 105&, U. S. DEPARTMENT OF AGRICULTURE. 



causing osmosis, is undoubtedly regulatory of all the conditions 

 within the cell. It is, indeed, through the study of soils that the 

 present conception of the nature of osmosis and of the importance of 

 the colloids of the plant in this process is arrived at. The work of 

 Bouyoucos (109) followed by Hoagland (127) has been especially 

 instructive on this point. Bouyoucos and McCool (106) first at- 

 tempted to determine directly the concentration of soluble salts in 

 the soil by measuring the freezing-point depressions of soils in their 

 natural conditions of moisture and with definite amounts of water 

 added to them. At the outset, no doubt, he supposed that, allowing 

 for increased solubility of the soil substances with increased moisture, 

 the freezing-point depressions would increase proportionately as the 

 concentration of the solutes increased by reduction of the whole 

 moisture of the soil. This he found not to be the case, for, while the 

 effect of increasing concentration was shown in greater freezing-point 

 depressions as the moisture was reduced, a point was rather suddenly 

 reached at which no freezing at all occurred. From this Bouyoucos 

 concluded that there must be at all times in the soil a certain amount 

 of " unf ree " water, probably not in a liquid state but so adsorbed or 

 chemically combined that at no time does it constitute a part of the 

 soil solution. This water was conceived to be that which is held 

 within the colloidal or clay masses of the soil, but it is fairly evident 

 that, since pure sand may contain a small amount of such water, it 

 may be in part water in extremely thin films, or more probably in 

 the form or independent molecules, on the surfaces of the crystalline 

 particles. 



In his later work Bouyoucos (109) succeeded in measuring the 

 volume of this unfree or combined water indirectly, by noting the 

 volume expansion of the whole mass at the instant when freezing- 

 occurred. By this means he was able to determine just what propor- 

 tion of the whole amount of water entered into the freezing process. 

 Adding in every case 5 cubic centimeters of water to 25 grams of air- 

 dried soil, he found that the proportion of this 5 cubic centimeters 

 which did not freeze was only 2 per cent (0.10 cubic centimeters) in 

 the case of a quartz sand, but 60 per cent (3 cubic centimeters) in the 

 case of a clay, and 74 per cent for heavy silt loam. 



The exact amount of unfree water for a given soil was found to be 

 dependent on several experimental conditions, which led to the belief 

 that under certain conditions it could be transformed into free water. 

 In other words, there is no fixed point at which the water begins to 

 be unfree, but under given conditions each soil exhibits certain definite 

 characteristics. Thus there was in all soils, except the quartz sand, 

 less water which failed to freeze when 10 cubic centimeters had been 

 added to a standard sample than when only 5 cubic centimeters were 

 used. There was also less when the supercooling was carried below the 



