390 Journal of Agricultural Research vol. xx. No. s 



That there exists some sort of definite and reversible state of equilib- 

 rium between the soil mass and the soil solution for any given set of 

 conditions is suggested by another experiment. Two soils were treated 

 with water in the proportion of i part of dried soil to i part of water. 

 After the soil and water were thoroughly mixed the resultant mixtures 

 were allowed to dry at room temperature until they reached the optimum 

 moisture content. Freezing-point depressions were then made and com- 

 pared with determinations made on samples of the same soils simply 

 moistened to optimum water content. The data given below show 

 that the agreement is, at least in these two cases, almost perfect. 



Table IV. — Freezing-point depressions of soil at optimum moisture content and of 

 treated soil evaporated to optimum moisture content 



Description of soil. 



Soil iC at optimum moisture content 



Soil iC after mixing i to i with water and allowing to evaporate to optimum 



moisture content 



Soil 9 at optimum moisture content 



Soil 9 after mixing i to i with water and allowing to evaporate to optimum 



moisture content 



Freezing- 

 point 

 depressions. 



' C. 

 O.063 



. 062 



•045 



.047 



In other words, although several times as much material was brought 

 into solution as was contained in the soil solution at optimum water 

 content when the excess water was added, these dissolved substances 

 were immediately removed from solution on lowering the moisture con- 

 tent. This, of course, does not mean that the concentration of the soil 

 solution may not easily be altered by the addition of soluble salts, as will 

 be discussed presently. 



If the general method of studying soils by means of their water extracts 

 is of value, then it becomes of considerable importance to determine the 

 most suitable conditions for making the extract. The technic might be 

 based on either one or two general objectives, first the attainment of 

 equilibrium (as nearly as possible final) for a given proportion of water, 

 and, second, the limitation of the extract as far as was practicable to the 

 material actually existing in the soil solution. In the first case a long 

 period of contact and continuous shaking would be essential; in the 

 second case the time would be limited to that necessary for complete 

 admixture of soil solution and added water. In order to determine the 

 magnitudes of dissolved substances under varying conditions, extracts of 

 3 soils were made by various methods as follows : (a) 1 part soil to 5 parts 

 water, as described by the Bureau of Soils of the United States Depart- 

 ment of Agriculture; (b) 1 part soil to 5 parts water, shaking for 1 week; 

 (c) 1 part soil to 1 part water, as in (a) ; (d) 1 part of soil to 1 part water, 

 shaking for 1 week. 



