382 



Journal of Agricultural Research 



Vol. XX, No. 5 



intervals during the growing season at least. This introduces certain 

 practical difficulties in the application of presssure methods. The 

 freezing-point method is most rapid and convenient as a means of study- 

 ing the approximate total concentrations , but it can not give any informa- 

 tion concerning the individual solutes. The method of water extraction 

 has been used rather frequently in . past investigations with the in- 

 tent to determine the amounts of plant foods available to the plant. 

 One of the writers (9) has carried out an extensive investigation in which 



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Fig. i. — Graph showing relation of freezing-point depressions in soil (calculated to 22 per cent moisture) 

 to total solids extracted by 5 parts of water to 1 of soil. Individual data from six soils composited. 



the very significant effect of season and crop growth on water extracts 

 of soils was made clear. At the same time the freezing-point method of 

 Bouyoucos and McCool was applied to the soils under investigation, and 

 a general agreement was noted between the values obtained by this 

 method and by the water-extraction method (6). Thus the effect of the 

 crop in diminishing the concentration of the soil solution was definitely 

 shown by both methods. At the present time the study of water ex- 

 tracts offers such promise that it has seemed highly important to attempt 



