THE SOIL SOLUTION 279 



eentration and composition so readily that the addition of ex- 

 traneous material or the exertion of unnatural pressure defeat 

 the object of the determination. Moreover, the soil solution is 

 probably not homogeneous and unless practically all of it is 

 removed a sample of value cannot be obtained. The signifi- 

 cance of such a sample, if it were attained, is questionable, 

 as it is impossible to know the proportion of the soluble nu- 

 trients that may actually be appropriated by the growing- 

 plant. 



The method of obtaining soil extracts has been used to a 

 greater extent than any other in studying the soil solution. 

 Water is the usual solvent. The Bureau of Soils filter method ^ 

 is commonly followed. As might be expected, it is purely 

 arbitrary in its procedure, the idea being to make the results 

 comparative rather than strictly quantitative. Soil and water 

 in the proportions of 1 to 5 are mixed, stirred three minutes 

 and allowed to stand twenty minutes. The supernatant liquid 

 is then forced through a Pasteur-Chamberland filter and a 

 clear extract obtained for analysis. 



The solution obtained is not representative of the soil-water 

 and its solutes. It is only an extract of the soil. The addi- 

 tion of a large amount of water is a disturbing factor. The 

 concentration of the extract is also modified by the absorptive 

 power of the soil, being relatively greater for a sandy than 

 for a clayey soil. Moreover, the differential influence of the 

 solvent comes into play, for as soon as solution begins, the 

 solvent is no longer pure water but a solution of constantly 

 changing efiiciency. Nevertheless, the work of Hoagland, 

 Stewart and Burd ^ indicates that there is not only a relation- 



^Schreiner, 0., and Pailyer^ G. H., Colometrie, TurUdity and Titra- 

 tion Methods Used %% Soil Investigations; U. S. Dept. Agr., Bur. Soils, 

 Bui. 31, 1906. 



* Hoagland, D. E., T}ie Freezing Point Method as an Index of Varia- 

 tions in the Soil Solution Due to Season and Crop Growth; Jour. Agr. 

 Bes., Vol XII, No. 6, pp. 369-395, 1918. 



Stewart, G-. R., Effect of Season and Crop Growth in Modifying the 

 Soil Solution; Jour. Agr. Res., Tol XII, No. 6, pp. 311-368, 1918. 



