EXTRACTION WITH PURE WATER 277 



used by plants are dissolved from it, but in very small 

 quantities. It has been proposed to use this extract for 

 soil analysis on the ground that it involves no artificial 

 solvent, the presence or amount of which in the soil 

 is doubtful, but shows those substances which are un- 

 doubtedly in a condition to be used by plants. By 

 determining the water content of the soil and using 

 a known quantity of water for the extraction, the per- 

 centage of the various constituents in the soil water 

 or in the dry soil may be calculated. 



The substances dissolved from the soil by extraction 

 with distilled water are probably only those contained 

 in the soil-water solution, including a part of the solutes 

 held by absorption. The aqueous extract does not con- 

 tain all of the nutritive salts in solution in the soil water, 

 and hence is not a measure of the fertility held in that 

 form. An undetermined amount of nutrients is retained 

 in the water in the very small spaces and on the surface 

 of the soil particles. It is, however, a fair comparative 

 measure of the content of available nutrients. 



141. Influence of absorption. The quantity of ex- 

 tracted material depends upon the absorptive properties 

 of the soil, and upon the amount of water used in the 

 extraction, or upon the number of extractions. Analyses 

 of the aqueous extract of a clay and of a sandy soil on 

 the Cornell University Farm serve to illustrate the 

 greater retentive power of the former for nitrates. 

 Sodium nitrate was applied to a clay soil, and to a sandy 

 loam soil at the rate of 640 pounds per acre. Analyses 

 of aqueous extract; some ninety days later, showed 

 the following: 



