340 SOILS: PROPERTIES AND MANAGEMENT 



retained by adsorption, and as this varies with soils of 

 different texture a fair comparison of such soils is impos- 

 sible. 



243. Extraction with pure water. — When soil is di- 

 gested with distilled water, all the mineral substances 

 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 that are undoubtedly 

 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 percentage 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 adsorption. The aqueous extract does not con- 

 tain the entire quantity 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 quantity 

 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. 



244. Influence of absorption. — The quantity of ex- 

 tracted material depends on the absorptive properties of 

 the soil and on the amount of water used in the extrac- 

 tion, or on 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 



