388 



Journal of Agricultural Research 



Vol. XX, No. 5 



Table II. — Extraction of soil with concentrated extract containing added potassium 



sulphate (K 2 S0 4 ) 



Soil 

 No. 



Description of extracts. 



Composition of extracts. 



Total 

 solids. 



Potas- 

 sium 

 (K). 



Cal- 

 cium 



(Ca). 



Mag- So- 

 nesium dium 



(Mg). (Na). 



Ni- 

 trate 

 (NOs). 



Phos- 

 phate 

 (POO. 



Sul- 

 phate 

 (SO<). 



Silica 

 (SiOj). 



Concentrated extract plus 



potassium sulphate 



Same after passing through 



soil 



Increase or decrease in con- 

 centration 



Concentrated extract plus 



potassium sulphate 



Same after passing through 



soil 



Increase or decrease in con- 

 centration 



P. p.m. P.p.m. 



1,652 

 1,896 

 + 244 

 2.408 

 2,592 

 + 184 



76 



52 



- 24 



277 



119 



P.p.m. 

 212 



280 



+ 68 



236 



300 



P.p.m. 



33 

 31 

 — 2 

 41 

 41 



-158 + 64 



P.p.m. 



122 

 129 



+ 7 

 72 

 109 



+ 37 



P.p.m. 

 312 

 333 

 +21 



P.p.m. 



+ 1 

 4 

 3 



— 1 



P.p.m. 

 166 

 161 



- 5 

 413 

 369 



— 44 



P.p.m. 



40 

 40 



All the experiments just discussed would seem to justify the conclusion 

 that in a concentrated extract the relation between the various elements 

 may be very similar to that existing in the soil solution. In seeking 

 an explanation of the results it is first essential to describe the components 

 which probably enter into a water extract of a soil. In the first place, 

 these would include the constituents of the soil solution diluted by the 

 added water. This diluted soil solution would then tend to bring into 

 solution constituents which were not present in the soil solution itself. 

 Thus, in no case would the solvent be pure water but rather a solution 

 the composition and concentration of which would vary with the soil 

 solution. It is plausible to assume that the solvent thus formed would 

 bring into solution principally either "adsorbed" salts or easily soluble 

 chemical compounds, originally derived from the more resistant minerals. 

 Finally, a certain quota of this very slightly soluble material would come 

 into solution, and the total quantity dissolved would depend at least in 

 part on the total volume of water as well as on time and temperature. 

 This latter fraction of the soil extract would ordinarily form only a small 

 portion of the total dissolved material. Evidence for this view has been 

 presented previously (6, g) and is also upheld by certain experiments of 

 Bouyoucos with regard to the solubility of soil minerals (2). It would 

 follow, therefore, that if the adsorbed or immediately soluble material 

 has the same relative composition as that already present in the soil 

 solution, then the water extract might also retain similar relations. It is 

 impossible at present to obtain direct evidence to this effect, but an 

 experiment was carried out from which certain inferences may be drawn. 

 A large quantity of moist soil (silty clay loam 1) was placed in a 

 Buchner funnel and leached with the least possible proportion of distilled 

 water, about y& part of water to 1 part of soil. Two subsequent leach- 

 ings were made with similar proportions of water. These three extracts 

 were then analyzed for the most important elements, and the ratios 



