SOLUBILITY OF SOIL CONSTITUENTS 269 



is never determined in any of the acid solutions, but 

 by a separate process. A deficiency of any particular 

 substance may be discovered in this way, but nothing 

 can be learned as to the ability of the plant to obtain 

 nutriment from the soil. A rock may show as much 

 mineral plant-food material as a rich soil. Such an 

 analysis is used only to ascertain the ultimate limita- 

 tions of a soil or its possible deficiency in any essential 

 constituent. 



128. Digestion with strong hydrochloric acid. Analy- 

 ses made with hydrochloric acid of 1.115 specific gravity 

 are those usually called "chemical soil analyses." 

 They are supposed to show the amount of plant food 

 at the time the analysis is made, which is in a condi- 

 tion to be ultimately used by the plant, and the plant- 

 food materials not dissolved by treatment with hydro- 

 chloric acid are assumed to be in a condition in which 

 plants can not use them. It may reasonably be ques- 

 tioned whether these relations hold under field condi- 

 tions. In fact, it is quite certain that some of them 

 do not hold. In other words, while treatment with 

 hydrochloric acid of a given strength marks a definite 

 point in the solubility of the compounds in the soil, it 

 does not bear a uniform relation to the natural processes 

 by which these compounds become available to the 

 plant. 



129. Interpretation of results of analysis of hydro- 

 chloric acid solution. This method of analysis was 

 originally thought to give some indication of both the 

 permanent fertility and the immediate manurial needs 

 of a soil; but for both purposes the accuracy of the 



