EXTRACTION WITH ORGANIC ACID 273 



substances may be dispensed with. It must be stated, 

 however, that there are cases for which these deductions 

 do not hold, owing to the intervention of other factors. 



134. Estimation of deficiency of ingredients. In a 

 soil in which the other conditions are normal, one would 

 suppose it possible to prescribe, with some degree of 

 accuracy, the content of certain constituents below 

 which a deficiency exists. The use of a manure contain- 

 ing this constituent should therefore be expected to 

 produce beneficial results. However, opinions differ 

 so widely, depending, apparently, upon the soils with 

 which the respective analysts have had to deal, that it is 

 difficult to decide where to set the limit. It is evident 

 that, as the content of any constituent becomes less, 

 the probable need for its application becomes greater, 

 and it thus suggests a practice without assuring its 

 success. 



135. Conclusions. An analysis of the hydrochloric 

 acid extract, therefore, cannot be taken as a guide to 

 the fertilizer needs of the soil, and of itself should not 

 be relied upon; but in connection with other knowledge, 

 particularly that derived from fertilizer tests, it may be 

 useful. 



136. Extraction with dilute organic acids. Other 

 methods used for dissolving soils for analysis depend 

 upon extraction with some dilute organic acid, as citric, 

 acetic, oxalic or tartaric acid. The assumption upon 

 which these methods are based is that the dilute organic 

 acids correspond to the solvent agents in the soil, and 

 thus take from it the amounts of those materials that 

 the plant could take up if it came in contact with all 



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