VI.] t/SE OF WEAK ACID SOLVENTS 161 



attained depends not only upon the strength of the acid 

 solution and the nature and amount of the phosphoric 

 acid compounds in the soil, but also on the nature and 

 amount of the bases that are there present. If, for 

 example, the citric acid solution is filtered off after it has 

 extracted all the phosphoric acid it can, and a second 

 portion of solution is added and the soil extracted afresh, 

 then more phosphoric acid will go into solution, the 

 amount being smaller than before but still considerable. 

 A third, a fourth, and even a fifth extraction does not 

 remove from the soil all the phosphoric acid that will 

 go into solution in the dilute citric acid solution. Thus 

 it is impossible to say that the dilute citric or any other 

 acid dissolves out and measures the "available" phos- 

 phoric acid or potash ; it does, however, provide a figure 

 indicating the comparative rate at which the soil is likely 

 to yield up its nutrient constituents to the normal solvent 

 actions going on in the soil. The results, then, of this 

 method of analysis are not to be regarded as absolute 

 amounts, but as empirically obtained figures which must 

 be interpreted in the light of experience. The type of 

 the soil plays a part ; for example, a quantity of citric acid 

 soluble phosphoric acid that would indicate poverty in a 

 strong loam or in a soil rich in organic matter like an 

 old pasture, would be ample for ordinary crop purposes 

 if the soil were light and sandy. Again, the crop must 

 be taken into account ; a percentage indicating enough 

 available phosphoric acid in the soil for wheat or man- 

 golds would indicate deficiency when the swede crop 

 came round. 



In certain cases, by continuing the extraction with 

 citric acid until the amount going into solution at each ex- 

 traction becomes approximately constant at some low 

 figure, it is possible to differentiate between the phos- 

 phates in the soil that are easily soluble and may 



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