vi.] USE OF WEAK ACID SOLVENTS 183 



for example, that remains in solution and that which 

 has been reabsorbed by the soil. All acids begin 

 by dissolving the same amount of phosphoric acid, 

 the amount depending upon their strength and the 

 nature of the phosphoric acid compounds in the soil, 

 but the precise equilibrium that is afterwards attained 

 is determined by the absorptive power of the soil 

 for phosphoric acid in the presence of the particular 

 acid employed. If 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" phosphoric 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. 



