160 THE CHEMICAL ANALYSIS OF SOILS [chap. 



mined by the processes previously described. The time 

 of extraction may be shortened to twenty-four hours if 

 the bottle be put in a good end-over-end shaking 

 machine which will keep the soil and the solvent 

 thoroughly agitated. 



An examination of the citric acid solution shows that 

 all the compounds of phosphoric acid that have been 

 indicated as existing in the soil are more or less 

 attacked; at any rate, the resulting solution contains 

 organic matter and salts of aluminium and iron, in 

 addition to calcium. It has been suggested that the 

 varying amounts of calcium carbonate contained by soils 

 will much affect the material dissolved by the citric acid, 

 some of which becomes neutralised by the calcium 

 carbonate. But though the amount of phosphoric acid 

 dissolved from a given soil by the citric acid solution will 

 be diminished if the calcium carbonate in the soil is 

 increased, a very similar reduction will be effected in the 

 natural processes of solution of the soil phosphates under 

 field conditions. No attempt should be made to add an 

 extra amount of citric acid to combine with the calcium 

 carbonate ; secondary solvent actions are set up both by 

 the carbon dioxide evolved and by the calcium citrate 

 formed, moreover, the real comparative basis of the 

 method of analysis is destroyed. 



It must not be supposed that the citric acid solution, 

 nor indeed any of dilute acid solvents that have been 

 proposed for this purpose, are real differential solvents, 

 which extract the material in the soil which is available 

 for the plant and leave untouched whatever is combined 

 in some other form. In reality, as soon as the acid has 

 been for a sufficient time in contact with the soil a state 

 of equilibrium is attained between the phosphoric acid, 

 for example, that has gone into solution and that which 

 remains in the solid state. The precise equilibrium 



