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THE CHEMICAL ANALYSIS OF SOILS [chap. 



In certain cases, by continuing the extraction with 

 citric acid until the amount going into solution at each 

 extraction 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 

 therefore be termed " available " because they possess a 

 comparatively high solubility factor, and other phosphates 

 which would yield, under natural conditions, solutions 

 too dilute to nourish the crop efficiently. 



For example, the following table shows the amounts 

 of phosphoric acid dissolved by successive extractions of 

 certain Rothamsted soils with i per cent, citric acid 

 solution, from which it will be seen that at about the fifth 

 extraction the quantity dissolved begins to approach a 

 constant. 



Phosphoric Acid, Mgms. per ioo Grams Soil. 



The successive amounts going into solution in the 

 first four or five extractions of the soil from the plots 

 which had received soluble phosphoric acid every year 

 are found to be decreasing in a logarithmic series, and 

 this may be supposed to indicate that the solvent is 



