VIII.] 



PHOSPHORIC ACID 



2I<) 



tion of citric acid, and found that both the surface and 

 the subsoil down to a depth of 27 inches contained more 

 of this readily soluble potash where it had been applied 

 as manure, than did the companion plot receiving no 

 potash, as will be seen from the following table : — 



These determinations show that soluble potash salts 

 applied to the land are retained chiefly by the surface 

 soil, as much as one-half of the estimated additions of 

 potash during fifty years' manuring being found there. 

 Some of the potash, however, sinks further and is 

 retained in the subsoil ; in the top 27 inches a large 

 proportion — nearly one-quarter of the whole — remains 

 in such a loose state of combination that it is soluble 

 in 1 per cent, citric acid, and so may be regarded as 

 available for the plant 



Absorption of Phosphoric Acid. 



The retention of soluble phosphoric acid by the soil 

 is more easily intelligible, for there are present several 

 substances capable of forming insoluble compounds 

 with phosphoric acid — e.g., calcium carbonate, hydrated 

 ferric oxide, and the hydrated silicates of alumina which 

 make up so much of clay. Sand and powdered silicates 

 like felspar have been found to possess little or no 

 power of removing phosphoric acid from solution, nor 



