158 



SOIL CONDITIONS AND PLANT GROWTH 



acids are all of the same type, but they show two remarkable 

 peculiarities: (i) some of the strong acids, such as hydro- 

 chloric and nitric, bring out less P2O5 than the weaker citric 

 and oxalic acids at equivalent concentration ; (2) the amount 

 of action is not always proportional to the time, and in the 

 case of some acids, e.g. hydrochloric and nitric, there is 

 actually less PgO^ dissolved after 24 hours' action than after 

 20 minutes (Fig. 19). 



Amount of PaOsextrdcted (rom soil by HNOi 

 of varying concentration acimg for 



different times. 



0-55 Hr. 



o 



O 

 O ^ 





O 



\ - 



•06 



•20 



•08 -10 -la -14 -16 -JS 



initial acidity, equivatents HNO3 per litre. 



Fig. 19. — Amount of P2O5 extracted from soil by HNO3 of varying concentra- 

 tion acting for different times. 



This second result indicates that a reverse action is coming 

 into play, proceeding more slowly than the direct action : so 

 that after 20 minutes the effect is determined largely by the 

 direct action, while after 24 hours it is determined by the 

 reverse action. 



The reverse action was eliminated by a diffusion method. 

 When this was done all dilute acids were found to act in a very 



