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 P 2 O 5 than tfre 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 P 2 O 5 dissolved after 24 hours' action than after 

 20 minutes (Fig. 19). 



Amount of P a 5 extracted from soil by HN0 5 

 of varying concentration acting for 



different times. 



0-55 h 



o 

 o 



pu 



a. * 



to 



e 



^ 



o 



d 



04 



06 



2.0 



08 -10 -ia -14 -16 -18 



Initial acidity, equivalents HW0 3 per litre. 



FIG. ig. Amount of P 2 O 5 extracted from soil by HNO 3 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 



