E. J. Russell and J. A. Prescott 



91 



above the Time ---= curve, and is approaching, and shows signs of soon 

 crossing, the 2 hour curve. Sulphuric acid behaves in the same way, 

 but the points of crossing occur at still lower concentrations; the 

 2 hour curve has already crossed the Time = curve at concentrations 

 lower than those we used, while the 24 hour curve crosses it just before 

 N/10. At higher concentrations the 24 hour curve crosses the 2 hour 

 curve, 'and at N/5 the longer the time of action the greater the amount 

 of P2O5 extracted. Citric acid always gives this result at all of the 

 times and concentrations used by us, but we have not tried extremely 

 dilute acids. 



In comparing the various acids, therefore, a steady gradation can 

 be detected : nitric acid shows the remarkable falling off in net action, 

 and therefore an increase in the reverse action with the time at all 

 concentrations up to N/5 and apparently beyond; hydrochloric acid 

 shows it nearly up to N/5 ; sulphuric acid only up to N/10 ; while 

 citric acid either does not show it, or if it does, only at concentrations 

 below N/20. 



This marked difference in behaviour between citric and nitric acids 

 is not due to any special decomposition effected by citric and not by 

 nitric acid, for after the soil has been treated 24 hours with citric acid 

 it behaves in the same way towards nitric acid. 



A quanitty (300 gms.) of Agdell soil was extracted for 24 hours with 

 N/10 citric acid. It was then divided into three lots : one was extracted 

 with HNO3 for half an hour ; the second for 24 hours ; the third with 

 N/10 citric acid for 24 hours. The results were as follows : 



1st ( xtraction, N/10 citric acid 

 10-4 mgms. PjOo per 100 gms. of soil 



2nd extraction 



The half-hour treatment with nitric acid still brings out more P2O5 

 than the 24 hours' treatment. 



The falling off in solvent action with the time is not peculiar to our 

 soil. Surprisingly few out of the vast number of soil analyses bear on 



