94 Dilute Acids and Pliosplioms Compounds in the Soil 



y = amount of substance adsorbed, 



c = concentration of the substance in the solution when equilibrium 

 is reached, 



m = amount of adsorbent, 



then — = KcP, where K and p are both constants, 

 m 



This curve is parabolic: it becomes the ordinary parabola in the 

 special case where p = 2. 



This curve has been found to hold for the most diverse cases; for 

 blood charcoal and various acids, chlorine, bromine, etc. ; for wool 

 and dyes; for filter paper and acids; and numerous others. It 

 does not hold invariably, however, which indicates that there may 

 be various types of adsorption, but it stands for the most usual type. 



In applying this equation to the case in hand we are confronted 

 with the difficulty of determining y, — the amount of P2O5 adsorbed. 

 It is, of course, easy to find out what proportion of added P2O5 is 

 adsorbed (as shown in Table IV) but this takes no account of the 

 adsorption of the P2O5 given up by the soil to the solution. 



Two methods were therefore adopted. 



1. The extractable P2O5 was carefully removed from a sample of 

 Agdell field soil by extracting it twice with N/5 II2SO4, and then seven 

 times with 2 % caustic soda in the cold. The resulting material gives 

 up no P2O5 when treated with dilute acids. It was shaken with a mixture 

 of HCl (0-06 N) and sodium phosphate for 24 hours at 23° C. 



Direct analysis of the resulting solution gives c in the above equation : 

 subtraction from the amount originally added gives y: there is here 

 no complication from the amount initially present in the soil because 

 all this was removed in the preliminary treatment. The results are 

 given in Table V. When these values are substituted in the equation 

 it is found that 



K = 20'2; p = 2-4 for the hydrochloric acid experiment 



and K^ 8-8; ^j = 2-1 for the citric acid experiment. 



Thus the two equations become 



y = 20-2 0-'* 



and y = 8-8 C^'^ respectively. 



The curves are drawn on Fig. 3 : it is clear that they fit the experi- 

 mental points very well. 



