76 Dilute Acids and Phospliorus Compounds in the Soil 



acids yield dark coloured solutions containing organic matter previously 

 adsorbed by the soil ; sulphuric acid yields lighter coloured solutions 

 containing less; hydrochloric and nitric acids, on the other hand, 

 displace very little organic matter, and yield almost colourless solutions. 

 Citric acid brings out more iron than either hydrochloric or nitric acids. 



14. There is not, however, a rigid parallelism between the dis- 

 placement of adsorbed organic matter and that of adsorbed phosphate. 

 Thus, of all acids and salts investigated, ammonium oxalate gave the 

 darkest colovired solutions, but it did not dissolve most phosphate. 

 Again, sodium fluoride gave a dark coloured solution, but dissolved 

 only little phosphate. 



The change in adsorption with the time. 



15. The amount of adsorption depends not only on the acid but 

 on the time. Figs. 4 and 5 show that the continued action of N/10 

 nitric acid causes increased adsorption both by Hoos and Agdell soils. 

 At first sight N/5 acid appears to show the contrary behaviour, but 

 on closer inspection it becomes clear that the curves will cross, so that 

 the 24 hour will ultimately come above the one hour as in the N/10 

 curve. And in point of fact the N/10 curves themselves also show this 

 relationship in the early part of their course. 



Citric acid, however, behaves differently : the adsorption after one 

 hour is greater than after 24 hours, and it is not obvious from inspection 

 that the curves will ever cross. 



It might be supposed from the N/10 nitric acid curve that adsorption 

 was a slow business. This does not appear to be the case, however. 

 After any given interval a definite equilibrium is attained expressed 

 by an equation with definite constants. The phenomena are entirely 

 consistent with the view that the adsorption itself is instantaneous 

 (as is almost invariably the rule with the other adsorbents), but that 

 the constants change with the time. In the curves of Figs. 4 a and 5, 

 p tends to fall as the time increases, so that the curve opens out. 



16. The changes in the amount of adsorption occur simultaneously 

 with a change in the soil and in the solution. The acid causes continuous 

 decomposition of the soil, the extent of which may be seen from the 

 reduction in strength of the acid. Table III shows the amounts of 

 nitric acid neutralised by soil after different intervals : these are con- 

 siderably greater than corresponds with the calcium carbonate, phos- 

 phates, etc., present. Much silica is liberated during the action. 



