72 Dilute Acids mid Pliospliorus Connpoiinds in the Soil 



however, the curves cross one another, and the action finally increases 

 with the time. 



4. Thus if acid of sufficiently low concentration (N/20 to N/10) is 

 allowed to act on soil for different periods of time the amount of 

 phosphorus compound extracted does not increase with the time but 

 decreases, so that less is extracted after 24 hours than after 10 minutes. 



This result indicates that a reverse reaction is coming into play 

 removing the phosphorus compounds from the solution, but that it 

 operates more slowly than the direct action of the acid in dissolving 

 the phosphorus compounds from the soil. Thus in the 10 minutes 

 experiment the net result is mainly determined by the direct action ; 

 after 24 hours the reverse reaction has become more pronounced and 

 reduced the amount of phosphorus left in the solution. 



5. The reverse reaction does not appear to be due to any preci- 

 pitating out of the phosphorus compounds from the solution by any 

 substance slowly extracted from the soil. For no precipitation of 

 phosphorus compound occurs when an extract obtained by shaking 

 soil with nitric acid for 10 minutes, and therefore rich in phosphorus, 

 is mixed with one obtained after 24 hours' shaking, and therefore rich 

 in any phosphorus-precipitating compound, if such is present. The 

 seat of the reverse action, therefore, is not in the solution, but in the 

 soil. 



6. The phenomena can be reproduced by adding sodium phosphate 

 to the mixture of soil and acid. Some of the phosphate is absorbed 

 by the soil, notwithstanding the presence of excess of acid. The absorp- 

 tion is found to take place in presence of both N/10 and N/5 acid, 

 though it fell off at the higher concentration (Table VII). Thus, the 

 reverse reaction of § 4 is not confined to acids of low concentration, but 

 is general: with acids of low concentration the absorption is great 

 relative to the extraction of the phosphoric oxide ; with acids of higher 

 concentration it is small relative to the amount of P2O5 extracted. 



7. The absorption of the P2O5 ion from solution in presence of acid 

 is found to follow precisely the ordinary lines of adsorption by colloids, 

 and is completely expressed by the ordinary adsorption formula 



y ^ 



— z=Kc^ , where ?/ = the amount adsorbed by a quantity m of the 



soil, c = the concentration of P2O5 in the solution when equilibrium is 

 established, and K and f are constants (Figs. 3, 4 and 5). In one 

 respect only is there any notable difference : adsorption by colloids is 

 usually an instantaneous process, whilst the reverse reaction observed 



