214 POWER OF THE SOIL TO ABSORB SALTS [chap. 



absorbents of the sewage material, but are unsuitable 

 because they do not admit of percolation. 



Absorption of Ammonium Salts. 



The absorption of free ammonia follows the lines 

 indicated above for the absorption of the organic com- 

 pounds of nitrogen ; but the salts of ammonia are 

 retained by the soil by purely chemical processes which 

 result in the formation of insoluble salts of ammonia in 

 the nature of double silicates and humates. Way and 

 Voelcker first found, that when either the sulphate, 

 chloride, or nitrate of ammonia in solution is allowed to 

 remain in contact with soil, the base is absorbed, but the 

 acid portion of the salt remains in solution in combina- 

 tion with lime. Voelcker also showed that when soil 

 was shaken up with dilute solutions of ammonium salts, 

 the withdrawal of ammonia from the solution was never 

 complete, but varied both with the nature of the soil and 

 the strength of the solution, a greater proportion being 

 taken from weak than from strong solutions. 



The absorption of the ammonium salts by the soil is 

 now known to be due to the combined effects of at least 

 three actions — upon the zeolites, upon the humus, and 

 upon calcium carbonate. With the zeolites a double 

 decomposition takes place, ammonium becomes insoluble, 

 and equivalent amounts of calcium, magnesium, potas- 

 sium (sodium also on occasion) enter into combination 

 with the acid in the solution, for no acid is absorbed 

 and the whole solution remains neutral. 



The reaction is a reversible one, but the clay con- 

 taining the zeolites is not capable of absorbing more 

 than a certain small amount of ammonium from the 

 strongest solutions of its salts. The following table 

 shows the ammonia absorbed by ioo grams of very 



