250 POWER OF THE SOIL TO ABSORB SALTS [chap. 



be far more effectual 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 retention of salts of 

 ammonia may be interpreted either as a colloid 

 adsorption process or as a purely chemical reaction 

 in which the double silicates or zeolites and calcium 

 humate take part. 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 combination 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 readiest way of explaining this action is to 

 suppose that a double decomposition takes place with 

 the zeolitic silicates. Ammonium enters into the com- 

 pound and turns out equivalent amounts of calcium, 

 magnesium, potassium (sodium also on occasion), which 

 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 



