252 POWER OF THE SOIL TO ABSORB SALTS [chap. 



is adsorbed and the acid ion set free combines with the 

 calcium which is present as a base in the calcium 

 "humate." 



With the calcium carbonate of soils a double decom- 

 position of the type 



(NH 4 ) 2 S0 4 + CaCO 



3 <- 



~^- 



(NH 4 ),C0 3 + CaS0 4 



takes place, not only with such substances as 

 ammonium sulphate, but also with humic and zeolitic 

 compounds of ammonium, and though the proportion 

 converted into carbonate is small, it is constantly 

 renewed as the ammonium carbonate is nitrified, so that 

 eventually the whole of the ammonium salt applied to 

 the land undergoes this change. 



In consequence, the continued use of ammonium 

 salts as a fertiliser results in the depletion of the stores 

 of calcium carbonate in the soil, as may be seen from 

 the following determinations of the rate of disappearance 

 between 1865 and 1904, of calcium carbonate from 

 some of the soils of the Rothamsted wheat field where 

 the calcium carbonate is of artificial origin and is con- 

 fined to the surface layer of the soil. 



