216 POWER OF THE SOIL TO ABSORB SALTS [chap. 



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. 



Thus the use of ammonium salts increases the 

 normal loss of calcium carbonate experienced by the 

 soil (due to solution as bicarbonate), and the amount 

 removed increases with the larger applications of 

 ammonium salts. Taking the mean of these and other 

 results obtained at Rothamsted, 200 lbs. of ammonium 

 salts causes a removal of about 120 lbs. calcium 

 carbonate, whereas the amount calculated from the 

 equation given above would be about 160 lbs. That 

 the loss from the plots receiving sodium nitrate and 

 dung is less than from the unmanured plot, is due, in 

 the former case, to the base left in the soil by the growth 

 of plants which derive their nitrogen from sodium nitrate, 

 and in the latter, to calcium carbonate formed by 

 bacterial action from organic calcium salts in the dung. 



