238 NITRIFICATION 



also present in but small amounts, even in the plots receiving 

 a great annual excess of this substance, while potash was 

 found in sHghtly greater quantities. The mean annual loss, 

 however, cannot be estimated at more than about 2 lb. of 

 phosphoric acid and 10 lb. of potash per acre, both of which 

 in normal cases Avould be arrested in the subsoil below the 

 drains. Dr Bernard Dyer's analyses of the Rothamsted soils 

 and subsoils would also indicate that all the excess of phos- 

 phoric acid, applied as a manure and not removed in the 

 crop, still remains in the soil very near the sm-face, the potash 

 having sunk a little further, and being present to some degree 

 in the third depth of 9 inches below the surface. 



The chief constituent of the drainage water from the 

 unmanured plots consists of calcium carbonate, the amount of 

 which is increased in the water from the dunged plot, owing to 

 the greater production of carbonic acid from the decay of the 

 dung and crop residues. Where ammonium-salts like the 

 sulphate and chloride are applied as a manure the soil suffers a 

 great loss of calcium carbonate, the calcium being removed in 

 the drainage water combined with the sulphuric or hydrochloric 

 acid of the manure. This reaction is the necessary precedent 

 to the arrest of the ammonia in the soil and its subsequent 

 nitrification. In the absence of a sufficiency of calcium 

 carbonate in the soil to bring about this reaction, ammonium- 

 salts become injurious to plant life. The salts of potassium, 

 like the sulphate and chloride, may also increase the loss of 

 calcium carbonate to the soil, for they react with it in the same 

 way as do the ammonium-salts, forming calcium sulphate and 

 chloride, which are no longer retained by the soil. 



Since the healthy condition of the soil depends on a due 

 proportion of calcium carbonate being present, these losses 

 caused by the use of natural and artificial manures are of the 

 greatest importance ; many of om- fertile soils may easily lose 

 much of their power of producing crops unless their proportion 

 of calcium carbonate is restored by judicious liming at intervals. 



Determinations of the calcium carbonate in samples of the 



