62 FERTILISERS CONTAINING NITROGEN [CHAP. 



change by the humus in the soil. But experiments 

 with pure kaolin and samples of natural humus — peats 

 of various age and origin — show that the reaction which 

 takes place is one of double decomposition whereby 

 ammonium displaces calcium and magnesium in clay 

 and humus. With kaolin and clays the reaction 

 takes place with the double hydrated silicates or 

 zeolites, with humus with certain natural calcium com- 

 pounds of the insoluble complex organic humic acids 

 produced during decay. A certain amount of the 

 ammonia is also taken up at once by soil bacteria and 

 converted into more organised and insoluble forms like 

 proteins. 



It is the calcium carbonate in the soil that finally 

 suffers loss, because before nitrification takes place the 

 ammonium compounds just described have to be 

 decomposed by calcium carbonate with the produc- 

 tion of ammonium carbonate, which alone can be 

 attacked by the nitrification organisms. 



Referring again to the analyses of the Rothamsted 

 wheat soils, p. 56, it will be seen that the long con- 

 tinued use of ammonium salts has reduced the proportion 

 of calcium carbonate below that of the unmanured plots 

 by amounts which are approximately those to be 

 expected if interaction between the salts and the calcium 

 carbonate had taken place according to the equation set 

 out above. 



The Rothamsted wheat soils started with sufficient 

 calcium carbonate to withstand this loss, but on soils 

 initially poor in calcium carbonate its removal by 

 sulphate of ammonia soon induces a condition approach- 

 ing actual sterility. The best example is afforded by 

 the experimental plots on the farm of the Royal 

 Agricultural Society at Woburn, where through the con- 

 tinued . use of ammonium salts as manure, the soil 



