DECOMPOSITION OF ORGANIC MATTER IN THE SOIL 393 



favorable action of lime is readily explained by its ability to neutralize 

 organic and mineral acids and to render, thereby, the soil reaction 

 favorable for the rapid growth of ammonifying, as well as of nitrifying 

 bacteria. Furthermore, the reserve of basic material serves to neutral- 

 ize the acid formed by the bacteria and prevents thus the accumulation 

 of an undue amount of acidity. 



The role of certain mineral salts in promoting nitrification is quite 

 significant. Small amounts of sodium chloride have been found to favor 

 nitrification in the experiments of Pichard and also those of Lipman. 

 The former showed also that sulphates not only promote nitrification, 

 but that different sulphates display marked variations in this respect. 

 In the same manner nitrate formation was shown to be favorably 

 affected by phosphates in bone meal, Thomas slag, and acid phos- 

 phates. Generally speaking, therefore, nitrifying bacteria are stimu- 

 lated in their development by a proper supply of available mineral 

 nutrients. 



The exact relation of organic matter in the soil to the activities of 

 nitrifying bacteria is but beginning to be properly understood. Earlier 

 observations made it manifest that heavy applications of animal 

 manures, or of green manure may not only retard nitrification, but may 

 actually cause the disappearance of a part or of all of the nitrate in the 

 soil. Subsequent experiments by Winogradski and by Winogradski 

 and Omelianski showed that in pure cultures the presence of even slight 

 amounts of soluble organic matter may depress or even suppress the 

 development of the nitrifying bacteria. It was, therefore, concluded 

 by these authors that relatively small amounts of soluble organic 

 matter may inhibit nitrification. These conclusions, based on the 

 study of liquid cultures only, were given a very broad application by 

 many writers on agricultural topics. More recent experiments make 

 it certain, however, that in the soil itself small amounts of soluble 

 organic matter, e.g., dextrose, are not only harmless, but may really 

 stimulate nitrification. It was shown, likewise, that humus and 

 extracts of humus may, under suitable conditions, stimulate nitrifica- 

 tion to a very striking extent. 



Certain substances in the soil may exert a toxic effect on nitrifying 

 bacteria. Ferrous sulphate, sulphites and sulphides may thus act in- 

 juriously, as may also calcium chloride and excessive concentrations of 

 sodium carbonate, sodium bicarbonate, sodium chloride, magnesium 



