262 MICROBIOLOGY OF SOIL. 



sequent experiments by Winogradski and by Winogradski and Ome- 

 lianski 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 condi- 

 tions, stimulate nitrification 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 

 sulphate, etc. Injury by ferrous compounds, as well as by organic acids, 

 is not uncommon in low-lying fields and bogs; while injury from excessive 

 concentration of soluble salts may occur in the so-called alkali lands. 



Finally nitrification in the soil should be considered from the stand- 

 point of the organisms themselves. There is no doubt that continued 

 growth under extremely favorable conditions leads to the development in 

 the soil of nitrifying bacteria, possessing a very marked physiological effici- 

 ency. On the other hand, in ill-aerated, sour soils the environment would 

 depress the physiological efficiency of the nitrifying bacteria. Differences 

 are thus undoubtedly established under actual field conditions, as is made 

 probable by the variable behavior of soils from different sources when 

 used as inoculating material in recently reclaimed or peat swamp lands. 



Accumulation and Disappearance of Nitrates. As shown above, the 

 rate of formation of nitrates in the soil is dependent upon moisture, 

 temperature and aeration, as well as on the presence of organic matter 

 and basic substances. On the other hand, the accumulation of nitrates 

 depends, under any given conditions, largely on the character of the 

 growing crop. Observations on the rain gauges at Rothamsted showed 

 an average annual loss 14 kg. (31.4 pounds) of nitric nitrogen per acre 

 in the drainage water from uncropped soil. In one of King's experi- 

 ments, land that had been fallowed contained 137 kg. (303.24 pounds) 

 of nitric nitrogen per acre, to a depth of four feet. Adjoining cropped 



