332 MICROBIOLOGY OF SOIL 



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 

 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: develop- 

 ment in the soil of nitrifying bacteria, possessing a very marked phy- 

 siological efficiency. On the other hand, in ill-aerated, sour soils the 

 environment would depress the physiological efficiency of the nitrify- 

 ing 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 4 feet. Adjoining cropped 

 land contained only 26 kg. (57.56 pounds) of nitric nitrogen per acre 

 to the same depth. Stewart and Greaves found in hmestone soil in 

 Utah 64 kg. (142 pounds) of nitric nitrogen per acre, under corn; 



