394 MICROBIOLOGY OF SOIL 



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 of 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 limestone soil in 

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

 98 pounds under potatoes, and only 12 kg. (27 pounds) under alfalfa. 

 Under the same conditions fallow land contained 74 kg. (165 pounds) 

 of nitric nitrogen per acre. The smaller amount of nitric nitrogen found 

 under alfalfa bears out the observations already made by a number of 

 other investigators that the accumulation of nitrates under legumes is 

 smaller than it is under non-legumes. While several explanations have 

 been offered to account for this fact, it is generally agreed that legumes 

 assimilate nitrate nitrogen more rapidly than non-legumes. Unusual 

 circumstances may favor, at times, the accumulation of quantities of 

 nitrate large enough to destroy all vegetation. It is reported, for 



