186 Bacteria in Relation to Country Life 



fifty tons of manure per acre are at times made, there 

 is some probability of injury thus resulting. When such 

 injury does occur, it cannot, however, be designated 

 as denitrification. 



The depression of the crop-yields arising out of bac- 

 teriological causes may be due to the suppression of 

 nitrification in the soil, to the transformation of the 

 nitrate into organic nitrogen by certain classes of bac- 

 teria, or to the actual reduction of the nitrate by soil 

 or manure bacteria. It is well known that nitrifying 

 bacteria are quite susceptible to the presence of soluble 

 organic substances. This may be easily demonstrated 

 by the addition of a solution of sugar to a soil in which 

 nitrification is taking place. When large amounts of 

 manure are added to the soil, the soluble materials 

 in it will discourage nitrification. The striking influence 

 of soluble organic materials on nitrification in the soil 

 is particularly apparent when sewage-irrigation is 

 practiced. 



Effect not permanent. — The depressing effect on 

 nitrification is not permanent. After a longer or shorter 

 interval, the nitrifying bacteria become active again, and 

 the supply of nitrates proceeds normally. The length of 

 the period during which nitrification is suspended, or is 

 very feeble, is determined by the amount of organic 

 matter applied; hence very large applications of manure 

 will suppress nitrification for a longer period than smaller 

 applications. During this time the conditions for crop 

 growth are not favorable. When ordinary applications of 

 manure are made, that is, in amounts not exceeding 

 twenty or even thirty tons per acre, the interference with 



