IO4 AGRICULTURAL BACTERIOLOGY. 



which have no power of active growth unless supplied with a 

 good quantity of organic food. 



On the other hand at the end of the decomposition changes, 

 when the high organic compounds have all disappeared, and 

 in their place is found a quantity of ammonia salts and other 

 simple nitrogen compounds, we have just the proper condition 

 for the action of nitrification. So long as there is any trace of 

 free ammonia left the only nitrobacteria which can act are those 

 which produce nitrons acid from the ammonia, since the bac- 

 teria which form nitrates are prevented from growing by 

 the very merest trace of ammonia. First the nitrobacteria 

 begin to work, oxidizing the ammonia, and soon, after all 

 traces of free ammonia disappear, the nitric bacteria them- 

 selves begin their action of completing the nitrification, 

 producing nitrates from the nitrites formed. After these 

 nitrates are produced they are not in much danger of being 

 reduced by the denitrifying bacteria even though they be still 

 present in the soil. These latter demand considerable organic 

 material for their growth and this has been all used up by the 

 previous decomposition. Denitrification will not begin again 

 unless more organic material is added to the soil to furnish the 

 denitrifying bacteria with their needed food. The nitrates will 

 be reserved for the next generation of plants growing in the 

 soil. If, however, more organic material, like manure, should 

 be placed upon the soil after the nitrification has been com- 

 pleted, this would result probably in the destruction of most 

 of the nitrates formed. A practical lesson from these facts is 

 that animal manure should never be mixed with nitrates as a 

 fertilizer. The nitrates are useful but are pretty sure to be 

 destroyed by the denitrifying bacteria if mixed with manure. 



NITRIFICATION NOT IN ALL SOILS. 



Although only a few of these nitrifying bacteria are as yet 

 known, these few are veiy widely distributed. They are 



