450 SOILS: PROPERTIES AND MANAGEMENT 



is that they cannot be grown in artificial media containing 

 organic matter. This property for a long time prevented 

 the isolation and identification of these organisms, as it 

 was hardly conceivable that organisms living in the dark, 

 where energy cannot be obtained from sunlight, could 

 exist without using the energy stored by organic matter. 

 It has been suggested, in explanation of this, that the 

 energy produced by the oxidation involved in the process 

 of nitrification makes possible the growth of the organisms 

 under these apparently impossible conditions. Some 

 experimenters report having grown nitrobacteria in or- 

 ganic media, but it is generally believed at present that 

 this is not possible and that there has been some error in 

 the work of these experimenters. 



The presence of peptone in the proportion of 500 

 parts per million completely prevents the development 

 of nitrobacteria, and one-half that quantity checks it; 

 while 150 parts of ammonia to the million has a similar 

 effect. In a normal soil the quantity of soluble ammo- 

 nium salts is well below this amount, as must also be that 

 of soluble organic matter. In confirmation of the inhibit- 

 ing effect of organic matter on the nitrobacteria, cases 

 have been reported of soils very rich in organic matter 

 in which no bacteria of this type exist. 



It has also been stated that very heavy manuring 

 with organic manures results in decreased nitrification 

 in the soil. While this may be true where farm manure 

 is used in the quantities sometimes applied in gardening 

 operations, it is not likely to be the case in soils on which 

 ordinary field crops are grown. The principle is well 

 illustrated by the dry-earth closet. Manure mixed with 

 earth in relatively small proportions and kept aerated 

 by occasional mixing undergoes a very thorough decom- 



