NITRIFICA TION 415 



ment of nitrobacteria and one half that quantity checks 

 it, while 150 parts of ammonia per million has a similar 

 effect. In a normal soil, the quantity of soluble am- 

 monium salts is well below this amount, as must also 

 be that of soluble organic matter. In confirmation of 

 the inhibiting effect of organic matter on the nitrobac- 

 teria, cases have been reported of soils very rich in 

 organic matter in which no bacteria of this type occur. 



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 garden- 

 ing operations, it is not likely to occur 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- 

 position of the manure but without any corresponding 

 increase in nitrates. On the other hand, under field con- 

 ditions, manure used in relatively small amounts does 

 not undergo this serious loss. 



The application of twenty tons of farm manure 

 per acre to sod on a clay loam soil for three consecu- 

 tive years, at Cornell University, resulted in a larger 

 production of nitrates on the manured soil than upon 

 a contiguous plat of similar soil left unmanured. 

 This was true during the third year of the applications, 

 when the land was in sod, and also the fourth year 

 when no manure was applied to either plat, and when 

 both were planted to corn, as may be seen from the 

 following table: 



