NITRIFYING POWER AND PRODUCTIVENESS 121 



certain time are determined. By working under uniform conditions 

 (which each investigator fixes for himself) the results obtained are 

 comparable for the series of soils under investigation. The amount 

 of nitrate produced by unit weight of soil is called the " nitrifying 

 power ". The actual figure is obviously arbitrary, depending on the 

 conditions selected, and it has meaning only in relation to the other 

 soils in the same set of experiments. Several investigators, however, 

 have found that nitrifying power shows some relation to plant growth, 

 the soils most favourable to plants having on the whole the highest 

 nitrifying powers. The results obtained at Rothamsted by Ashby 

 [5] and at Fallen, Nevada, by Kellerman and Allen l are as 

 follows : 



TABLE XLVII. NITRIFYING POWERS OF VARIOUS SOILS OF KNOWN 

 PRODUCTIVENESS. 



Withers and Fraps (3140) have modified the method, and use 

 sterilised soil as the medium for the growth of the organisms. 



A careful distinction must be made between the nitrifying power 

 ascertained from culture media and the rate at which nitrates accumu- 

 late in the soil. The experiments in culture media measure the rate 

 of nitrification under the circumstances of the experiment : the accu- 

 mulation of nitrate in the soil on the other hand measures the rate of 

 ammonia production (p. 88). 



The " ammonifying power " or " putrefactive power " is determined 

 by inoculating soil into a I per cent peptone solution, and determining 

 the ammonia formed after incubation at 20. Remy found that 

 certain soils known to give good crop returns for organic manures also 

 possessed high putrefactive power. He incubated for four days, 

 but Russell and Hutchinson (240^) found that better results were 

 obtained by taking definite intervals and plotting curves showing the 



1 Bacteriological Studies of the Soil of the Truckee-Carson Irrigation Project, Karl F. 

 Kellerman and E. R. Allen. U. S. Dept. of Agric. Bureau of Plant Industry, Bull. No. 

 211, 1911. See also Ehrenberg (936). 



9 



