NITRIFICATION 125 



Stirring and pulverizing the soil is therefore of great im- 

 portance, as shown by the following. A number of pots were 

 filled with soil. Part of them were allowed to stand undisturbed 

 while the others were poured out upon the floor and frequently 

 stirred. Those stirred invariably contained from ten to forty 

 times as much nitrates as did the unstirred. Summer-fallowed 

 land or land ^plowed in the fall contains more nitrates than 

 cropped or unplowed soil. The difference at times is apparent 

 thruout the following summer. Therefore when the farmer 

 cultivates his soil he is in reality ventilating the laboratory of the 

 nitrifiers, as they, like men and women work best in ventilated 

 work shops. 



Temperature. — The nitrifying bacteria are heat-loving and 

 light-avoiding. Therefore, we are not surprised at King's re- 

 sults where he found that there was 1.26 times as much nitrates 

 formed at 9^ C. as at l^ C, 2.76 times as much at 20°, and 

 6.24 times as much at 35° as at l° C. 



The significance of these figures is brought out more fully 

 when we examine the amount of nitrogen appearing as nitrates 

 in some cases. At 1° C. there were formed 120 pounds per 

 acre J at 9°, 150 pounds per acre; at 20°, 329 pounds per acre; 

 while at 35° there were formed 747 pounds per acre. When 

 we remember that the growing plant feeds on nitrates we are 

 not surprised they grow rapidly in a warm sandy soil and slowly 

 'in a heavy cold clay. In the latter case the nitrifiers' laboratory 

 is not sufficiently warm for efficient work. 



Moisture. — Long before the process of nitrification was known 

 to be due to microorganisms it was known that there must be a 

 certain proportion of water in the medium in which nitrates were 

 being formed. It was found that when small quantities of water 

 were present the process was slow, and if too great an amount 

 was present the nitrates disappeared completely. The quantity 

 of water required for optimum nitrification varies with the soil, 

 being higher for clay than for lighter soil. The Senior author, 

 however, has found that nitrification is at its maximum in all 



