II] THE SOIL AND PLANT FOOD 17 



found in normal soils. It therefore follows that the 

 rate at which nitrites are oxidised is greater than 

 the rate at which they are formed. 



We have, then, three reactions going on : 



Nitrogenous plant residues change to ammonia, 



Ammonia changes to nitrite, 



Nitrite changes to nitrate. 



Of these the last is the quickest, the second is slower 

 and the first is slowest. The first change, therefore, 

 limits the rate at which nitrates are produced : if we 

 could speed up this change we should hasten the 

 others. 



Further, the quickest change (the third) is most 

 susceptible to external influences. Tlie organisms 

 are very sensitive, they are more easily kiUed than 

 the rest and they stop working more readily. Am- 

 monia producers, on the other hand, are very re- 

 sistant and will tolerate somewhat rough treatment. 

 All three compounds, ammonia, nitrites and nitrates 

 can be used as plant food, but in normal conditions 

 the plant does not get the chance of using anything 

 but nitrates, the other two being only transitory 

 products. 



Four important facts have thus been established 

 with regard to nitrification : 



(1) Ammonia is changed to nitrite and this is 

 then changed into nitrate in the soil, the conversion 

 being almost complete. 



R. 2 



