102 AGRICULTURAL BACTERIOLOGY 



obtained through the oxidation of ammonia or nitrous acid, 

 depending on the group of nitrifying bacteria concerned. 

 The nutrient solution must contain the potassium, sulphur, 

 and phosphorus needed for the structure of the bacteria, 

 but these may be present in inorganic form. The am- 

 monia in the soil or in the nutrient solution may be in the 

 form of a neutral . salt, such as ammonium sulphate. 

 Through the activity of the bacteria, an alkaline-reacting 

 radicle, ammonium, is changed to an acid nitrous acid. 

 This, together with the sulphate radicle, causes the rapid 

 increase of acidity in the soil or in the solution in which 

 the first step in nitrification is going on. It is essential to 

 have a neutralizing agent present, such as calcium carbon- 

 ate, else tiie process will soon stop. An abundant supply 

 of oxygen is also essential, as it is for all oxidizing pro- 

 cesses. The organisms grow best at about 30 to 35 C., 

 (86- 95 F,). 



Nitrification in the soil. If nitrification is to go on rap- 

 idly in the soil, conditions that will permit the rapid growth 

 of the causal bacteria must be established. One of the 

 most important conditions is a well aerated soil. If the 

 pores are filled with water, anaerobic conditions prevail and 

 no nitrification can go on. In those soils that naturally are 

 well drained, as sandy soils, or in which the excess of water 

 is removed by drainage or otherwise, the air supply will 

 be greater, and other conditions being equal, the oxidation 

 of the ammonia formed by the ammonifying bacteria will 

 go on quickly. It is difficult to establish the most favor- 

 able condition with reference to oxygen in our soils. Only 

 by frequent cultivation can the nitrifying process be kept 

 at its maximum. The formation of nitrates is most rapid 

 in the soil on which a cultivated crop is growing, such as 

 corn or roots. This accounts, in part at least, for the 

 large amount of dry matter these crops will produce per 



