BIOLOGY OF SOILS 157 



Simultaneous Action. Although the successive changes 

 in the process are caused by different organisms, the latter 

 are, for the most part, similarly influenced by like condi- 

 tions. The changes, therefore, take place almost simul- 

 taneously, and the intermediate compounds do not accumu- 

 late. Accumulation of ammonium compounds has a 

 peculiar effect upon both the nitrous and the nitric fer- 

 ments. Within certain limits, the larger the proportion 

 of ammonium compounds present the better for the deve- 

 lopment of the nitrous ferment. But excessive quantities, 

 i.e., more than about one part per thousand of neutral 

 salts of ammonia, and less than half that quantity of 

 alkaline ammonium carbonate, inhibit its action altogether. 

 Much smaller quantities of ammonium salts inhibit the 

 action of the nitric ferment, but as these are gradually 

 converted into nitrous :acid by the action of the nitrous 

 ferment, it resumes its activity and converts the nitrous 

 into nitric acid. 



Habitat and Conditions of Growth. The soil is the 

 natural habitat of the nitrifying bacteria. They are 

 usually present in very large numbers, especially in soils 

 moderately rich in organic matter. For this reason, and 

 also because they require oxygen, they are much more 

 abundant in the surface soil, i.e., that part of the soil 

 which comes under cultivation, than in the subsoil. They 

 are found at a greater depth in open, sandy soils, into 

 which the air penetrates more freely, than in stiff clays. 

 As a rule very few are found at a depth greater than 

 3 feet, and none at all at a depth greater than 9 feet. 

 The largest numbers are not, however, found actually on 

 the surface, because they grow best in the dark strong 

 sunlight destroys them and because the soil at the sur- 

 face is often too dry. A plentiful supply of moisture is 

 necessary for their growth, but excessive quantities hinder 

 it by excluding the oxygen necessary for oxidation. It 



