CHAPTER XVII 



BACTERIA AND THE TRANSFORMATION OF 

 , SOIL-NITROGEN 



Nitrogen is one of the essential elements. There can 

 be no life without it. It makes up nearly four-fifths of 

 the gaseous envelope surrounding the earth, there 

 being about thirty-five thousand tons of it over every 

 acre of ground. Like the air above the ground, the soil 

 atmosphere contains almost four-fifths nitrogen. This 

 nitrogen, however, cannot ordinarily serve as food 

 for plants. It must be combined with other elements 

 in order to do so. 



The source of nitrogen in the soil. — The crops growing 

 on the land must depend for their supply of nitrogen on 

 the humus, for this contains practically all of the com- 

 bined nitrogen in the soil. There is but one exception 

 to this, — the plants of the legume family. These possess 

 the power of utilizing free nitrogen for their growth. 

 But even they are deprived of this power in the absence 

 of certain bacteria. 



Proportion of nitrogen in the soil. — Cultivated soils 

 contain, on the average, 0.1 to 0.2 per cent of nitrogen in 

 the surface portion. Taking the average weight of an acre 

 of ground to a depth of nine inches to be about 3,000,000 

 pounds, we find 3,000 to 6,000 pounds of combined 



(155) , 



