154 THE SOIL: ORGANIC MATTER 



All nitrogen in combination on the earth came at one time 

 or another from the atmosphere. The nitrate of soda beds, 

 coal, and many other forms, all owe their nitrogen to the 

 air. In other words, there is and has been some natural 

 agency for combining atmospheric nitrogen. Nitrogen gas 

 is very inert and does not combine easily with other elements. 

 A small portion unites with oxygen under the influence of 

 lightning. But aside from this there exist in the soil 

 certain bacteria which can combine nitrogen from the air 

 with carbon, hydrogen, and oxygen, and so put it in a form 

 that can be used. This process by which atmospheric 

 nitrogen is fixed, or made into stable compounds, is called 

 nitrogen fixation, and is probably the most important single 

 process taking place in the soil, all things considered. 



There are two kinds of bacteria which can fix nitrogen: 

 First, those which act independently of other living things, 

 like most of the soil bacteria; and second, those which act 

 most energetically when living with some other plant. In 

 both cases the bacteria derive energy for combining nitro- 

 gen from the oxidation of carbohydrates, and for their most 

 efficient work large quantities of soluble carbohydrates, 

 sugars probably, are necessary. The nitrogen so fixed is 

 then used by the bacteria in part, although more is fixed 

 than the bacteria need for their own growth. 



(a) Non-Symbiotic. — Those bacteria which act indepen- 

 dently are non-symbiotic in character, that is, they do not 

 live with any other plant to the mutual advantage of both. 

 They occur in most soils apparently and are able to fix some 

 nitrogen which is left in the soil after their death. Not 

 very much is known about these bacteria, but it is not prob- 

 able that in ordinary farming they play much part in adding 

 nitrogen to the soil. 



(b) Symbiotic. — This class is by far the more important 

 and is familiar to all farmers. The nodules on the roots of 

 leguminous plants like clover, alfalfa, peas, beans, and 

 vetch, are abnormal root growths formed to accommodate 

 the colonies of these nitrogen-fixing bacteria which are living 

 with the legumes to the mutual benefit of both (Figs. 33, 34, 

 35). The legumes supply the bacteria with soluble carbo- 



