TRANSFORMATIONS OF CARBON AND NITROGEN. 69 



mum salts. Nitrates are the most useful and most easily made 

 use of, while ammonia salts can also be easily utilized, perhaps, 

 however, after being converted into nitrates, as we shall see 

 later. On the other hand plants are unable to use the highly 

 complex nitrogen compounds, like proteids or other organic 

 substances ; nor are they able to make use of the simple com- 

 pounds of nitrous acid, called nitrites. Nitrates and ammonium 

 salts are then plant foods ; nitrites, organic nitrogen and free 

 nitrogen are useless for plants. Our problem is to discover 

 the means of maintenance of the supply of nitrates and am- 

 monium salts. 



The primitive origin of soil nitrates is at present purely a 

 matter of conjecture. The atmosphere, as is well known, con- 

 tains nitrogen in great quantity, about 77 per cent, of the air be- 

 ing free nitrogen. It is hence almost certain that the primitive 

 soil nitrates must have come from the atmosphere, and have 

 been^/fcm/ by some agency in the soil in the form of nitrogen 

 compounds. Several different processes are known which can 

 fi.\ free nitrogen and which may have contributed to the forma- 

 tion of the first nitrates in the soil. 



1 . It is known that certain alkaline bodies in porous soil can 

 hold and fix a small quantity of nitrogen. 



2. Electric discharges are known to induce the atmospheric 

 nitrogen to enter into certain compounds which become fixed 

 as combined nitrogen and serve as a source of nitrates. 



These two factors have in past years been looked upon as 

 the chief agencies for fixing nitrogen. But it seems somewhat 

 doubtful whether they are sufficient to account for the great 

 stores of nitrogen found in certain parts of the world. In re- 

 cent years there has been a tendency to regard a third factor 

 as of more importance. 



3. Bacteria under certain conditions, to be noticed later, have 

 the power of fixing atmospheric nitrogen. 



The reasons for regarding this last factor as of the most im- 



