118 AGRICULTURAL BACTERIOLOGY 



Chemical processes have been devised by which the nitro- 

 gen and the oxygen of the air can be brought into combi- 

 nation. The most successful of these is the fixation of 

 nitrogen by electric discharges. Where cheap electric 

 power can be had, nitrates can be made at prices that en- 

 able them to compete with the natural product from Chile. 

 Large quantities of such nitrates are made in the Scandi- 

 navian countries, where water power is abundant and con- 

 ditions are not such as to enable the power to be used for 

 other purposes. Constant progress is being made in the 

 development of methods by which the nitrogen of the air 

 is made to combine with other elements, and it is highly 

 probable that the world has little to fear from an insufficient 

 supply of combined nitrogen in the years to come. 



Nitrogen added to the soil by rains. Every electric dis- 

 charge occurring in the atmosphere results in the produc- 

 tion of oxides of nitrogen. These and the ammonia in the 

 air are returned to the soil in the rain water. It has been 

 determined that from three to six pounds of nitrogen are 

 thus added to each acre in a year. About 70 per cent, of 

 this is in the form of ammonia, the remainder in the form 

 of oxides of nitrogen. 



Nitrogen fixation in soil. The same crop can be grown 

 on the land for hundreds of years ; the yield will soon reach 

 a level below which it will not fall. This level is usually 

 established by the rate at which some one element is made 

 available. The limiting factor most frequently is nitro- 

 gen. The yield of the crop is usually larger than could be 

 accounted for by the amount of nitrogen added to the soil 

 in the rain water. It would thus seem that there must be 

 factors at work in the soil that tend to maintain the nitro- 

 gen content. In the latter part of the last century, Berthe- 

 lot, a French chemist, studied the increase of nitrogen in 

 soils on which no crop was growing, and which were pro- 



