CHAPTER XXIV. 

 SYMBIOTIC NITROGEN FIXATION. 



FROM the earliest day of agricultural practice it has been the 

 experience of practical men that legumes under appropriate condi- 

 tions render the soil more productive. It was the practice of the 

 Roman farmers to plow under lupines in order to enrich their soil. 

 This practice has persisted through all the succeeding ages by the 

 farmers of Europe and Asia. But it is only within the memory of 

 men now living that we have been able to state the cause of the 

 increased fertility. 



Early Theories. Liebig, by applying the exact methods of chemis- 

 try to agriculture, was able to demonstrate that plants get their 

 carbon from the carbon dioxid of the air and not from the carbon 

 compounds of the soil. He came to regard the ammonia of the air 

 as analogous to the carbon dioxid and taught the doctrine that the 

 plants are able to derive their nitrogenous food from the atmosphere. 

 He wrote: "If the soil be suitable, if it contains a sufficient quan- 

 tity of alkalies, phosphates, and sulphates, nothing will be wanting. 

 The plants will derive their ammonia from the atmosphere as they 

 do carbonic acid." Liebig considered all crops capable of securing 

 the nitrogen from the air, but the legumes and other broad-leafed 

 plants were especially fitted for this task, as is witnessed by the 

 fact that they benefit the succeeding cereal crops and do not respond 

 as readily to nitrogenous fertilizers. 



It was soon proved that the ammonia and other nitrogen com- 

 pounds of the air which were brought down by snow and rain were 

 very small and would account for only a small fraction of the nitro- 

 gen removed by the crops. 



Lawes and Gilbert (1855) reached the conclusion that non- 

 leguminous plants require a supply of some nitrogenous compound, 

 nitrates and ammonium salts being about equally effective. The 

 amount of ammonia obtainable from the atmosphere is insufficient 

 for the need of crops. Leguminous plants behave abnormally. 



They took the precaution of calcining the soil and removing all 

 of the ammonia from the air before it was admitted to the vessel in 

 which the plants were grown. Their results and those of Boussin- 

 gault agree fully in pointing to the conclusion that free nitrogen of 

 the air was not available to the plants. These conclusions were 

 accepted as decisive for a number of years, although much evidence 



