CHAPTER XV 

 SYMBIOTIC NITROGEN FIXATION 



The atmosphere over every acre contains about one-half ton of 

 carbon and 25,000 tons of nitrogen. Plants never lack carbon, 

 but they often starve in the midst of this vast ocean of nitrogen. 

 All plants can obtain carbon from the atmosphere. Are there 

 none v^hich can obtain nitrogen from the air? If there are, 

 which are they and under what conditions can they obtain it? 

 These questions have been asked by many, and the unfolding of 

 the answer constitutes one of the most interesting and practical 

 phases of agricultural development. 



Belief of the Seventeenth Century. — ^Today one cannot re- 

 frain from smiling at the conclusion reached by van Helmont 

 in the seventeenth century. He planted a willow weighing five 

 pounds in two hundred pounds of soil. This he watered regularly 

 for five years. At the end of this time he removed the soil from 

 the roots and found its weight unchanged, whereas the willow 

 had gained 1 64 pounds. From this he concluded that the willow 

 had been produced from the water. He had overlooked the 

 small quantity of carbon dioxid which a century and a half later 

 Priestley concluded came from animal life and was removed by 

 plant life. And what was more natural than for Priestley to 

 also conclude that plants get their nitrogen from the air? His 

 conclusions rested on crude qualitative tests j hence, it was not 

 until the last quarter of the eighteenth century that it was fully 

 established that plants obtain their carbon from the atmosphere. 



Later Work. — Liebig considered all crops capable of securing 

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

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

 fact that they benefit the succeeding cereal crop and do not re- 

 spond as readily to nitrogenous fertilizers. 



