Hugh Spencer 



The swellings are inhabited by a vast 

 number of tiny one-celled organisms that 

 feed upon carbohydrates produced by 

 the alfalfa plant. These guests absorb 

 nitrogen from the air and combine it 

 with material taken from the host, pro- 

 ducing proteins. The alfalfa plant makes 

 use of the excess protein. Nitrogen- 

 fixing soil bacteria form similar tubercles 

 on the roots of peas, beans, clover and 

 other plants of this family. The bacteria 

 produce much more protein than they 

 can use, just as most green plants pro- 

 duce much more sugar or starch than 

 they can use. As a result of this part- 

 nership the plants of the legume family 

 contain much larger proportions of 

 nitrogenous compounds than those of 

 any other family. And a crop of such 

 plants leaves more nitrogen in the soil 

 than there was at the start 



BACTERIAL SWELLINGS ON ROOTS OF ALFALFA 



lizer. We have only to plant a crop of peas or alfalfa, and to make sure of 

 the special kinds of bacteria that form the tubercles on the roots of these 

 plants. It is now possible to buy cultures of the species of bacteria that are 

 known to thrive best on any particular legume species. 



In the course of the summer the bacteria in the tubercles will "fix" a 

 large quantity of nitrogen from the air. Part of this they will make into 

 proteins and consume in growth. Another part will be taken from them 

 by the plants upon which they grow. And at the end of the season there 

 will be present in the soil and above the soil (in the green plants) a great 

 deal more nitrogen in combined form than there was at the beginning. 

 The clover or alfalfa can be plowed under, and the nitrogen compounds 

 in the plants thus added to the soil. After another season of this kind of 

 crop enough nitrogen will be restored to the soil to support several crops 

 of grain. This rotation of crops has been practiced by experienced farmers 

 for many centuries, but it is only within the last fifty or sixty years that 

 the significance of rotation has been understood. 



Industrial Fixation of Nitrogen For the chemical solution of the 

 nitrogen problem we are indebted to a Swedish scientist, Svante Ar- 

 rhenius (1859-1927). Arrhenius worked out a process for making nitro- 

 gen combine with other elements under the action of electric currents. 

 A process for combining nitrogen from the air with hydrogen, forming 

 ammonia, was worked out by the German chemist Fritz Haber (1868- 



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