SYMBIOTIC NITROGEN FIXATION 113 



Large amounts of phosphorus are required by Azotobacter 

 cells for their growth. Phosphorus is by far the most important 

 of the mineral substances absorbed during growth of the 

 cells. The amount of the element required by the bacteria 

 is in direct proportion to the amount of nitrogen fixed. This 

 is borne out by a simple analysis of the composition of the 

 microbial cell which, in a washed condition (freed from adher- 

 ing capsular material, largely hemicelluloses), contains about 

 10 per cent nitrogen and 5 per cent P2O5. It has been calculated 

 that for every gram of glucose used by Azotobacter as a source 

 of energy, 2.5 mgm. of phosphorus are required. This offers a 

 good method for determining the amount of available phosphorus 

 in the soil. For this purpose a definite amount of soil is added to 

 a definite amount of culture solution, free from phosphorus, 

 and the solution is then inoculated with Azotobacter; the amount 

 of nitrogen fixed under these conditions is taken as an index of the 

 quantity of available phosphorus in the given quantity of soil 

 added to the solution. A very practical procedure for determining 

 the available phosphorus in soil, based upon this phenomenon, is 

 discussed in detail later. 



Symbiotic Nitrogen Fixation. — It has been known since 

 olden times that the growth of leguminous plants, such as clover, 

 alfalfa, peas, or beans, leaves the soil richer for the succeeding 

 crops. The reason for this phenomenon was not known, but the 

 practical farmer recognized that the growth of such a crop was 

 equivalent to the appfication of manure as far as the succeeding 

 crop was concerned. It was only toward the middle of the nine- 

 teenth century that it became an established fact that this bene- 

 ficial effect of a leguminous crop is due to an increase in the supply 

 of soil nitrogen; the leguminous plants were found capable of 

 utilizing the nitrogen gas of the atmosphere, changing it thereby 

 into combined forms in some mysterious fashion. It was soon 

 estabhshed that when the soil in which the legume is grown is 

 previously heated or sterihzed, the leguminous plant behaves like a 

 cereal plant and is unable to use the gaseous nitrogen. When a 

 little fresh soil is added to the previously sterihzed soil, the legume 

 is once more able to draw upon the store of gaseous nitrogen 

 (Fig. 52). After a series of investigations, to which botanists, 

 bacteriologists, agronomists, and chemists have contributed, it 

 became definitely established toward the end of the last century 



