33 



under favourable conditions to fix more or less atmospheric 

 nitrogen independent of legumes. They reach their highest 

 efficiency, however, when growing in the roots of legumes (clovers, 

 alfalfa, peas, beans, &c.,) where they usually form nodules. The 

 origin of the species is possibly from a solid form, Radiobacter, 

 commonly growing in association with Azotobacter. 



The value of leguminous crops as soil improvers has been well 

 known for centuries, and they are regularly used for this purpose, 

 especially in the older agricultural countries. It is, however, 

 only since the work of Hellriegel and Wilfarth in 1888 that it 

 has been universally recognized that the ability of these crops to 

 grow in soil devoid of nitrogen is due to the presence of certain 

 bacteria in the root nodules. These bacteria have been carefully 

 studied by many investigators in Europe and in this country, 

 and much valuable information has been secured regarding them. 

 The literature of the subject has been reviewed so many times in 

 various publications that it is not necessary to go over it again.* 

 It has been amply demonstrated, not only by hundreds of years 

 of actual experience, but by numberless carefully conducted 

 experiments in many countries, under widely varying conditions, 

 that clovers and numerous other legumes supplied with tubercle 

 bacteria obtain from the air through the agency of these bacteria 

 under favourable conditions, all the nitrogen they require, and 

 that they leave in the soil considerable quantities for succeeding 

 crops. In Germany the amount of nitrogen added to the soil by 

 legumes, besides that taken off in the crop, is estimated at 200 

 pounds per acre. In the United States the average for sixteen 

 States is 122 pounds, equivalent to not less than 800 to 1,000 

 pounds of nitrate of soda per acre.* These effects, of course, are 

 secured where the conditions for fixation are favourable, viz., 

 where the soil is abundantly supplied with nodule bacteria of 

 high efficiency and where the available nitrogen content of the 

 soil is low and the soil is well supplied with carbonate of lime or 

 its equivalent, and when the phosphates and other elements of 

 available plant food are present in sufficient quantity. If the soil 

 is already rich in nitrates, the leguminous crop may do no more 

 than maintain the nitrogen equilibrium. This is an important 

 thing to do, however, inasmuch as this nitrogen will be required 

 by subsequent crops in the rotation and the requirement of ex- 

 pensive nitrogenous manures is thereby reduced. 



VARIETIES AND EFFICIENCY OF ROOT-NODULE BACTERIA. 

 It is now pretty well agreed that the nodule bacteria on most 

 legumes belong to the same species. However, there are well- 

 defined strains or varieties especially adapted to certain genera and 

 species of legumes which adapt themselves with more or less dif- 

 ficulty to other genera and species. The immediate efficiency of 

 the bacteria will, therefore, depend upon the natural or artificial 

 inoculation of the particular legume with the best strain of bac- 

 teria adapted to it. In soils, therefore, where it is desired to cul- 



* See Bulletin 71 Bureau of Plant Industry. 



