SOIL ORGANISMS 433 



nitrogen fixation. The maintenance of a fair supply of soil 

 organic matter is, therefore, as important as the regulation 

 of the temperature, the oxygen, the moisture, and the reac- 

 tion of the soil. While the presence of nitrates in small 

 amounts seems to stimulate azofication, large quantities of 

 nitric nitrogen tend to lessen nitrogen fixation. 



The amount of nitrogen fixed in the soil by organisms func- 

 tioning independently of higher plants is, as might be ex- 

 pected, a variable quantity. Hall * considers it to be on the 

 average about 25 pounds yearly to the acre, Greaves 2 25 

 pounds, Lbhnis 3 36 pounds, and Lipman 4 from 15 to 40 

 pounds. As a basis for calculation 25 pounds is perhaps a 

 conservative and reasonable figure. A comparison of this 

 figure with the 6 pounds of nitrogen brought down yearly 

 in rain-water, indicates that the free-fixing organisms are 

 four or &ve times more important than rainfall as a source of 

 nitrogen. 



239. Bacillus radicicola and its relationship to the host 

 plant. — It has long been recognized by farmers that certain 

 crops, as clover, alfalfa, peas, beans, and some others, im- 

 prove the soil, making it possible to grow larger crops of 

 cereals after these plants have occupied the land. Within 

 the last century the benefit has been traced to the fixation 

 of nitrogen through the agency of bacteria contained in nod- 

 ules on the roots. The specific plants so affecting the soil 

 were found to be, with a few exceptions, those belonging to 

 the family of legumes. It has furthermore been demonstrated 

 that the host plant is generally able to appropriate some of 

 the nitrogen so fixed and thus benefit by the relationship. 

 The phenomenon was fully explained in 1886 by Hellreigel 



1 Hall, A. D., On the Accumulation of Fertility by Land Allowed to 

 Bun Wild; Jour. Agr. Sci., Vol. I, pp. 241-249, 1905. 



3 Greaves, J. E., Azofication; Soil Sci., Vol. VI, No. 3, pp. 163-217, 

 1918. 



3 Lohnis, F., and Westermann, F., Tiber SticTcstojf fixierende Bakterien, 

 IV. Centrbl. f. Bakt., II, Bd. 22, S. 234-254, 1909. 



4 Lipman, J. G., Marshall's Microbiology, p. 343, 1917. 



