FIXATION OF ATMOSPHERIC NITROGEN 595 



less than 4.5. 146 Alfalfa yields were found to show an increase with an 

 increase in the pH value of the soil from 3.8 to 7.0. The plants experi- 

 ence difficulty in becoming established in soil with low pH values, but, 

 after becoming established, they make an excellent growth at as low 

 a pH as 3.8. With increasing hydrogen-ion concentrations (decrease 

 of pH), nodule formation is decreased. According to Bryan, 147 

 alfalfa organisms do not survive a greater acidity than pH 5.0, red 

 clover organisms pH 4.5 to 4.7 and soybean organisms pH 3.5 to 3.9. 

 The optimum growth for all strains of the alfalfa organism was found 

 to be at pH 7.0, with a limiting acid reaction at pH 5.5. 145 



Nodule formation and nitrogen fixation. When leguminous plants are 

 grown on soil properly limed, containing the necessary amount of min- 

 erals, and inoculated with the proper organism, considerable nitrogen 

 is found in the plant. Most of this nitrogen comes from the atmo- 

 sphere. The presence of nitrates in the soil will retard and may even 

 prevent, if present in sufficient quantities, nodule formation by the 

 plant. 148 The injurious influence of nitrate upon nodule formation is 

 due largely to the fact that it offers a source of available nitrogen to the 

 plant; it may also have a specific action upon the plant juice. But other 

 nitrogen compounds, such as ammonium salts or ammonia-producing 

 substances, may also reduce or even inhibit nodule formation. 149 This is 

 probably due to the direct assimilation of the combined nitrogen by the 

 plants rather than to any inhibition of bacteria from penetrating into the 

 roots. Carbonaceous substances, such as carbohydrates, certain or- 

 ganic acids and alkaloids stimulate nodule production in the soil. 

 However, appreciable amounts of nitrogen will be fixed even in the pres- 

 ence of considerable quantities of available nitrogen, including nitrates, 

 in the soil. 150 



The mechanism of nitrogen fixation by the plant was a subject of early 



146 Joffe, J. S. The influence of soil reaction on the growth of alfalfa. Soil 

 Sci., 10: 301-307. 1920. 



147 Bryan, O. C. Effect of acid soils on nodule forming bacteria. Soil Sci., 

 15: 37-^0. 1923. 



148 Strowd, W. H. H. The relation of nitrates to nodule production. Soil 

 Sci., 10: 343-356. 1920. 



149 Wilson, J. K. Physiological studies of Bacillus radicicola of soybean (Soja 

 max Piper) and of factors influencing nodule production. N. Y. (Cornell) Agr. 

 Exp. Sta. Res. Bui. 386. 1917. 



150 Albrecht, W. A. Symbiotic nitrogen fixation as influenced by the nitro- 

 gen in the soil. Soil Sci., 9: 275-319. 1920. 



