124 TRANSFORMATION OF NITROGEN BY SOIL MICROBES 



responsible for the very limited development of the organisms once 

 they have entered the plant. The final result is a lack of nodule 

 development and consequently no fixation of nitrogen within the 

 plant. 



Amounts of Nitrogen Fixed. — Since legumes absorb large 

 amounts of nitrogen from the soil, if the nitrogen exists in available 

 forms, it is not possible to state without qualifications how much 

 nitrogen will be fixed through the use of legumes in agricultural 

 practice. Under conditions where the legumes make good growth 

 and the nitrogen supplied by the soil is small, there are usually 

 between 100 and 200 pounds of nitrogen fixed per acre (Table 27). 

 Even larger amounts of nitrogen may be fixed under particularly 

 favorable conditions. However, when the crop is removed for 

 hay or similar purposes, there may be practically no increase in 

 the combined nitrogen of the soil, since most of the nitrogen of the 

 plant (often as much as 75 per cent) is in the stems, leaves, and 

 seeds, and only a small amount in the roots. The nitrogen con- 

 tained in the roots may represent no more than such nitrogen as 

 has been absorbed from the soil during growth. Where a large 

 portion of the plants is not removed but becomes incorporated with 

 the soil, there may be great increases in the combined soil nitrogen. 



Influence of Soil Conditions upon Nitrogen Fixation. — 

 Although no other single factor appears to exert such a pronounced 

 influence on the degree of nitrogen fixation as the nitrate content of 

 soils, there are numerous other factors which influence the process. 

 Among these may be mentioned, moisture, soil reaction, tempera- 

 ture, soil texture, and addition of fertilizer salts. In general, with 

 the exception of the influence of nitrogen compounds, it maj^ be 

 stated that there is a close correlation between the effects of these 

 factors on nodule formation and on the growth of the legumes. 

 Those conditions most favorable to vigorous development of the 

 host plants favor nodulation and nitrogen fixation. Conditions 

 which are unfavorable to the plants lower nodulation and fixation 

 of nitrogen. 



With most legumes, growth is best at reactions close to neu- 

 trality, and nodulation is greatest under such conditions. Some 

 legumes are quite acid tolerant, and good nodule development 

 occurs in acid soils. In general, the degree of acidity which 

 limits development of the bacteria is also one that is known to be 

 injurious to the host plant (Table 28). Of the various fertihzing 



