INOCULATION 157 



formation on the young roots. On clover and alfalfa the 

 nodules are very small like a pinhead or small bean, but 

 on some beans and cowpeas they are very large, even reaching 

 the size of baseballs on the velvet bean. When the legume is 

 harvested and the roots die the nodules decompose, the 

 accumulated fixed nitrogen going back to the soil. The 

 bacteria remain for the most part inactive until more legumes 

 are grown. The amount of nitrogen added to the soil by 

 plowing under the legume crop varies considerably with the 

 crop, season, and condition of soil, but it is safe to say that 

 the ordinary clover crop adds 40 to 50 pounds per acre, and 

 alfalfa 75 to 100 pounds per acre. 



Although there seems to be evidence that most of these 

 symbiotic bacteria belong to but one or two species, as a 

 matter of practical fact they are so differentiated by habit 

 of growth that there are several classes. For instance, the 

 bacteria which live on alfalfa roots are not fitted to live on 

 red clover roots, nor do the bacteria of beans live on vetch 

 roots. Xot every legume has its own special bacteria, how- 

 ever, for alfalfa and sweet clover can interchange bacteria; 

 white, alsike, and red clovers apparently have the same 

 bacteria; the vetches all seem to use the same organism. 



As far as crop plants are concerned only legumes have 

 nitrogen fixing bacteria on their roots, but there are some 

 plants such as the alder, New Jersey tea, buffalo berry, 

 sweet fern, and a few others which also have bacterial nodules 

 on their roots. The importance of this fact lies in the 

 ability of waste lands to accumulate nitrogen through the 

 agency of wild plants. 



131. Inoculation. — Practically all soils contain the nitrogen 

 fixing bacteria for the common clovers, peas, and beans, so 

 that a failure to have nodules develop on these legumes is 

 due rather to other causes than to lack of the bacteria in the 

 soil. For example, lime may be lacking, the soil may need 

 drainage, too much available nitrogenous compounds like 

 nitrates may be present, plant diseases may infect the 

 legumes. But where a new legume is tried, such as alfalfa 

 or serradella, and the crop fails under ordinarily beneficial 

 conditions, it may be necessary to inoculate the soil with 

 the proper bacteria. 



