SOIL CONDITIONS AND NITROGEN FIXATION 125 



materials, phosphates and calcium exert the most pronounced 

 influences on the legumes, although any element essential to plant 

 growth which is present in insufficient amounts lowers nodulation. 



TABLE 28 



Critical Reaction (/jH) for the Bacteria of Some Legumes (from Fred) 



Alfalfa and sweet clover 4.9 



Peas and vetches 4.7 



Clovers and beans 4.2 



Soybeans 4,0 



Lupine 3.2 



Since nitrogen fixation is dependent upon the presence and 

 activity of specific bacteria, it is essential that the organisms be 

 present in soils in order that the plants may make use of atmos- 

 pheric nitrogen. Where a legume has never been grown in a cer- 

 tain soil it is Hkely that the specific bacterium is not present, at 

 least in sufficient numbers to thoroughly inoculate the plants. 

 Consequently, the seeds or the soils in which the seeds are planted 

 are frequently inoculated with the bacteria at the time of planting. 

 Once the legume has been well nodulated during growth, the bac- 

 teria persist in the soil for a number of years, provided that the 

 soil reaction is not too acid. In the absence of the host plant, 

 the organism will survive longer in a neutral or slightly acid soil 

 than in a more acid soil. 



In addition to the leguminous plants, several non-legumes are 

 also capable of forming nodules on their roots. These include 

 such plants as the alder, sweet gale, and red root. The microbe 

 responsible for the formation of these nodules was found to belong, 

 in most cases, to the Bac. radicicola group, namely, to the same 

 group which includes the organisms that cause nodule formation 

 on the roots of legumes. However, it is still unknown whether 

 or not nitrogen becomes fixed in these non-leguminous plants. 



Some tropical plants, such as Pavetta and Ardisia, form nod- 

 ules either on the upper or on the lower part of their leaves. Bac- 

 teria were found to be the responsible agents. These bacteria 

 are aerobic, rod-shaped cells, developing abundantly in the intra- 

 cellular spaces of the plants and said to be present even in the 

 embryo sacs of the seed. With the germination of the seed, they 

 develop through the plants, so that there is unbroken continuity 

 of the association throughout the life cycle of the plants. In these 



