202 FIXATION OR ASSIMILATION OF NITROGEN 



returns after inoculation seems to occur where the land 

 is supplied with an excess of easily assimilated nitro- 

 genous compounds which check the growth of the nodule 

 organisms, and also on acid soils and those deficient in 

 phosphates and potash. 



Inoculation is likely to be effective upon soils poor in 

 nitrogen, especially where leguminous crops have not 

 been grown previously, and also on land which has 

 given meagre crops of this class, with roots devoid of 

 nodules. 



Much careful and continued experiment is needed to 

 determine the best kinds of leguminous crop to grow, 

 how to grow them, and how to use them, so as to bring 

 to the farm the greatest amount of nitrogen at the least 

 expense. These are problems which should be con- 

 sidered along with the cultivation and application of 

 pure cultures of the various nodule organisms if the 

 work is to be of economic interest. 



Pseudomonas radicicola (Beijk.), Moore. Ps. radicicola is an 

 aerobic bacterium without spores, and grows best at about 

 15 C, being easily killed at a temperature of 60 to 70 C. As 

 previously indicated, the size and form of the organisms present 

 in the nodules of the Leguminosae vary with the condition of 

 development of the nodule, and the kind of plant from which 

 they are taken. In the infection threads and young parts of the 

 nodule, the bacteria are always short non-motile rods about 

 i to 2 /& long and .5 to .6 {* broad. The bacteroids differ con- 

 siderably in different plants. Those of Pkaseolus, Hedysarum, 

 and Genista are thickish rods, straight or curved, but rarely 

 branched : in Vicia> Pisum and Trifolium, when fully developed, 

 they become T and Y shaped, 2 to 4 ^ long, .5 to .8 p broad, 

 while those of Lupinus are often extensively and irregularly 

 branched. 



