UTILIZATION OF NITROGEN-FIXING POWERS OF LEGUMES. 105 
species grow better in some climates than others, and certain soils 
seem to be, for some reason, better adapted for particular species, 
quite independent of the question of the presence of the proper soil 
bacteria. By the proper consideration of the facts of his experience 
the farmer can, without much difficulty, determine what species of 
legume grows best in his soil. The most vigorously growing 
legume is the best. In clay soils red and yellow clover, lupin, sera- 
dilla, horse beans, and vetches are successfully grown. Which of the 
varieties is to be selected must be determined by the conditions of 
the soil and the needs of the farmer for the particular crop which he 
raises. The essential feature must be that the species selected 
should be one that will grow well in the soil in question, otherwise 
the advantage of the nitrogen fixation will not be obtained. 
2. Insuring Presence of Proper Bacteria. In order that the soil 
may increase its nitrogen store it is evidently necessary for tubercles 
to develop in large numbers on the roots of the legumes. For this 
purpose, of course, it is necessary that the proper variety of bacteria 
shall be present in the soil, otherwise no tubercles will be formed, or 
the tubercles formed will be few and small. To insure this result 
may sometimes require a little experimenting and observation. 
Some species of legume find in a certain soil the tubercle organism 
adapted to them, while other species of legume may not find the 
proper organisms in the same soil. The soy bean is a most excellent 
crop for nitrogen gathering since it is an extremely luxurious grow- 
ing legume, producing abundant tubercles and a large fixation of 
nitrogen when supplied with the organisms which produce tubercles. 
But in order to make use of this crop it may be necessary to import 
the proper bacteria from other soils. On the other hand, there are 
some species of legumes, like most kinds of peas, which are capable 
of growing in most soils and producing an abundance of tubercles. 
Further, a legume, which, during the first season produces only a 
small number of tubercles, may succeed better the second year than 
the first and may fix more nitrogen. The growth of the crop in the 
soil during the first year apparently either increases the number of 
soil organisms appropriate to this particular legume or produces 
such changes in the physiological character of the bacteria present 
