374 ANNUAL REPORT OP THE Off. Doc. 



acquire a certain immunity against infection by bacteria of a certain 

 degree of virulence. 



This may account for the failure of land in many instances to grow 

 clover, so called "Clover sickness/' when there is reason to believe 

 that the soil is well supj)lied with the specific organisms. 



To return to the question of species among different root tuber- 

 cle organisms, it may be concluded that distinct species do not exist, 

 and that there is but one species of variable virulence, capable of 

 infecting the roots of an}^ of the legumes either feebly or readily. 

 That this species, which may at first attack a given host but feebly, 

 has its virulence so increased by repeated growth in that host that 

 it eventually is able to produce ready infection. 



5. Relation of the Soil to Tubercle Production. 



It has been stated that legumes will grow readily in soils where 

 there is an abundance of available nitrogenous food independent of 

 their power to assimilate atmospheric nitrogen. The legumes, how- 

 ever, are so rich in nitrogen, and such vigorous growers as a rule, 

 that their demands for nitrogen are greater than those of any other 

 class of agricultural plants. Hence, in ordinary soils, legumes must 

 utilize their nitrogen assimilating function to a greater or less de- 

 gree. 



In poor soils, and these are the ones in which we desire to grow 

 legumes as soil enrichers, there is often at first a struggle on the 

 part of the legumes for existence, particularly is this true when a 

 new legume is introduced. 



It is assumed that in most soils root tubercle organisms of one 

 variety or another exist, but often in such few numbers or in such an 

 attenuated form as to feebly affect the plant. In this event an in- 

 troductory crop of that legume, however small, may be necessary 

 for the purpose of stocking the soil not only with the necessary num- 

 ber of organisms but with those of the proper degree of virulence. 

 Soils, therefore, become adapted to the growth of any legume and 

 this adaptation consists in an abundant supply of organisms of the 

 proper virulence to infect the roots. 



Nothing directly is known regarding the relation of soil acidity 

 to the life of root tubercle bacteria therein, but Salfeld found that 

 the addition of lime to land was beneficial to the development of 

 root tubercles on field peas, lentils and garden peas. 



