VII.] SOIL INOCULATION 139 



pulled and not cut, thus depriving the soil of a consider- 

 able residue of root and stubble. Of course it should 

 not be forgotten that leguminous plants can, and do to 

 a large extent, in nature feed upon the combined 

 nitrogen in the soil, and it is probable that a plant like 

 a bean utilises the nitrogen of the soil until it is driven 

 by starvation to make the nitrogen of the nodules its 

 source of supply. 



In view of the enormous importance to practical 

 agriculture of the nitrogen-collecting power of the 

 leguminous nodule bacteria, a number of experiments 

 have been made all over the world to see if anything 

 could be gained by introducing an increased or possibly 

 a more vigorous supply of these organisms into the 

 soil. It has been shown, in fact, that although there is 

 only one species, as it may be called, of bacterium 

 associated with leguminous plants, yet that species 

 possesses a certain amount of racial adaptation, so that 

 clover which has been infected from a clover nodule 

 grows better than if it had been infected from a bean 

 nodule. In some cases this specialisation has gone so 

 far that the particular leguminous plant can only be 

 infected from another plant of the same kind or by soil 

 in which it has been grown ; it responds very indif- 

 ferently to the neutral form of organism that is present 

 in cultivated soils and adapts itself to most of the 

 leguminous crops it meets. Moreover, from time to 

 time, particularly in new countries or where heath or 

 bog or salted alkali land is being reclaimed for the first 

 time, soils are met with which do not contain any 

 nodule organisms, so that when leguminous plants are 

 sown they neither develop nodules nor fix nitrogen. 

 These considerations have led to the artificial inocula- 

 tion of the soil with the organism appropriate to the 

 desired crop, either by spreading over the land a small 



