FIXATION OF NITROGEN BY BACTERIA 199 



position of the soil and its chemical reaction influence 

 nodule formation, and affect the vitality and virulence of 

 the nodule bacteria. The latter are rarely abundant 

 in acid soils, and for the healthy exercise of their 

 physiological powers they need an adequate supply of 

 mineral substances, especially compounds containing 

 lime, phosphates, and potash ; without these their 

 virulence is reduced and their ability to assist the 

 leguminous crop is diminished. These facts are of 

 great importance in the preparation and use of pure 

 cultures for agricultural purposes. 



5. Soil inoculation. From what has been said pre- 

 viously it is obvious that the growth of a good legu- 

 minous crop is of the greatest importance to the 

 farmer. In itself it provides valuable food for farm 

 stock, and at the same time leaves the land well 

 supplied with nitrogen, which vastly assists the growth 

 of cereals and other crops succeeding it. The certain 

 production of a good leguminous crop is therefore 

 very desirable. In most cases the sowing of the 

 proper seed is enough to ensure success, the nodule 

 bacteria being very abundant in the majority of soils. 

 In some soils, however, certain kinds of leguminous 

 plants have been found to grow very poorly : no nodules 

 have appeared upon their roots, and the land has been 

 no more benefited than it would have been by the growth 

 of a cereal crop. In such instances the right nodule- 

 organism is missing from the land, and its introduction 

 is necessary before satisfactory results can be expected. 

 Experiment has shown that this can be achieved by 

 taking soil from a field which has grown a good crop of 

 the particular plant in question, and spreading it over the 

 land on which the crop has failed. Such a process of 



