26 



SOIL CONDITIONS AND PLANT GROWTH 



(247) showed that the weight of nitrogen absorbed from the 

 air was approximately equal to the gain by the plant and the 

 soil, and thus finally clinched the evidence. 



The organism was isolated by Beijerinck (p. 204) and called 

 Bacterium radidcola. 



Thus another great controversy came to an end, and the 

 discrepancy between the field trials and the laboratory experi- 

 ments of Lawes, Gilbert, and Pugh was cleared up. The 

 laboratory experiments gave the correct conclusion that legu- 

 minous plants, like non-leguminous plants, have themselves 

 'no power of assimilating gaseous nitrogen ; this power belongs 

 to the bacteria associated with them. But so carefully was 

 all organic matter removed from the soil, the apparatus, and 

 the air in endeavouring to exclude all trace of ammonia, 

 that there was no chance of infection with the necessary 

 bacteria. Hence no assimilation could go on. In the field 

 trials the bacteria were active, and here there was a gain of 

 nitrogen. 



The general conclusion that bacteria are the real makers 

 of plant food in the soil, and are, therefore, essential to the 

 growth of all plants, was developed by Wollny (317^) and 

 Berthelot (28). It was supposed to be proved by Laurent's 

 experiments (160, see also 87). He grew buckwheat on 

 humus obtained from well-rotted dung, and found that plants 

 grew well on the untreated humus, but only badly on the 

 humus sterilised by heat. When, however, soil bacteria were 

 added to the sterilised humus (by adding an aqueous extract 

 of unsterilised soil) good growth took place. The experi- 

 ment looks convincing, but is really unsound. When a rich 

 soil is heated some substance is formed toxic to plants. The 

 failure of the plants on the sterilised humus was, therefore, 

 not due to absence of bacteria, but to the presence of a toxin. 



