18 SOIL CONDITIONS AND PLANT GROWTH 



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 (159, see also 86). 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) he 

 got good growth again. The experiment 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. 

 No one has yet succeeded in carrying out this fundamental experiment 

 of growing plants in two soils differing only in that one contains bac- 

 teria while the other does not 



The close connection between bacterial activity and the nutrition of 

 plants is, however, fully justified by many experiments, and forms the 

 basis of our modern conception of the soil as a producer of crops, as 

 will appear in the following chapters. 



