i6 



SOIL CONDITIONS AND PLANT GROWTH 



supplied as manure plants get practically nothing but nitrate as food. 

 This closed the long discussion as to the nitrogenous food of non- 

 leguminous plants : in natural conditions they take up nitrates only 

 (or at any rate chiefly), because the activities of the nitrifying organisms 

 leave them no option. The view that plants assimilate gaseous 

 nitrogen has from time to time been revived^but has not been taken 

 seriously. 



The apparently hopeless problem of the nitrogen nutrition of 

 leguminous plants was soon to be solved. In a striking series of 

 sand cultures Hellriegel and Wilfarth (130) showed that the growth of 

 non-leguminous plants, barley, oats, etc., was directly proportional to 

 the amount of nitrate supplied, the duplicate pots agreeing satisfac- 

 torily ; while in the case of leguminous plants no sort of relationship 

 existed and duplicate pots failed to agree. After the seedling stage 

 was passed the leguminous plants grown without nitrate made no 

 further progress for a time, then some of them started to grow and 

 did well, while others failed. This period of no growth was not seen 

 where nitrate was supplied. Two of their experiments are given in 

 Table III. 



TABLE III. RELATION BETWEEN NITROGEN SUPPLY AND PLANT GROWTH. 

 HELLRIEGEL AND WILFARTH (130). 



Analysis showed that the nitrogen contained in the oat crop and 

 sand at the end of the experiment was always a little less than was 

 originally supplied, but was distinctly greater in the case of peas ; the 

 gain in three cases amounted to -910, 1-242 and 789 grm. per pot re- 

 spectively. They drew two conclusions : (i) the peas took their 

 nitrogen from the air ; (2) the process of nitrogen assimilation was 

 conditioned by some factor that did not come into their experiment 

 except by chance. In trying to frame an explanation they connected 

 two facts that were already known. Berthelot (26) had made experi- 

 ments to show that certain micro-organisms in the soil can assimilate 

 gaseous nitrogen. It was known to botanists that the nodules on the 



1 e.g. see (226a). 



