168 



had elapsed since the land had been under the plough show the 

 following changes: 



NITROGEN, LB. PER ACRE, on soil left wild. 



The very remarkable gain of nitrogen in the soil of these two 

 plots must be put down to the action of bacteria; on Broadbalk 

 there are leguminous plants with which are associated the nodule 

 bacteria (Pseudomonas radicicold), but that these are not the only 

 or even the main agents in fixing nitrogen is seen from the gain 

 of nitrogen in the soil of the Geescroft field, which is almost 

 devoid of leguminous plants. The main factor has been the Azo- 

 tobacter, the bacterium which fixes nitrogen when free in the soil, 

 and its presence has been verified in the soil from both plots. 

 The reason for its activity on these pieces of land lies in the fact 

 that the yearly growth of vegetation is allowed to die back and 

 fall on to the land. Thus the soil receives an annual contribution 

 of purely carbonaceous material previously elaborated by the plant 

 from the carbon dioxide of the atmosphere, and by the oxidation 

 of this carbonaceous material the Azotobacter organism derives the 

 energy necessary to bring the tree nitrogen gas into combination. 

 In the laboratory Azotobacter must be supplied with sugar or si- 

 milar carbohydrates and fixation of nitrogen will then take place 

 to an extent that is proportional to the amount of sugar oxidised; 

 in Nature the requisite oxidisable carbohydrate is supplied by the 

 debris of previous vegetation. We have seen that on the adjoining 

 unmanured plot of Broadbalk from which the wheat is removed 

 every year, fixation is so small that it only just balances the yearly 

 loss of nitrogen due to drainage, etc. ; fixation is kept down at this 

 low level because, beyond the small root and stubble residue of 

 the wheat plant, there is no carbonaceous material supplied for 

 the Azotobacter. The much greater nitrogen fixation in the Broad- 

 balk than in the Geescroft soil may be set down to the presence 

 of a fair amount, 2 to 3 per cent of calcium carbonate, a substance 



