1 1 8 Conservation of the Fertility of the Soil, [may, 



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 free 

 nitrogen gas into combination. In the laboratory Azotobacter 

 must be supplied with sugar or similar 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, &c. ; 

 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 Broadbalk than in the 

 Geescroft soil may be set down to the presence of a fair 

 amount, 2-3 per cent., of calcium carbonate, a substance 

 which is almost absent from the Geescroft soil, yet without it 

 the Azotobacter cannot function properly. 



It is to the activity of Azotobacter when thus supplied with 

 carbohydrate by the annual fall of vegetation that we may 

 attribute the accumulation of nitrogen in virgin soils. The 

 higher plants alone, however long they might have occupied 

 the land, could only restore what they had previously taken 

 from the soil,^ and thus could originate no such vast stores 

 df nitrogen as are found in the virgin soils like the black 

 steppe soils of Manitoba and the North- West. This con- 

 clusion is strengthened by the fact that such steppe soils are 

 always well supplied with calcium carbonate, a necessary 

 factor in the action of Azotobacter. The organism itself has 

 also been isolated from all such soils. 



We are now in a position to see how far these various 

 examples can be made to interpret the conditions which 

 prevail in practice. 



In the first place, it is clear that the growth of successive 

 cereal crops which are wholly removed from the land will 

 rapidly reduce the stock of nitrogen originally in the soil, 

 not only by the amounts withdrawn in the crop, but also 

 because of the oxidising actions which the cultivation sets 

 up in the land. Moreover, the richer the land to begin with, 



