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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. Jlie 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 of nitrogen as are found in the 

 virgin soils like the black steppe soils of Manitoba and the North- 

 West. This conclusion 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 withdrawn in the crop, but also because of the oxidising actions 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, the greater will be the annual losses ; when 

 the land gets anywhere near the pitch of impoverishment represented 

 by the Broadbalk unmanured plot, not only is the annual con- 

 version from dormant into available plant food small, but the 

 wasteful oxidation is similarly reduced, and the stock of nitrogen 

 is only slowly depleted. If instead of cropping continuously with 

 cereals a more conservative system of farming is introduced, in 

 which leguminous crops become a regular feature in the rotation, 

 and a certain amount of carbonaceous matter is returned to the 

 land, as by the folding off of green crops by sheep, then the re- 

 cuperative agencies fixing nitrogen become sufficient to repair the 

 losses due to the crops and the waste by drainage and oxidation, 

 and a moderate level ot fertility may be maintained indefinitely 

 without the introduction of any extraneous source of nitrogen. 



Such, indeed, was the state of affairs in Europe prior to the 

 discovery of artificial manures and foods ; the farm had to be self- 

 supporting, the nitrogen that came back to the land in the farm- 

 yard manure had all been taken from the land previously; it was 

 less than that which left the land by the amounts in the corn, 

 meat, milk, and wool sold off the farm, and by all that was lost 

 and wasted in making the farmyard manure. These losses were 

 however, so far balanced by the gains of nitrogen due to bacterial 



