518 



Mr. A. D. Hall 



[May 24, 



contains less nitrogen than the less rich virgin soils of the prairies, 

 three times as much nitrogen are wasted every year as is converted 

 into crop, and the same or an even greater rate of wastage must attend 

 the conversion of the rich virgin soils into land growing a succession 

 of cereal crops. 



We may now turn to another plot on the same field to illustrate 

 the recuperative actions of which I have spoken. This is a part of 

 the field that has Vieen running wild since 1881, when the wheat it 

 carried was not harvested but allowed to seed itself. A very few 

 years sufficed to eliminate the wheat, which was unable to maintain 

 itself against the competition of the weeds, and the land now carries 

 a miscellaneous vegetation consisting mostly of grass. A soil sample 

 was taken at starting, and when compared with another sample taken 

 twenty-three years later showed that in the intei'val the land had 

 gained nitrogen at the enormous rate of 92 lbs. per acre per annum. 

 Making every allowance for possible errors in sampling and analysis, 

 the accumulation of nitrogen is in marked contrast to its steady 

 depletion in the equally unmannred arable land alongside. Now, the 



Broadbalk Field, Rothamsted. 

 Land allowed to run wild. Nitrogen in Soil, lb. per acre. 



difference between the two plots lies in the fact that on the land run- 

 ning wild the vegetation is never removed, but allowed to die down 

 naturally. Hence not only is the nitrogen taken out by the crop 

 returned to the soil, but also a large stock of carbonaceous matter 

 assimilated from the atmosphere, and this carbonaceous matter fur- 

 nishes a bacterium present in the soil, Azotohacter chroococcum. with 

 the source of energy which will enable it to fix atmospheric nitrogen. 

 Azotohacter is equally present in the soil of the unmanured wheat plot ; 

 but, as there the crop is removed and only a little root and stubble 

 left behind, there is hwi little carbonaceous matter for the Azotohacter 

 to work upon, and a correspondingly small fixation of nitrogen, 

 sufficient only, as we have seen, to repair the casual losses by drainage 

 and weeding. This plot gives us a clue to the source of the vast 

 accumulations of nitrogen in the old prairie soils. Vegetation alone, 

 however long continued, cannot increase the stock of nitrogen in 

 the soil ; there is only a circulation of the initial stock removed 

 by the plants and then put back when the plant dies in situ. But 

 if the conditions are also favourable to the development of Azoto- 



