192 



THE EOTHAMSTED EXPERIMENTS. 



Grass- 

 land, rich, 

 arable 

 land, poor 

 in nitro- 

 gen and 

 carbon. 



Accumu- 

 lated fer 

 tility. 



Reduction 

 of yield of 

 wheat from 

 prairie 

 land. 



ber of Eussian soils ranged in percentage from 0.130 to 0.607. 

 It is further seen that the percentages of carbon, and the 

 amount of carbon to 1 of nitrogen, are higher in the grass- 

 land than in the arable soils, and higher still in the rich 

 prairie soils. 



From these various results there can be no doubt that a 

 characteristic of a permanent grass surface-soil, or of a rich 

 virgin-soil, is a relatively high percentage of nitrogen and of 

 carbon, and a high relation of carbon to nitrogen. On the 

 other hand, a soil that has been long under arable culture is 

 much poorer in these respects ; whilst arable soils, under con- 

 ductions of known agricultural exhaustion, show a very low 

 percentage of nitrogen and of carbon, and a low relation of 

 carbon to nitrogen. 



It has sometimes been maintained that a soil is a laboratory 

 and not a mine. But not only the facts ascertained in our 

 own and in other investigations, but the history of agriculture 

 throughout the world, so far as it is known, clearly show 

 that a fertile soil is one which has accumulated within it 

 the residue of long periods of previous vegetation ; and that 

 it becomes infertile as this residue is exhausted. Such ac- 

 cumulations are truly enormous in many of the prairie lands 

 of the American continent ; sometimes, indeed, extending to a 

 considerable depth. But, even after the comparatively few 

 years which most of them have been under cultivation, it is 

 alleged by some that they are already showing exhaustion. 



In view of the facts both as to the percentage of nitrogen, 

 and the annual yield of wheat without manure over 40 or 50 

 years in the Eothamsted experimental field, it is indeed very 

 difficult to believe that the rich prairie lands of the American 

 continent, which yield so large a proportion of the wheat ex- 

 ported from the United States and Canada, can in so much 

 less a time have become exhausted of available nitrogen. 

 Thus it is probable that at the commencement the surface- 

 soil of none of these lands contained less than twice, and few 

 of them less than three times, as high a percentage of nitrogen 

 as the Eothamsted wheat-field soil; whilst frequently the 

 subsoils would, to a considerable depth, be richer than the 

 Eothamsted surface-soil. Yet it is estimated that over a 

 period of 40 years, from 1852 to 1891 inclusive, the produce of 

 the Eothamsted soil without manure has only reduced by an 

 average of about ^ bushel per acre per annum due to exhaus- 

 tion, irrespectively of fluctuations due to season ; and when 

 we consider how much shorter a time most of the rich prairie 

 lands have been growing wheat without manure, it seems that 

 some other reason than exhaustion must be found for their 

 alleged reduction in yield. 



