58 RESULTS OF EXPERIMENTS AT ROTHAMSTED, 



be stated, that whilst the surface-soil of the grass land at Rothamsted 

 contains from 0*25 to 0-30 per cent, of nitrogen, that of the corresponding 

 arable land contains only from 0*1 to 0-15 per cent. The arable soil 

 has, in fact, originally been covered with natural vegetation of some 

 kind, with comparatively little removal, and consequent accumulation ; 

 whilst, under arable culture, much of the accumulated nitrogen has 

 been used up, and the loss has not been compensated by free nitrogen 

 brought into combination within the soil, either under the influence of 

 electricity or of lower organisms. Indeed, whether or not there is any 

 restoration of the kind supposed, a consideration of the origin of soils 

 generally, and of the history of agriculture in different countries, leads 

 to the conclusion that the losses of combined nitrogen by cropping, and 

 in other ways, are not compensated by corresponding amounts of free 

 nitrogen constantly brought into combination. 



The Rothamsted Field Experiments have now been continued long 

 enough to afford some pertinent examples bearing upon this subject. 



Thus, in the case of the plots under continuous wheat, continuous 

 barley, alternate wheat and fallow, and continuous root-crops, with 

 mineral, but without nitrogenous manure, the average annual yield of 

 nitrogen in the crops, has only been about or under 201bs per acre ; the 

 amount has declined to less than the average in the later years ; and, 

 coincidently with the continuous and diminishing growth, the percentage 

 of nitrogen in the surface-soil has been considerably reduced. The loss 

 by the removal of even such small crops, together with that by 

 drainage, has, therefore, as a matter of fact, not been compensated by 

 free nitrogen brought into combination, either by the plants, or within 

 the soil. 



In the field where the Leguminous crop Beans, had been grown 

 25 years out of 32, with mineral but without nitrogenous manure, and 

 had yielded less than average agricultural crops, the per-centage of 

 nitrogen in the surface-soil was also greatly reduced. 



In another field, where the Leguminous crop Red Clover, had been 

 sown 12 times in 30 years, the Clover failed many times, the yield of 

 nitrogen in the crops very greatly diminished, and the per-centage of 

 nitrogen in the surface soil was greatly reduced. 



Again, in rich garden-soil, where Red Clover has been grown for 36 

 years consecutively, and has yielded throughout good, but gradually 

 diminishing crops, it was found, after the first 22 years, that the 

 nitrogen in the surface-soil had been reduced from 0*5095 to 0*3634 

 per cent., or by about one-third. 



Even in an actual course of rotation, of Turnips, Barley, Clover or 

 Beans, and Wheat, with mineral, but without nitrogenous manure, the 

 per-centage of nitrogen in the surface-soil has been much reduced j 

 whilst, in a parallel rotation in which Fallow takes the place of the 

 Clover or Beans, the reduction is still greater. 



Thus, in all the cases cited, including Gramineous, Cruciferous, Cheno- 

 podiaceous, and even Leguminous crops, and a rotation of crops, grown 

 for many years in succession without nitrogenous manure, and yielding 



