104 
SIR J. B. LA WES AND PROFESSOR J. H. GILBERT ON THE 
are less rich than the prairie lands, for the most part found in the valleys or on the 
plains. Taking the vast areas of fertile natural prairie on the American continent, for 
example, sometimes of several feet in depth, it may be estimated that, m such cases, 
each foot of depth will contain from 6000 to 10,000 lbs., or even more, of combined 
nitrogen per acre (= 6720 to 11,200 kilog. per hectare); and the probable time of 
these accumulations entirely obviates the necessity of calling in the aid of the free 
nitrogen of the atmosphere, brought into combination either under the influence of 
the plants themselves or of micro-organisms within the soil. 
Further, the history of agriculture so far as it is known, indicates that soils under 
cultivation without supplies by manure from external sources, do, as a matter of 
fact, gradually become less fertile. This, as a rule, will take place more rapidly 
in undulating or high forest lands, than in the natural grass or prairie lands of 
the plains. 
Again, if we compare the amount of nitrogen in the surface soil of permanent grass 
land, with that of adjoining land of the same original character, but which has for 
some tune been under arable culture, we find that the latter is much poorer in 
nitrogen. In illustration, it may 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 
correspouding arable land only contains from OT to 0T5 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, under the influence 
either of electricity, or of organisms within the soil. Whether or not there is any 
restoration of the kind supposed, we believe that a consideration of the origin of soils 
generally, and of the history of agriculture in different countries, will lead 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 indeed now been continued long enough to 
afford some pertinent examples bearing upon this subject. 
Thus, in the case of the fields under continuous wheat, continuous barley, alternate 
wheat and fallow, and continuous root-crops, the average annual yield of nitrogen in 
the crops with mineral, but without nitrogenous manure, has only been about or 
under 20 lbs. per acre (= 22'4 kilog. per hectare); 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 soiJ. 
