ON THE GROWTH OF LEGUMINOUS CROPS. 57 



nitrogen accumulated in the surface-soil of land laid down to grass may 

 not be due to such an action. On this point it may be remarked, that 

 the " nodules " have been observed on the roots of some Leguminosse 

 growing in the mixed herbage of grass-land. On the other hand, it 

 has to be borne in mind, that the proportion of leguminous plants in 

 such mixed herbage is comparatively small. Further, it has yet to be 

 determined whether the source of the nitrogen of the bacteroids in the 

 nodules is exclusively free nitrogen, when the development proceeds in 

 soil and sub-soil containing a large amount of combined nitrogen. 



As to the supposition that the gains of nitrogen in argillaceous 

 matters of very low initial nitrogen-contents, and which are attributed 

 to the fixation of free nitrogen, serve to explain the gradual formation of 

 vegetable soils, there cannot be any doubt that, so far as nitrogen is 

 concerned, the natural fertility of most soils is at any rate mainly due 

 to the accumulation of ages of natural vegetation, with little or no 

 removal of it, by animals or otherwise ; and if the amounts of nitrogen 

 even now brought into combination over a given area under the 

 influence of electricity in equatorial regions, were not exceeded in the 

 earlier periods of the history of our globe, that would probably be 

 sufficient, with growth and with little or no removal, through the ages 

 which modern science teaches us to reckon upon, to account for most, 

 if not the whole, of the accumulations in natural grass, or forest lands ; 

 and it is these which have to a great extent furnished us with our 

 meadows and pastures, and arable soils. 



Frequently the natural forests have been on the more elevated, or 

 the more undulating lands, and the soils they have formed 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, in such cases, each foot of depth will contain 

 from 6,000 to 10,000 Ibs., or even more, of combined nitrogen per 

 acre ; and the probable time of these accumulations at any rate obviates 

 the necesssity of assuming the intervention of the free nitrogen of the 

 atmosphere, brought into combination by the soil, or by the plants 

 themselves. So far, however, as leguminous growth may have 

 contributed to the result, it is, in view of existing evidence, to be 

 supposed that some at any rate of the accumulation may have been due 

 to fixation under the influence of the symbiotic growth of lower 

 organisms and the higher plants. 



Further, the history of agriculture so far as it is known, indicates 

 that arable soils 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 time been under arable culture, we 

 find that the latter is much poorer in nitrogen. In illustration, it may 



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