26 



leguminous than where gramineous crops had grown. The results pointed to the 

 conclusion that, under the influence of leguminous growth and crop residue, the con- 

 ditions were more favorable for the development of the nitrifying organisms, and, es- 

 pecially in the case of deep-rooting plants, of their distribution, thus favoring the 

 nitrification of the nitrogen of the subsoil, which so becomes a source of the nitrogen 

 of such crops. 



(11) An alternative was that the plants might take up at any rate part of their 

 nitrogen from the soil and subsoil as organic nitrogen. Direct experimental evidence 

 leads to the conclusion that fungi take up both organic nitrogen and organic carbon, 

 but there is at present no direct experimental evidence in favor of the view that 

 green-leaved plants take up either nitrogen or carbon in that form from the soil ; whilst 

 there are physiological considerations which seem to militate against such a view. 



(12) In the case of plots where Tri folium repens (white clover) and Vicia saliva 

 (tares or vetches) had been sown, each for several years in succession, on soil to which 

 no nitrogenous manure had been applied for about thirty years, and the surface soil 

 had become very poor in nitrogen, both the soil and subsoil contained much less ni- 

 trogen as nitric acid where good crops of Vicia saliva had grown, than where the 

 more shallow-rooted Trifolium repens had failed to grow ; and the deficiency of nitric 

 nitrogen in the soils and subsoils of the Vicia sativa plots, compared with the amount 

 in those of the Trifolium repens plot, was, to the depth examined, sufficient to account 

 for a large proportion of the nitrogen of the Vicia crops. 



(13) It may be considered established, that much, if not the whole, of the nitrogen 

 of crops is derived from nitrogen within the soil, accumulated or supplied ; and that 

 much, and in some cases the whole, of the nitrogen so derived, is taken up as nitrates. 



(14) An examination of a number of United States and Canadian prairie soils showed 

 them to be very much richer in both nitrogen and carbon, to a considerable depth, 

 than the surface soils of old arable lands in Great Britain, and about as rich, to a much 

 greater depth, as the surface soil of permanent pasture land. 



(15) On exposure of portions of some of these rich prairie soils, under suitable con- 

 ditions of temperature and moisture, for specified periods, it was found that their 

 nitrogen was readily susceptible of nitrification, and so of becoming easily available 

 to vegetation. 



(16) After several extractions, the subsoils almost ceased to give up nitric acid; 

 but on seeding them with a tenth of a gram of rich garden soil containing nitrifying 

 organisms, there was a marked increase in the rate of nitrification. This result af- 

 forded confirmation of the view that the nitrogen of subsoils is subject to nitrifica- 

 tion, if under suitable conditions, and that the growth of deep-rooted plants may 

 favor nitrification in the lower layers. 



(17) Under favorable conditions of season and of cultivation the rich prairie soils 

 yield large crops; but, under the existing conditions of early settlement, they do not, 

 on the average, yield crops at all commensurate with their richness, when compared 

 with the soils of Great Britain which have been under arable culture for centuries. 

 But so long as the laud is cheap, and labor dear, some sacrifice of fertility is unavoid- 

 able in the process of bringing these rich virgin soils under profitable cultivation. 



(18) A comparison of the percentages of nitrogen and carbon in various soils of 

 known history, showed that a characteristic of a rich virgin soil, or of a permanent 

 pasture surface soil, was a relatively high percentage of nitrogen and carbon. On the 

 other hand, soils which have long been under arable culture are much poorer in these 

 respects; whilst arable soils under conditions of known agricultural exhaustion, 

 show a very low percentage of nitrogen and carbon, and a low proportion of carbon 

 to nitrogen. 



(19) Not only the facts adduced in this and former papers, but the history of agri- 

 culture throughout the world, so far as it is known, clearly shows that, pre-eminently 

 so far as the nitrogen is concerned, a fertile soil is one which has accumulated within 

 it the residue of ages of natural vegetation, and that it becomes infertile as this resi- 

 due is exhausted. 



