ON THE GROWTH OF LEGUMINOUS CROPS. 47 



laid down in sainfoin and mixed grasses ; and M. Deherain points out 

 the practical significance of such facts. 



M. Deherain concluded that the loss of nitrogen by arable soil, that 

 is by soil that is mechanically worked, is due to the slow combustion of 

 the nitrogenous organic matter of the soil ; the nitrogen being either 

 evolved as free nitrogen, or oxidated into nitric acid and carried down 

 into the sub-soil, or into the drains. As to the gain by the surface-soil, 

 he considers, as is doubtless the case, that part is due to the action of 

 deep-rooted plants, in taking up the nitric acid accumulated in the 

 lower layers, and leaving a nitrogenous residue near the surface ; and 

 that as to the gains not so to be accounted for, it is not yet settled 

 whether they are due to the ammonia of the atmosphere, as supposed 

 by M. Schlcesing ; or to free nitrogen, as supposed by M. Berthelot. 



It may be remarked, that, if the losses in ordinary agriculture were 

 in amount anything like those which M. Deherain's figures show, even 

 such large gains as are also indicated, would be far from sufficient to 

 compensate them. It would indeed be necessary to seek for other 

 sources of restoration, if our arable surface-soils are not to lose their 

 nitrogen much faster than the evidence at command leads us to suppose 

 is the case in actual practice. That they do, however, slowly suffer 

 reduction in their stock of nitrogen, when there is no restoration from 

 without, there can be no doubt. In other words, in actual practice 

 without restoration from external sources, the losses are not fully 

 compensated. 



In conclusion in regard to M. Deherain's experiments, I may add that 

 he has quite recently reiterated his results and conclusions ; but he 

 does not say anything that appears to us to obviate our objections to 

 his quantitative estimates. 



M. Joulie made numerous vegetation experiments in which the 

 soils and the plants were, with certain precautions, exposed to the free 

 air, and in which known amounts of combined nitrogen were supplied. 

 He found very variable, but in some cases very large, gains of nitrogen. 

 He considered that the variations of result were largely due to the 

 varying conditions as to mineral-supply in the different experiments. 



M. Joulie concluded that microbes probably play an important part 

 in the fixation of nitrogen. He did not think that his results were 

 favourable to the supposition that the plants themselves effected the 

 fixation. For the present he limits himself to the establishment of the 

 great fact of the fixation of the free nitrogen of the atmosphere, 

 leaving to the future the exact explanation. 



It is to be observed that the large gains shown were chiefly with a 

 polygonous plant, Buckwheat, and not with plants of the Leguminous 

 family, which are reputed to be " nitrogen collectors." 



To show the practical importance of the fixation of free nitrogen, 

 M. Joulie calculates what, would be the gain per hectare according to 

 some of his results. It may be confidently affirmed, however, that 

 such gains as he so estimates, do not take place, either with or without 

 vegetation, in ordinary soils, in ordinary practice. 



