ON THE GROWTH OF LEGUMINOUS CROPS. 37 



the shallow-rooting and comparatively low nitrogen yielding Trifolium 

 repens. Indeed, the nitric acid is nearly exhausted in the deep-rooting 

 Medicago saliva plot ; there remaining, to the total depth of 9 feet, 

 only about 17 Ibs. of nitric-nitrogen, against more than 100 Ibs. to the 

 same depth in the Trifolium repens soil. The total deficiency of nitric- 

 nitrogen in the Medicago as compared with the Trifolium repens soil, is 

 seen to be 85'69 Ibs. according to one set of determinations, and 83*94 

 Ibs. according to the other. 



As already said, we cannot know what was the stock of nitric- 

 nitrogen in the soil at the commencement of the growth of the season, 

 or the amount formed during the growing period. But, with so much 

 more Medicago growth for several previous years, it seems reasonable 

 to assume that there would be much more nitrogenous crop-residue for 

 nitrification than in the case of the Trifolium repens plot. 



But even supposing, for the sake of illustration, that each year's 

 growth would leave crop-residue yielding an amount of nitrogen as 

 nitric acid for the next crop, or succeeding crops, approximately equal 

 to the amount which had been removed in the crop, the increasing 

 amounts of nitrogen yielded in the crops from year to year could not 

 be so accounted for ; and there would remain the amount of nitrogen 

 in the crop-residue itself, still to be provided in addition. In fact, 

 assuming the proportion of nitrogen in the crop-residue to that in the 

 removed crop to be as supposed in the above illustration, nearly 700 

 Ibs. of nitrogen would have been required for the Medicago crop and 

 crop-residue of 1884 ; or, if we assume the nitrogen in the residue to 

 be only half that in the crop, about 500 Ibs. would have been required. 

 Doubtless, however, some of the nitrogenous crop-residue would 

 accumulate from year to year. 



The results can leave no doubt that the Trifolium repens, and the 

 Medicago sativa, have each taken up much nitrogen from nitric acid 

 within the soil, and that, in fact, nitric acid is an important source of 

 the nitrogen of the Leguminosse. Indeed, existing direct experimental 

 evidence relating to nitric acid, carries us quantitatively further than 

 any other line of explanation. But, it is obviously quite inadequate to 

 account for the facts of growth, either in the case of the Medicago sativa 

 after the Clover, or in that of the Clover after the Beans. 



It is obvious that, if nitric acid were the source of the whole, 

 there must have been a great deal formed by the nitrification of the 

 nitrogen of the sub-soil. A difficulty in the way of the assumption 

 that nitric acid is the exclusive, or even the main source of the nitrogen 

 of the Leguininosse is, that the direct application of nitrates as manure, 

 has comparatively little effect on the growth of such plants. In the 

 case of the direct application of nitrates, however, the nitric acid will 

 percolate chiefly as sodium or calcium nitrate, unaccompanied by the 

 other necessary mineral constituents in an available form ; whereas, in 

 the case of nitric acid being formed by direct action on the sub-soil, 

 it is probable that it will be associated with other constituents, 

 liberated, and so rendered available, at the same time. 



