134 THE KOTHAMSTED EXPEKIMENTS. 



and at most very much less, nitrogen as nitric acid remaining 

 in the soil than where so much less had been removed in the 

 Trifolium repens crops. The difference is distinct even in 

 the upper layers, but it is very striking in the lower depths. 

 Thus there is, on the average, not one-twelfth as much nitric- 

 nitrogen in the lower ten depths of the soil of the deep-root- 

 ing and high nitrogen-yielding Medicago saliva, as in those of 

 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 lb. of nitric-nitrogen 

 against more than 100 lb. 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 lb. according to one set of determinations, and 

 83.94 lb. 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 nitrifica- 

 tion than in the case of the Trifolium repens plot. 

 Increasing But, even supposing for the sake of illustration, that each 

 amounts of year's growth would leave crop-residue yielding an amount of 

 le account*- nitrogen as nitric acid for the next crop, or succeeding crops, 

 ed for. 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, as- 

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

 in the removed crop to be as supposed in the above illustra- 

 tion, nearly 700 lb. of nitrogen would have been required for 

 the Medicago crop and crop-residue of 1884. Or, if we as- 

 sume the nitrogen in the residue to be only half that in the 

 crop, about 500 lb. would have been required. Doubtless, 

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

 late from year to year. 

 Nitric acid The results can leave no doubt that the Trifolium repens, 

 an import- an( j fa e Medicago saliva, have each taken up much nitrogen 

 of nitrogen from nitric acid within the soil, and that, in fact, nitric acid 

 for legu- j s an important source of the nitrogen of the Leguminosae. 

 cr0 p S , 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 



