LEGUMINOUS CROPS. 135 



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 Another 

 whole, there must have been a great deal formed by the s °jj£™£j[ 

 nitrification of the nitrogen of the subsoil. A difficulty in 

 the way of the assumption that nitric acid is the exclusive, 

 or even the main source of the nitrogen of the Leguminosse 

 is, that the direct application of nitrates as manure has com- 

 paratively 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, un- 

 accompanied by the other necessary mineral constituents in 

 an available form ; whereas in the case of nitric acid being 

 formed by direct action on the subsoil, it is probable that it 

 will be associated with other constituents, liberated, and so 

 rendered available, at the same time. 



Numerous direct experiments have been made at Eoth- Nitrifica- 

 tion i\ — 

 clays. 



amsted to determine whether the nitrogen existing in a tl 



comparatively insoluble condition in raw clay subsoil was 

 susceptible of nitrification; and the methods and results 

 have been described in various papers. It was established 

 that the nitrogenous matters of raw clay subsoils, which con- 

 stitute an enormous store of already combined nitrogen, are 

 susceptible of nitrification if the organisms, with the other 

 necessary conditions, including a sufficient supply of oxygen, 

 are present. It was further indicated, not only that the 

 action was more marked under the influence of leguminous 

 than of gramineous growth and crop-residue, but that the 

 organisms become distributed to a considerable depth, even 

 in raw clay subsoils, especially where deep-rooted and free- 

 growing Leguminosa3 have developed. 



But the data at command do not justify the conclusion 

 that. the essential conditions would be adequately available 

 in such cases as those of the very large accumulations of 

 nitrogen by the red clover grown after the beans, and of the 

 increasing and very large accumulations by the Medicago 

 sativa for a number of years in succession. 



The alternatives are — either that the plant may take up Nitrogen 

 nitrogen from the subsoil in some other way, as ammonia or {^J^ 

 as organic nitrogen ; or that the free nitrogen of the atmos- the air. 

 phere is in some way brought under contribution. 



In reference to the first of these alternatives, the question The power 

 suggested itself, whether roots, by virtue of their acid sap, f r ™nti° . 

 may not either directly take up, or at any rate attack and gen from 

 liberate for further change, the otherwise insoluble organic subsoil. 

 nitrogen of the subsoil? 



