

FIXATION OF FREE NITROGEN. 161 



Of what Importance to Agriculture is the newly -recognised 

 source of Nitrogen to Leguminous Crops ? 



The question yet remains, What is the practical importance Thepracti- 

 of the newly -recognised source of nitrogen to the Leguminosse, c ^Tof°tlie 

 considered in its bearing on the known facts of agricultural neio doc- 

 production, and especially on the question of the sources of trine - 

 the nitrogen, not only of leguminous crops themselves, but of 

 crops generally? Unfortunately, as in the matter of the 

 explanation of the action by which the nitrogen is fixed, 

 there is much yet to learn before an adequate answer can be 

 given. Still it is desirable to report progress. 



It has been stated that the characteristic nodules have been 

 found on the roots of various leguminous plants growing 

 among the mixed herbage of grass-land, and also on those of 

 others growing on arable land, in the ordinary course of 

 agriculture. There can be little doubt that when such 

 plants are growing in soil and subsoil containing an abun- 

 dance of combined nitrogen, they will obtain some of their 

 nitrogen from nitrates, or other ready-formed compounds of 

 nitrogen. An apparent difficulty in the way of the assump- 

 tion that much of the greater assimilation of nitrogen by the 

 leguminosse than by other plants is due to a supply of nitric 

 acid by the nitrification of the combined nitrogen of the 

 subsoil 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 condition ; whereas in the case of nitric acid 

 being formed as a result of action on the organic nitrogen of 

 the subsoil, it is probable that it will be associated with other 

 constituents liberated, and so rendered available, at the same 

 time. 



But, so far as the plants do obtain nitrogen derived from 

 the fixation of free nitrogen, the question arises, Under what 

 conditions will this supply come the more or less into play ? 



In the later series of experiments made at Eothamsted, Theforma- 

 those conducted in pits in the open air, to which brief ^ M °£j 00 *' 

 reference has been made, the general, though not the in- andfixa- 

 variable, result was, that there was a much greater number ^ °^ ree 

 of nodules formed on the roots of the plants growing in rich 

 soil than on those grown in sand. But whilst as a rule the 

 individual, but much fewer, nodules on the roots grown in 

 sand, developed to a much greater size, the much larger 

 number in the soil were very much smaller. 



As to the smaller number of nodules formed in sand than 

 VOL. VII. L 



