The Sources of the Nitrogen of mtr Leguminous Crops. G97 
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 abundance of combined nitrogen, 
they will obtain some of their nitrogen from nitrates, or other 
ready-formed compounds of nitrogen. It has further been sug- 
gested, that lower organisms may serve the higher, at any rate 
in part, by bringing the combined nitrogen existing in the soil 
and subsoil in a comparatively inert state, into a more readily 
available condition. An obvious difficulty in the way of the 
assumption, that much of the greater assimilation of nitrogen by 
the Leijuminoso} than by other plants is due to a supply of nitric 
:tcid by the nitrification of the combined nitrogen of the subsoil, 
ir< that the direct application of nitrates as manure has compara- 
tively 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 nitrate of soda or nitrate of lime, unaccom- 
panied by the other necessary mineral constituents in an avail- 
able condition ; whereas, in the case of nitric acid being formed 
by direct action on the organic nitrogen of the subsoil, it is 
probable that it will be associated with other constituents, liber- 
ated, and so rendered available, at the same time. But, so far 
j as the plants obtain nitrogen derived from the fixation of free 
; nitrogen, the question arises — under what conditions will this 
supply come the more or the less into play ? 
j In some of the experiments made in pots at Rothamsted. 
I the results of which have been given, there was a less develop- 
ment of nodules on the roots when soil containing an abundance 
of combined nitrogen was used, than when nitrogen-free sand 
was employed. But the less growth, and the less formation of 
nodules in the rich soil, was supposed to be due to clotting, and 
therefore to defective porosity, especially after watering. On 
the other hand, as already said, some experimenters have con- 
cluded that the activity of the process depends on the quantity 
of nitrogenous compounds at the disposal of the roots ; the 
nodules developing unhindered, and becoming large and typical 
in soils rich in nitrogen, whilst in soils poor in nitrogen they 
attain no great size. 
In the later series of experiments made at Eothamsted, 
those conducted in pits in the open air, to which brief reference 
has been made, the general, though not the invariable, result 
was, however, that there was a much greater number 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 
