658 The Sources of the Nitrogen of our Leguminous Orops. 
Avhen the subject of tlie sources of tlie niti'ogen of the plants 
of the different sub-divisions is in question (see p. 687). 
There can be no doubt that both the scientific interest and 
the practical value of our leguminous crops depend veiy much 
on the amount of nitrogen which they contain, and on the 
sources of their nitrogen ; and especially on the great differences 
in these respects between them and the representatives of the 
other families of plants with which they are grown, either in 
alternation in our rotations, or in association in our meadows 
and pastures. 
It is well known that, under the conditions in which crops 
are grown in ordinary agriculture, nitrogenous manures have 
very direct effects in increasing the produce of wheat, of barley, 
and of oats, of turnips, of mangel, and of potatoes. This is the 
case, notwithstanding that in the cereals the increased produce 
consists characteristically of the non-nitrogenous substances 
starch and cellulose ; in the root-crops of the non-nitrogenous 
substance sugar; and in potatoes of the non-nitrogenous[substance 
starch. Leguminous crops, on the other hand, not only as a 
rule accumulate much more nitrogen over a given area of land 
under equal soil conditions, and contain a higher percentage of 
nitrogen in their dry substance, than the crops above enumerated, 
but there is abundant evidence that they also derive much 
nitrogen from the combined nitrogen of the soil and subsoil, and 
further that they probably take up much as nitric acid ; yet it 
is generally recognised in practical agriculture that direct nitro- 
genous manures have comparatively little effect in increasing the 
produce of such crops. 
The influence of nitrogenous manures in increasing the pro- 
duction of the non-nitrogenous constituents of our crops is very 
strikingly illustrated by the results given in Table I. (p. 659). 
The calculations are based on the average produce obtained 
in the experiments at Rothamsted by the different manures — of 
wheat oyer twenty years, of barley over twenty years, X)f sugar- 
beet over three years, of mangel-wurzel over eight years, of 
potatoes over ten years, and of beans over eight years. 
As will be seen, the Table shows — the estimated amounts 
of carbon, per acre per annum, in the total produce (grain and 
straw) of wheat, and of barley, in the roots of sugar-beet and of 
mangel-wurzel, in the tubers of potatoes, and in the total produce 
(corn and straw) of beans, when each is grown by a complex 
mineral manure without nitrogen, and also when grown with the 
same mineral manure with nitrogenous manures in addition. 
Next is shown the estimated gain of carbon, that is, the increased 
amount of it accumulated in the crop under the influence of the 
