20 The History of a Field newUj laid down to Permanent Grass. 
supplied nitrogen, either by decomposition within the soil and 
evolution of free nitrogen, or by the drainage of nitric acid, the 
estimated excess in the manure over that in the crops removed 
would not account for the indicated gain of nitrogen by the 
surface-soil, and in the roots. We may also reckon that the 
meadow received nitrogen at the rate of about 5 lbs. per acre 
per annum, by cake or corn given to the animals feeding off the 
second crops. There will also be from 5 to 10 lbs. per acre per 
annum, due to combined nitrogen coming down in rain and the 
minor aqueous deposits from the atmosphere. 
Making full allowance for the various sources that have been 
enumerated, there would still appear to be a greater or less 
balance of the gain not so accounted for. If so, it must have 
its source, either in the subsoil, or the atmosphere, or both. 
There is much experimental evidence pointing to the con- 
clusion that, at any rate some deep-rooted leguminous plants 
derive a considerable quantity of nitrogen from the subsoil ; and 
there seems no reason to doubt that the deep-rooting plants of 
the mixed herbage of grass-land, whether leguminous or other- 
wise, may also avail themselves of subsoil nitrogen ; and, if so, 
it is to be supposed that they, like clover for example, will leave 
nitrogenous crop-residue in the surface-soil, the nitrogen of 
which has been derived fi'om the subsoil. 
It is, indeed, very probable that, at any rate the greater 
part, if not the whole, of the nitrogen gained by the surface- 
soil, and not accounted for by the excess supplied in manure 
over that removed in crops, or by combined nitrogen from the 
atmosphere, is due to nitrogen of the subsoil. But if the 
whole is not to be so accounted for, the question remains — 
whether some may not be derived in some way from the free 
nitrogen of the atmosphere ? 
On this point we think it may safely be concluded, from the 
results of the experiments of Boussingault, and of those made at 
Kothamsted, many years ago, that our agricultural plants do 
not themselves directly assimilate the free nitrogen of the air by 
their leaves. But in recent years the question has assumed 
quite a new aspect. It now is, whether the free nitrogen of the 
atmosphere is brought into combination within the soil, under 
the influence of micro-organisms, or other low forms, and so 
serving indirectly as a source of nitrogen to plants of a higher 
order ? 
Thus, Hellriegel and Wilfarth have found, in experiments 
with various leguminous plants, that if a soil free of nitrogen 
have addjBd to it a small quantity of soil-extract containing the 
organisms, the plants will fix much more nitrogen than was 
