198 
Report of Experiments on the 
more easily comprcliend why it should be necessary for a certain 
period of time to intervene before a^jain cultivatin<; certain crops 
on the same land ; for, we could easily understand that this might 
be requisite for the gradual formation and accumulation of a 
sufficient amount of the compounds in question. 
Whatever may be the precise chemical character of the carbon 
compounds of the soil, more complex than carbonic acid, there are 
numerous facts in horticulture, and even in agriculture, leading to 
the supposition that some plants take up a part at least of their 
carbon from some other form of combination than carbonic acid. 
In one of our experimental fields we have grown very large 
crops of wheat for 17 consecutive years, without the supply, by 
manure, of a single ounce of carbon. The crops have been consider- 
ably greater on some plots where no carbon has been supplied in the 
manure, than on others to which it had been very largely supplied. 
There are, indeed, good reasons for supposing that carbonic acid 
is, at any rate, the chief, if not the exclusive source, of the carbon 
of many of the plants yielding food largely to man and other 
animals — which, by their respiration, return so much carbonic 
acid to the atmosphere. Were it not so, as forests make way 
for the growth of food, the proportion of carbonic acid in the 
atmosphere would gradually increase. The cultivation of 
the cereal crops, wliich enter so largely into the food of man 
and other animals, seems admirably adapted for preserving the 
equilibrium in the composition of the atmosphere ; for an acre of 
wheat will decompose as much, or more, carbonic acid, liberating 
a corresponding amount of oxygen, as an acre of the forest which 
it may have supplanted. 
Provided the soil yield a sufficient supply of the necessary 
mineral constituents, the amount of carbonic acid decomposed 
by a cereal crop over a given area, will veiy much depend 
upon the amount of nitrogen, in an available condition of com- 
bination, and distribution, within the soil. But the direct supply 
of nitrogen to the soil in the form of ammonia, which so much 
increases the vigour of growth of Graminaceous crops generally, 
and consequently the amount of carbon which the plants will 
assimilate from carbonic acid, so far from effecting the same result 
in the case of Leguminous crops, is generally injurious to them. 
In the early years of our experiments, both upon Clover and upon 
Beans, the application of the fixed alkalies as manure, and espe- 
cially of potash, caused a considerably increased assimilation ol 
both carbon and nitrogen over a given area ; whilst the direct use of 
ammonia-salts, which are so efficacious in the case of our Grami- 
naceous crops, had either little or no such effect, or was more 
frequently injurious, in the case of these Leguminous crops. Where 
the supply of mineral constituents is sufficiently kept up, the 
supply of ammonia is as efficient as ever in enabling the wheat 
