468 
Agricultural Chemistry. 
of foreign corn might be altogether dispersed with after a short time. For 
these materials England is at present dependent upon foreign countries, and 
the higli price of guano and of bones ]irevents their general application, and in 
sufficient quantity. Every year the trade in these substances must decrease, 
or their price will rise as the demand for them increases. 
" According to these premises, it cannot be disputed, that the annual ex- 
pense of Great Britain for the importation of bones and guano is equivalent to 
a duty on corn : with this ditierence only, that the amount is paid to foreigners 
in money." — Letters, 3rd edition, p. 524. 
Now, that the efficacy of guano for the production of corn, is 
measurable by the amount of nitrogen, and not by that of the 
phosphates which it contains, is obvious from the simple facts, — 
that guano rich in nitrogen and relatively poor in phosphates, 
will command twice the price of one rich in phospliates and poor 
in nitrogen, — and that all experience shows, that the dearer 
guano, rich in nitrogen and relatively poor in phosphates, yields 
a far greater increase of grain, than that which is cheaper and 
contains more of phosphates and less of nitrogen. But, with 
regard to Baron Liebig's supposition, that if a rich and cheap 
source of phosphates were open to England, we might soon be 
independent of foreign corn, — since we now have such a source, 
this opinion is worth a little examination. It is probable, that 
from 20 to 25 per cent, of the corn consumed in this coun- 
try, is imported. Unless, therefore, a very much larger breadth 
of land were brought under corn, it is obvious that, to attain this 
liappy result, the whole of our wheat-fields must yield from one- 
fourth to one-third more produce than at present. W hether or 
not it is likely that they would do so by the use of phosphates, 
even if mixed with alkalies, piovided that available nitrogen 
were not at the same time added, the results not only of our own 
experiments, but the experience of every farmer who has ever 
used an artificial manure, will at once decide. 
Fourthly — We proceed to illustrate, by condensed summaries 
of an immense mass of experimental results, and a very rapid 
consideration of their indications, some prominent points con- 
nected with the action of manures on the different crops of 
rotation, and with the chemical circumstances involved in fallow 
and a rotation of crops. The first of these points, namely, 
the characteristic action of certain constituents of manure, upon 
the most important crops grown in our rotations, is illustrated 
in Table IV. which follows ; and, for the purpose in question, 
to/teat and barley have been selected as cereals ; turnips as a 
root-crop ; and beans and clover as leguminous plants. 
