Familiar Letters on Chemistry
by
Justus Liebig
Part 3 out of 3
there is scarcely any ashes of plants altogether devoid of them, and
those parts of plants which experience has taught us are the most
nutritious, contain the largest proportion. To these belong all
seeds and grain, especially the varieties of bread-corn, peas,
beans, and lentils.
It is a most curious fact that if we incinerate grain or its flour,
peas, beans, and lentils, we obtain ashes, which are distinguished
from the ashes of all other parts of vegetables by the absence of
alkaline carbonates. The ashes of these seeds when recently
prepared, do not effervesce with acids; their soluble ingredients
consist solely of alkaline phosphates, the insoluble parts of
phosphate of lime, phosphate of magnesia, and oxide of iron:
consequently, of the very same salts which are contained in blood,
and which are absolutely indispensable to its formation. We are thus
brought to the further indisputable conclusion that no seed suitable
to become food for man and animals can be formed in any plant
without the presence and co-operation of the phosphates. A field in
which phosphate of lime, or the alkaline phosphates, form no part of
the soil, is totally incapable of producing grain, peas, or beans.
An enormous quantity of these substances indispensable to the
nourishment of plants, is annually withdrawn from the soil and
carried into great towns, in the shape of flour, cattle, et cetera.
It is certain that this incessant removal of the phosphates must
tend to exhaust the land and diminish its capability of producing
grain. The fields of Great Britain are in a state of progressive
exhaustion from this cause, as is proved by the rapid extension of
the cultivation of turnips and mangel wurzel--plants which contain
the least amount of the phosphates, and therefore require the
smallest quantity for their development. These roots contain 80 to
92 per cent. of water. Their great bulk makes the amount of produce
fallacious, as respects their adaptation to the food of animals,
inasmuch as their contents of the ingredients of the blood, i.e. of
substances which can be transformed into flesh, stands in a direct
ratio to their amount of phosphates, without which neither blood nor
flesh can be formed.
Our fields will become more and more deficient in these essential
ingredients of food, in all localities where custom and habits do
not admit the collection of the fluid and solid excrements of man,
and their application to the purposes of agriculture. In a former
letter I showed you how great a waste of phosphates is unavoidable
in England, and referred to the well-known fact that the importation
of bones restored in a most admirable manner the fertility of the
fields exhausted from this cause. In the year 1827 the importation
of bones for manure amounted to 40,000 tons, and Huskisson estimated
their value to be from L 100,000 to L 200,000 sterling. The
importation is still greater at present, but it is far from being
sufficient to supply the waste.
Another proof of the efficacy of the phosphates in restoring
fertility to exhausted land is afforded by the use of the guano--a
manure which, although of recent introduction into England, has
found such general and extensive application.
We believe that the importation of one hundred-weight of guano is
equivalent to the importation of eight hundred-weight of wheat--the
hundred-weight of guano assumes in a time which can be accurately
estimated the form of a quantity of food corresponding to eight
hundred-weight of wheat. The same estimate is applicable in the
valuation of bones.
If it were possible to restore to the soil of England and Scotland
the phosphates which during the last fifty years have been carried
to the sea by the Thames and the Clyde, it would be equivalent to
manuring with millions of hundred-weights of bones, and the produce
of the land would increase one-third, or perhaps double itself, in
five to ten years.
We cannot doubt that the same result would follow if the price of
the guano admitted the application of a quantity to the surface of
the fields, containing as much of the phosphates as have been
withdrawn from them in the same period.
If a rich and cheap source of phosphate of lime and the alkaline
phosphates were open to England, there can be no question that the
importation of foreign corn might be altogether dispensed with after
a short time. For these materials England is at present dependent
upon foreign countries, and the high price of guano and of bones
prevents 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
expense of Great Britain for the importation of bones and guano is
equivalent to a duty on corn: with this difference only, that the
amount is paid to foreigners in money.
To restore the disturbed equilibrium of constitution of the
soil,--to fertilise her fields,--England requires an enormous supply
of animal excrements, and it must, therefore, excite considerable
interest to learn, that she possesses beneath her soil beds of
fossil guano, strata of animal excrements, in a state which will
probably allow of their being employed as a manure at a very small
expense. The coprolithes discovered by Dr. Buckland, (a discovery of
the highest interest to Geology,) are these excrements; and it seems
extremely probable that in these strata England possesses the means
of supplying the place of recent bones, and therefore the principal
conditions of improving agriculture--of restoring and exalting the
fertility of her fields.
In the autumn of 1842, Dr. Buckland pointed out to me a bed of
coprolithes in the neighbourhood of Clifton, from half to one foot
thick, inclosed in a limestone formation, extending as a brown
stripe in the rocks, for miles along the banks of the Severn. The
limestone marl of Lyme Regis consists, for the most part, of
one-fourth part of fossil excrements and bones. The same are
abundant in the lias of Bath, Eastern and Broadway Hill, near
Evesham. Dr. Buckland mentions beds, several miles in extent, the
substance of which consists, in many places, of a fourth part of
coprolithes.
Pieces of the limestone rock in Clifton, near Bristol, which is rich
in coprolithes and organic remains, fragments of bones, teeth, &c.,
were subjected to analysis, and were found to contain above 18 per
cent. of phosphate of lime. If this limestone is burned and brought
in that state to the fields, it must be a perfect substitute for
bones, the efficacy of which as a manure does not depend, as has
been generally, but erroneously supposed, upon the nitrogenised
matter which they contain, but on their phosphate of lime.
The osseous breccia found in many parts of England deserves especial
attention, as it is highly probable that in a short time it will
become an important article of commerce.
What a curious and interesting subject for contemplation! In the
remains of an extinct animal world, England is to find the means of
increasing her wealth in agricultural produce, as she has already
found the great support of her manufacturing industry in fossil
fuel,--the preserved matter of primeval forests,--the remains of a
vegetable world. May this expectation be realised! and may her
excellent population be thus redeemed from poverty and misery!
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