sed very considerably of late years, the same cannot be said
of nitrogen. According to Mr Hermann Voss, some 34,000[95] tons of
nitrogen were used in the form of artificial manures in 1873, while now
only about 28,000 tons are used--_i.e._, some 6000 tons less.

_Oil-seeds and Oilcakes._

There still remains a very important source of nitrogen which has not
yet been mentioned, in the shape of oil-seeds and oilcakes, used for
feeding purposes. Oilcakes are both manufactured in this country and
imported in large quantities. Recent Agricultural Returns show the total
imports of oilcakes at 256,296 tons; that of linseed at 370,000 tons;
that of rape-seed at 80,000 tons; and that of cotton-seed at 289,413
tons.

_Other imported Sources of Nitrogen._

We have further, in considering this question, to take into account the
large amount of maize, peas, beans, wheat, and oats which are imported
into this country, a certain quantity of which is used as cattle-food,
and will therefore go to enrich their manure. Also the imported straw
used for purposes of litter must not be forgotten. In 1887 this amounted
to 52,393 tons.

_Conclusion._

In conclusion, it may be asked how far are the artificial sources of
nitrogen able to make good the loss? In the opinion of such a reliable
authority as Sir John Lawes, they do not. There are some soils which
depend almost entirely upon imported fertility, and could not be
cultivated without it. Upon some of them it is possible that the imports
of nitrogen are in excess of the exports. Taking the agricultural
acreage as a whole, however, he is of opinion that there is a decided
loss of nitrogen, which he estimates at _from 15 lb. to 20 lb. per acre
per annum_.[96]

FOOTNOTES:

[63] The total amount of nitrogen in the air has been estimated
approximately at four million billion tons.

[64] See Introductory Chapter, pp. 40 to 45.

[65] Although ammonia is more abundant than nitrates and nitrites, it
only amounts to a few parts per million of air. According to Muentz, the
air at great heights contains more ammonia than in its lower strata. The
opposite, however, is the case with regard to nitrates, which are only
found in air near the surface of the earth. See p. 49.

[66] Nitric acid may also be formed by the oxidation of ammonia by
ozone, or peroxide of hydrogen.

[67] According to Schloesing, the chief source of the ammonia present in
the air is the tropical ocean, which yi