A further source of nitrogen is bones, which, of course, are chiefly valuable as a phosphatic manure, but which contain also some 3 to 4 per cent of nitrogen. Of this valuable manure we import at present about 30,000 tons, while about 60,000 tons are collected in this country, bringing up our total consumption to 100,000 tons.
The above mentioned are the most important of nitrogenous manures; there are, however, a number of other nitrogenous manures used in this country in very much smaller quantities. As most of these substances are made in this country, it is very difficult to estimate the amount of their annual production with exactness. These substances are as follows: fish-guano, meat-meal guano, dried blood, shoddy, scutch, horns and hoofs, hair, bristles, feathers, leather-scrap, &c. Of fish-guano, the total consumption per annum may be put down at about 8000 tons, of which a fourth is imported into this country, the remaining 6000 tons being manufactured at home. Of meat-meal guano, dried blood, hoof-guano, &c., about 2500 tons are annually imported, the home production bringing up the total amount to some 10,000 tons. Of shoddy, some 12,000 tons are manufactured in this country; while scutch—the name given to a manure manufactured from the waste products incidental to the manufacture of glue and the dressing of skins—is produced only to the extent of a few thousand tons annually.
It is a fact worthy of notice, that while the use of phosphatic manures has increased 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]