The loss due to our present system of sewage disposal has been already taken into account in dealing with the loss due to removal of crops. It may be well, however, to treat it from the sewage aspect. Taking the amount of nitrogen in the excreta of every individual as, on an average, half an ounce, the annual amount voided in the excreta of the total population of the British Isles would amount to 365,000,000 lb.[92]--of this, the amount in the London sewage alone being 91,000,000 lb.[93] By the water system, which is almost universally adopted in this country, the above quantity of nitrogen is entirely lost to the soil. A small portion of it, it may be argued, is eventually recovered in sea weed and fish, which may be used for manure. This, however, is to argue too much _sub specie aeternitatis_. Not all the nitrogen originally present in the excreta finds its way into the sea; for it is highly probable that a considerable quantity escapes in the process of the decomposition of the sewage as "free" nitrogen. From the above statement of the sources of loss and gain of nitrogen taking place in the soil, it may be pretty safe to conclude that while in a state of nature the gain balances the loss, if indeed it does not do more, under conditions of arable farming such is very far from being the case; and that if fertility of the land is to be maintained, recourse to nitrogenous manures must be had,--in short, that the application of artificial nitrogenous manures is a necessary condition of modern husbandry. _Our Artificial Nitrogen Supply._ Before concluding this chapter, it may be interesting to enumerate very briefly the chief sources of our artificial nitrogen supply. _Nitrate of Soda and Sulphate of Ammonia._ The most important artificial nitrogenous manures in use at present are nitrate of soda and sulphate of ammonia. Of the former, the annual exportation from Chili is close on one million tons, of which quantity about 120,000 tons is imported into the United Kingdom. Of sulphate of ammonia, on the other hand, the total production in this country is about 130,000 tons per annum,[94] the greater proportion of which is exported, leaving only from 30,000 to 40,000 tons for consumption. Nitrate of soda, it must be remembered, is not entirely used for manurial purposes, a small proportion of the above imports being used for chemical manufacturing purposes. _Peruvian Guano._ Peruvian guano is another important nitrogenous manure very much less abundant now than formerly, as the different guano-beds have become nearly exhausted. While the imports of this important manure into the United Kingdom amounted in 1870 to nearly 250,000 tons, at present not more than 11,000 tons are being imported. _Bones._