es, practically the whole of this nitrogen is converted into
"free" nitrogen. The loss is thus in a sense irreparable. In the paper
above, referred to, our total annual exports of this substance are
estimated at 19,000,000 lb.; while the total annual production of the
world is estimated at not less than 100,000,000 lb. The annual loss of
nitrogen due to this source alone would amount to about 10,000,000
lb.[91] Similarly, loss of nitrogen, although to a less extent, is
caused by the use of other explosives, as well as in the manufacture of
the other articles above mentioned.

_Loss due to Sewage Disposal._

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 a