ogen does
not (_i.e._, within a reasonable number of years) become available for
the crop's uses. What, indeed, becomes of the nitrogen is a mystery; but
it is highly probable that some such kind of retrogression as that above
referred to, whereby the nitrogen is converted into some inert organic
form, takes place.

_Artificial Sources of Loss of Nitrogen._

So far, the sources of loss of nitrogen considered have been what we may
term _natural_ sources. By this is meant that the loss of nitrogen from
the above sources takes place in a state of nature, and not merely under
conditions of cultivation. No doubt the loss due to drainage is very
much greater under arable farming than would be the case where
artificial drainage does not obtain; still, under any conditions, this
loss must be reckoned with. On the other hand, by _artificial_ sources
of loss are meant those entirely dependent on our modern system of
agriculture and our modern system of sewage disposal, whereby the
nitrogen contained in that portion of the produce of the farm which goes
to supply our food is not returned to the soil, but is totally lost.

_Amount of Nitrogen removed in Crops._

The modern tendency towards centralisation in large towns has rendered
this loss--despite all that has been said to the contrary--a necessity.
It is extremely difficult, however, to form any estimate of its amount.
We know, of course, the amount of nitrogen removed from the soil by
different crops. We cannot, however, estimate how much of this may find
its way back again to the soil. The amount of nitrogen contained in the
different crops will be fully dealt with in the chapter on the manuring
of different crops. It may be, however, not without interest to give
here some approximate indication of the amount of this loss, in order to
render the view of the subject as comprehensive as possible.

Recent agricultural returns for Great Britain give the total produce of
_wheat_ at over 76 million bushels, that of _barley_ at over 69 million,
and that of _oats_ at over 150 million. Calculating the amount of
nitrogen, these quantities of wheat, barley, and oats respectively and
collectively contain, and calculating also how much _sulphate of
ammonia_ and _nitrate of soda_ these amounts of nitrogen represent, the
following are the results:--

Nitrogen. Sulphate of Nitrate of
Ammonia. Soda.