he usual way, it generally contains some 3 per cent of nitrogen,
chiefly in the form of sulphate of ammonia, and small quantities of
potash and phosphates. A varying proportion of the nitrogen is present
in the form of ammonia salts; and this undoubtedly confers upon soot its
manurial value. It has long been used as a top-dressing for young grain
and grass, and has been applied at the rate of from 40 to 60 bushels per
acre. It has an indirect value as a slug-destroyer.

Many of the above-mentioned manures, of comparatively low value, will
probably be less used in the future than they have been in the past,
owing to the more abundant supplies of nitrate of soda and ammonia salts
which are now available. Many of these substances have probably been
used in mixed manures.

FOOTNOTES:

[240] See p. 324.

CHAPTER XVII.

SEWAGE AS A MANURE.

The value of sewage as a manure has been in the past enormously
overrated, and much misunderstanding has existed on the part of the
public on the question of the profitableness of the disposal of town
sewage as an agricultural manure. Not a few of the erroneous opinions
prevalent in the past regarding sewage have been due to statements made
by scientific and other writers as to the enormous wealth lost to the
world by many of the present methods of sewage disposal. Fortunately,
however, the sewage question is now increasingly regarded as a question,
in the first instance, of sanitary interest. As much has been written on
the subject, and many schemes have been devised, at the expense of much
ingenuity, for utilising its manurial properties, it may be desirable
here to say a few words on the purely agricultural side of the question.

The two most important points about sewage are its enormous abundance
and its extremely poor quality. If the most important consideration
were not the sanitary one, but its manurial value, then indeed our water
system, so universally used in towns, must be regarded as a most
wasteful one; for by its means the value of the excrementitious matter
from which it derives its manurial ingredients is tremendously lessened.
When we reflect that a ton of sewage, such as is produced in many
European cities, contains only 2 or 3 lb. of dry matter, and that the
total amount of nitrogen in this is only an ounce or two, while the
phosphoric acid is considerably less, and that it is on those two
ingredients that its value as a manure entirely depends, we see