sewage that to which it owes most largely its value as a
manure--viz., ammonia; and from this fact it deserves a first place in
the consideration of agriculturists. For however admirable other methods
may be from a sanitary point of view, it is obvious that a method which
would allow the ammonia in sewage wholly, or at least to over 90 per
cent, to be lost, cannot claim the same place in the judgment of
agriculturists as a method which can extract for the soil not only the
whole of this valuable constituent, but all else in the sewage which in
any way is of value to plant-life.

_Effects of continued Application of Sewage._

When sewage is continuously applied to the same land, what generally
takes place is this: At first the sewage is purified, and the soil
derives corresponding benefit from the valuable fertilising ingredients
it thus extracts. After a time, however, the land becomes what has been
termed "sewage-sick." The pores in the soil become choked up by the
slimy matter the sewage contains in suspension; the aeration of the
soil, which, as we have already mentioned, is so necessary, is
consequently to a large extent stopped; and the result is, that the land
rapidly deteriorates, and the sewage is no longer purified.

_Intermittent Irrigation._

This is obviated to some extent by intermittent irrigation. The land,
instead of receiving sewage continuously, only receives it at intervals,
and is allowed some time to recover between each dose. It is, however,
the opinion of those who have given the subject much attention, that
land, even although intermittently sewaged, never recovers its original
efficacy.

Irrigation, therefore, under favourable conditions, is a most successful
method of utilising the manurial value of sewage; but the great
difficulty in practice is to obtain those favourable conditions. It has
long been known that if soil is properly to discharge its function as a
purifier of sewage water, it must be properly aerated; and we now know
that in every fertile soil the process of nitrification must be
permitted free development. Now the application of large quantities of
sewage to a soil is apt to prevent this free development. As we have
already seen, absence of air and the lowering of the temperature of the
soil distinctly tend to retard nitrification; and these two conditions
accompany the application of large quantities of sewage.

_Crops suited for Sewage._

Another objection to irr