the nitrogen in these substances, however, is of an evanescent
character, and rapidly flies away in the form of carbonate of ammonia;
hence, a heap of farmyard manure, left for several years, loses much of
its value as manure, and guano should be kept in bulk as long as
possible, and protected from the atmosphere, or its ammonia will largely
disappear. One difficulty experienced by chemists and others in
preparing artificial manures is that of 'fixing' the needful ammonia, so
that it may be kept from being dissipated in the atmosphere, and at the
same time be always in a state in which it can be appropriated by the
plant. In all good manures, however, there is a certain proportion of it
in combination, and in many instances the percentage of nitrogen is made
the test of the value of a manure.

The importance of humus--the black earthy substance resulting from the
decay of vegetation--in a soil is that it contains in an assimilable
form many of the ingredients essential to plant life. Humus when it
decomposes gives off carbonic acid, which breaks up the mineral
substances in the soil and renders them available as plant food. When
vegetable refuse is burned, the nitrogen--one of the costliest
constituents--is dissipated and lost. But by burying the refuse the soil
gets back a proportion of the organic nitrogen it surrendered and
something over in the way of soluble phosphatic and potassic salts; and
as this organic nitrogen assumes ultimately the form of nitric acid, it
can be assimilated by the growing plant, to the great benefit of
whatever crop may occupy the ground.

The practical conclusion is, that in the treatment of the soil a skilful
gardener will endeavour to promote its fertility by affording the
natural influences of rain, frost and sun full opportunity of liberating
the constituents that are locked up in the staple; by restoring in the
form of refuse as much as possible of what the soil has parted with in
vegetation; and by the addition of such fertilising agents as are
adapted to rectify the natural deficiencies of the soil. Thus, instead
of following a process of exhaustion, the resources of the garden may be
annually augmented.

ARTIFICIAL MANURES AND THEIR APPLICATION TO GARDEN CROPS

Plants, like animals, require food for their sustenance and development,
and when this is administered in insufficient quantities, or unsuitable
foods are supplied, they remain small, starved, and unhealthy.