Thus far we have not even mentioned nitrogen, or its common form of salts of ammonia; nor have we mentioned carbon, or its very familiar form of carbonic acid.  These are important elements of plant growth; and they account for the efficacy of manures derived directly from the animal kingdom, as, for example, the droppings of animals, including guano, which consisted originally of the droppings of sea-birds.  Some of 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.