Compost is a mixture of manures, or of earths and manures, in such proportions, and of such qualities as are considered particularly applicable to the plant or crops to which the composition is to be applied. The subject of Composts has been studied but slightly at present, by men who combine science with practice, and what is here offered, must consequently be less satisfactory than the author is conscious it might be.

A correct preparation of Composts must be founded upon a due knowledge of the food of plants. This is ascertainable from their analyses, and these reveal what is sustained by practice, that there are some substances required by all plants as food, and that there are other substances which are beneficial to some plants, and useless, or even injurious to others.

Now the substances universally required by cultivated plants, are, carbon, hydrogen, oxygen, nitrogen, and phosphate of lime, and, perhaps, common salt. These are supplied to plants by the atmosphere, and by decaying animal and vegetable matters.

The substances required only by particular plants, are certain salts, such ns sulphate of lime, by the turnip; - sulphate of magnesia, (Epsom salt) by the potato; - nitrate of potass (saltpetre), by borage, and the lilac.

The food of plants, whether imbibed by their roots or their leaves, must be either in a liquid, or a gaseous form. In these states, most plants obtain a large portion of their food from the atmosphere, decomposing its carbonic acid, and water, and retaining only so much of their carbon, hydrogen, and oxygen, as they require. From the same source also they obtain nitrogen. Hut they do not obtain these from the atmosphere alone- The roots also obtain them from the soil. Animal and vegetable matters, as they decay, give out carbonic acid; and partly decompose into mucilage, soluble in water, both of which are rapidly imbibed by the roots. Composts, therefore, should contain these decomposing matters in such proportion, as to give the plants, to which they are applied, the vigour required. If leaves are required to be largely developed, the compost can be scarcely too rich; for the greater the quantity of food imbibed by the roots, the greater will be the surface of leaves requisite for its elaboration.

But if flowers and fruit, as well as leaves, are desired, the composts, if excessively rich, will cause these to diminish in number and size, the flower-buds passing into leaf-buds for the reason already alleged.

Composts must also duly regulate the amount of moisture supplied to the roots, totally independent of drainage, as compost retains to them moisture by itschemical and capillary powers. The richer in decomposing animal and vegetable matter, - the richer in alumina, (clay), - and the looser its texture, the better does a compost retain water. And this power is diminished in proportion as siliceous sand, or calcareous (chalky) matters preponderate.

Mr. Errington, (Gardener's Chronicle, 1845,) prepares his composts from strong tenacious loam; half-rotten leaf-mould; heath-soil; horse-manure; cow-manure; charcoal and wood ashes; bone-dust; sharp sand; burnt turf; and moss well scalded; and, from these materials there is no doubt that a compost could be prepared, embracing any desired degree of fertility. - See Manures and Potting.