This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
While sand, clay, lime, and peat are all useful and necessary ingredients of every good garden soil, each one by itself would be practically useless. When mixed together in certain proportions they are more valuable, but they still lack something to make them into a really good garden soil. It would be possible, for example, to obtain sand, clay, and limestone from the roadway when excavating for sewers and other purposes. But no one would dream of trying to grow plants upon such material, even if mixed in suitable proportions. There is evidently something lacking, and that something is of an organic, not a mineral, nature.
When the decayed remains of plants and the refuse from animals (including decayed leaves, peat, stable manure, etc.) are mixed with the mineral ingredients it is found that plants grow well. This plant and animal refuse in a thoroughly decomposed condition is known under the name of "humus". One of the most popular forms in which humus is added to the soil is leaf mould or leaf soil. Every crop would produce a large quantity of leaf mould every year, but much valuable material is wasted, and the deficiency must be made up by the purchase of stable and other manures.
The best kind of leaf mould is seen in natural woods of oak, beech, lime, etc, more especially in the ditches and hollows where great accumulations have taken place. Leaf mould is largely used in the cultivation of many kinds of stove, greenhouse, and hardy plants mixed with loam, sand, and peat in various ways. The beds on which the French maraichers grow their Lettuces, Endives, Carrots, Radishes, Cauliflowers, etc. (see Vol. IV), are almost entirely humus, with a certain amount of inorganic gritty soil; hence the luxuriant and rapid growth that is secured.