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.
The addition of humus to the soil has physical and chemical effects. Physically humus absorbs and detains moisture; it raises the temperature of the soil and maintains it in an equable condition; it keeps the particles of sand and clay asunder and therefore improves the aeration and porosity; it detains the heat, and thus prevents the roots of plants being frozen during hard frost. But humus performs other important functions in the soil, especially in connection with the nutrition of many trees and shrubs and green-leaved plants generally. It has been discovered that the roots of many plants (e.g. Oaks, Beeches, Poplars, Elms, Rhododendrons, Cranberries, Bilberries, Brooms, Heaths, Conifers, etc.) are invested with the filaments of certain fungi, which, instead of being injurious, are actually beneficial. These fungal threads are interwoven in the tissues of the feeding roots, and often look like root hairs, and perform similar functions of absorbing water from the soil together with the mineral salts and other compounds dissolved in it. The name of "mycorniza" has been given to these fungi which envelop the roots of many plants, and it has been proved that they are not only beneficial and essential to the plants on which they grow, but that they can only come into existence when humus is present in the soil. This accounts for the great esteem in which all gardeners hold leaf mould as an ingredient in the soils they use, and they know by actual experience that a soil without humus or leaf mould would be practically useless for their plants (fig. 83).
Fig. 83. - 1, Roots of White Poplar with mycelial mantle. 2, Tip of Root of Beech with closely adherent mycelial mantle x 100 (after Frank). 3, Section through a piece of wood of the White Poplar with the mycelium entering into the external cells, x 180.
Chemically, humus gives rise to living micro.organisms in the soil, when lime is present, during the process of fermentation and decay, if the temperature is favourable, and thus yields up a supply of organic food in the process of decomposition.
The following table shows the composition of three different kinds of humus: -
Organic matter (humus)............
Clay and silica (sand) .............