Humus
Humus is the substance obtained from the breakdown of plant material, usually referred to as composting.
The following text is taken from the Household Cyclopedia of 1881:
- "Soils contain a great amount of matter which results from the decay of vegetables and animals; to a compound of which with earthy material the name of humus is given. This was once incorrectly supposed to give the whole nutriment of the plant. Trees and plants, instead of abstracting carbon from the earth, really, by taking it from the air, and subsequently dying and decaying, annually by their leaves, and finally altogether, give carbon and other atmospheric elements to the soil. As above said, all plants by their leaves absorb carbonic acid from the air, and retain carbon, giving out oxygen. It is evident, therefore, that the leaves are of great importance to the plant. So are the roots, for their absorbing office. Thus it is true that the growth of a plant is always proportioned to the surface of its roots and leaves together. Vegetation, in its simplest form, consists in the abstraction of carbon from carbonic acid, and hydrogen from water; but the taking of nitrogen also, from ammonia especially, is important to them, and most of all, to those which are most nutritious, as the wheat, rye, barley, &c., whose seeds contain gluten and other nitrogenous principles of the greatest value for food. Plants will grow well in pure charcoal, if supplied with rain-water, for rain-water contains ammonia.
- "Animal substances, as they putrefy, always evolve ammonia, which plants need and absorb. Thus is explained one of the benefits of manuring, but not the only one as we shall see presently. Animal manure, however, acts chiefly by the formation of ammonia. The quantity of gluten in wheat, rye, and barley is very different; and they contain nitrogen in varying proportions. Even in samples of the same seed the quantity varies, and why? Evidently because one variety has been better fed with its own appropriate fertilizer than another which has been reared on a soil less accurately adapted by artificial means for its growth. French wheat contains 12 per cent. of gluten; Bavarian 24 per cent. Sir H. Davy obtained 19 per cent. from winter, and 24 from summer wheat; from Sicilian 21, from Barbary wheat 19 per cent. Such great differences must be owing to some cause, and this we find in the different methods of cultivation.
- "An increase of animal manure gives rise not only to an increase in the number of seeds, but also to a remarkable difference in the proportion of gluten which those seeds contain. Among manures of animal origin there is great diversity. Cow dung contains but a small proportion of nitrogen. One hundred parts of wheat, grown on a soil to which this material was applied, afforded only 11 parts of gluten and 64 of starch; while the same quantity of wheat, grown on a soil fertilized with human urine, yielded 35 per cent. of gluten, and of course a smaller proportion of less valuable ingredients. During the putrefaction of urine, ammoniacal salts are formed in large quantity, it may be said, exclusively; for under the influence of warmth and moisture, the most prominent ingredient of urine is converted into carbonate of ammonia."
Humus has many properties that encourage plant growth. It acts as a sponge, thus increasing the water-holding capacity of the soil. Aggregates of humus in heavy soil allow more rapid water penetration and the infiltration of air required for plant root health. It makes heavy soil more crumbly, so easier to till.
Humus is a colloid - a material that carries a negative electrical charge. Many essential plant nutrients occur as positively charged ions: calcium, magnesium, molybdenum, etc. The colloids in healthy soil bind to these nutrients due to the opposite electrical charges, preventing them from leaching by water movement, but allowing the plant roots to access them by emitting hydrogen ions.
Humus is also an Arabic dip popular all over the Middle East made of garbanzo bean paste with various additions (such as olives, olive oil, garlic, lemon juice, tehina). Humus is usually scooped up with bread (chunks of pita) to be eaten. Falafel is humus patties deep-fried in oil.