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Humus

2007 Schools Wikipedia Selection. Related subjects: Geology and geophysics

   Humus is a word actually used for two different things, which are both
   related to soil and thus get used interchangeably.

   First, in earth sciences "humus" (see http://www.suprahumic.unina.it/)
   is any organic matter which has reached a point of stability, where it
   will break down no further and might, if conditions do not change,
   remain essentially as it is for centuries, or millennia.

   Second, in agriculture, "humus" is often used simply to mean mature
   compost, or natural compost extracted from a forest or other
   spontaneous source for use to amend soil.

   The process of "humification" can occur naturally in soil, or in the
   production of compost. Chemically stable humus is thought by some to be
   important to the fertility of soils in both a physical and chemical
   sense, though some agricultural experts advocate a greater focus on
   other aspects of nutrient delivery, instead. Physically, it helps the
   soil retain moisture, and encourages the formation of good soil
   structure. Chemically, it has many active sites which bind to ions of
   plant nutrients, making them more available. Humus is often described
   as the 'life-force' of the soil. Yet it is difficult to define humus in
   precise terms; it is a highly complex substance, the full nature of
   which is still not fully understood. Physically, humus can be
   differentiated from organic matter in that the latter is rough looking
   material, with coarse plant remains still visible, while once fully
   humified it become more uniform in appearance (a dark, spongy,
   jelly-like substance) and amorphous in structure. That is, it has no
   determinate shape, structure or character.

   Plant remains (including those that have passed through an animal and
   are excreted as manure) contain organic compounds: sugars, starches,
   proteins, carbohydrates, lignins, waxes, resins and organic acids. The
   process of organic matter decay in the soil begins with the
   decomposition of sugars and starches from carbohydrates which break
   down easily as saprotrophs initially invade the dead plant, while the
   remaining cellulose breaks down more slowly. Proteins decompose into
   amino acids at a rate depending on carbon to nitrogen ratios. Organic
   acids break down rapidly, while fats, waxes, resins and lignins remain
   relatively unchanged for longer periods of time. The humus that is the
   end product of this process is thus a mixture of compounds and complex
   life chemicals of plant, animal, or microbial origin, which has many
   functions and benefits in the soil. Earthworm humus ( vermicompost) is
   considered by some to be the best organic manure there is.

Benefits of Humus

     * The mineralisation process that converts raw organic matter to the
       relatively stable substance that is humus feeds the soil population
       of micro-organisms and other creatures, thus maintaining high and
       healthy levels of soil life.
     * Effective and stable humus (see below) are further sources of
       nutrients to microbes, the former providing a readily available
       supply while the latter acts as a more long-term storage reservoir.
     * Humification of dead plant material causes complex organic
       compounds to break down into simpler forms which are then made
       available to growing plants for uptake through their root systems.
     * Humus is a colloidal as substance, and increases the soil's cation
       exchange capacity, hence its ability to store nutrients by
       chilation as can clay particles; thus while these nutrient cations
       are accessible to plants, they are held in the soil safe from
       leaching away by rain or irrigation.
     * Humus can hold the equivalent of 80-90% of its weight in moisture,
       and therefore increases the soil's capacity to withstand drought
       conditions.
     * The biochemical structure of humus enables it to moderate – or
       buffer – excessive acid or alkaline soil conditions.
     * During the Humification process, microbes secrete sticky gums;
       these contribute to the crumb structure of the soil by holding
       particles together, allowing greater aeration of the soil. Toxic
       substances such as heavy metals, as well as excess nutrients, can
       be chelated (that is, bound to the complex organic molecules of
       humus) and prevented from entering the wider ecosystem.
     * The dark colour of humus (usually black or dark brown) helps to
       warm up cold soils in the spring.

Humification of leaf litter and formation of clay-humus complexes

   Compost which is readily capable of further decomposition is sometimes
   referred to as effective or active humus, though again actual
   scientists would say that if it is not stable, it's not humus at all.
   This kind of compost is principally derived from sugars, starches, and
   proteins, and consists of simple organic (fulvic) acids. It is an
   excellent source of plant nutrients, but of little value regarding
   long-term soil structure and tilth. Stable (or passive) humus
   consisting of humic acids, or humins, on the other hand, are so highly
   insoluble (or tightly bound to clay particles that they cannot be
   penetrated by microbes) that they are greatly resistant to further
   decomposition. Thus they add few readily available nutrients to the
   soil, but play an essential part in providing its physical structure.
   Some very stable humus complexes have survived for thousands of years.
   Stable humus tends to originate from woodier plant materials, eg,
   cellulose and lignins.

   Retrieved from " http://en.wikipedia.org/wiki/Humus"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
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