These Mysterious Terms Seem to be Used Interchangeably. This is an Attempt to Explain.
Organic matter is a substance containing carbon compounds originating from living organisms and deposited on or within the soil.
Humus is a brown to black material, a complex of variable carbon-containing compounds that have come from the decomposition of plant and animal bodies, and have lost cellular structure and cannot be defined as carbohydrates, fats, waxes, alkanes, peptides, amino acids, proteins, lipids or organic acids. These non-humic substances are rapidly degraded by microorganisms within the soil, while, in contrast, the complex humic molecules of soil humus are slow to decompose.
Humic substances are the components of humus and as such are complex high molecular weight compounds that together form the brown to black hydrophilic (water-loving), molecules called humus.
Humic substances can be subdivided into three major fractions: Humin, Humic Acids, and Fulvic Acids based on their size and solubility in water at different pHs.
Humin complexes are macro organic (very large substances) with molecular weights in the range of 100,000 to 10,000,000.
The main benefits of humins in the soil are to improve the soil's water-holding capacity, improve soil structure, maintain soil stability, function as a cation exchange system, and to generally improve soil fertility. Because of these important functions, humin is a key component of fertile soils.
Biochar is made by heating organic material to high temperatures in low oxygen; charcoal is an example. It is like a humin in that it is high in carbon and improves soil's water holding capacity and provides large surface area for beneficial micro-organisms. But it has very little nutritional value.
The molecular size of humic acids ranges from approximately 10,000 to 100,000. Humic acid polymers readily bind clay minerals to form stable organic clay complexes. And they readily form salts with inorganic trace mineral elements. These trace elements are bound to humic acid molecules in a form that can be readily used by various living organisms.
Fulvic acids are smaller than humic adds, with molecular weights in the range 1,000 to 10,000. The relatively small size of fulvic acid molecules means they can readily enter plant roots, stems, and leaves. As they enter these plant parts they carry trace minerals from plant surfaces into plant tissues.
Humates are mineral salts of humic and/or fulvic acids mined from carbon-rich lignite-like rock. These humate deposits are high in carbon, natural minerals and trace elements.
Soil Fertility – Benefits of Humus
Humic substances are a good source of energy for soil organisms that lack the ability to photosynthesise and capture energy from the sun. Such organisms must survive on these (and other organic, non-humic, carbon-containing substances) on or in the soil. Humic compounds provide the energy and many of the mineral requirements of soil organisms (algae, yeasts, bacteria, fungi, nematodes, mycorrhizae, and small animals). These organisms benefit the soil and plants in a variety of ways. E.g. mycorrhizae aid plant roots in the uptake of water and trace elements and other fungi decompose vegetative matter releasing bound nutrients for other organisms. Available water is the most important property of fertile soil. The most important effect of humic substances in the soil is their ability to hold water during periods of drought.
Soils with good crumb structure have more open spaces which allow for gaseous interchange with the atmosphere, and for greater water infiltration. Humic substances are key components of a loose crumb soil structure.
Humic substances in soils help neutralize the pH of those soils. Both acidic and alkaline soils are neutralized; moved towards pH neutral (7.0).
Increasing Humic Substances in Soil
Humic substances form naturally in soils with certain production practices, such as planting legumes, ploughing in green manures, application of compost, crop rotation, balanced fertilisation programs, and using minimum cultivation. However, this process is very slow. The most rapid and practical solution to improving soil fertility is the addition of humates directly to the soil.