Amino acids exist in soil in several different forms, such as:
Free amino acids either in the soil solution or in soil micropores
Peptides or proteins or simple amino acids, bound to clay minerals on external surfaces and or internal surfaces.
Peptides or proteins or simple amino acids, bound to humic colloids
e.g., H-bonding and van der Waals' forces, or
in covalent linkage as quinoid-amino acid complexes
(short-lived) existence in soil. Since they are readily decompose the amounts present in the soil at any point in time represent a balance between synthesis and destruction by microorganisms.
Weather conditions, the soilís moisture status, fluctuations of organic residues, condition of the soil culture, and stage of growth and type of plants present, dramatically affect the the soilís free amino acids content. http://www.up.wroc.pl/~weber/no.htm
Humic matter is completely decomposed organic matter and it is readily soluble in acids or bases. Humification is the biological process of converting organic matter into humic substances. [Humintech website]
Humic substances can be subdivided into three major fractions:
(1) HUMIC ACIDS
(2) FULVIC ACIDS and
These sub-divisions are arbitrarily based on the solubility of each fraction in water adjusted to different acid/alkaline (pH levels) conditions. Humic substances are a colloid and coat soil particles, acting as a cushion and an interface between the non-living mineral of the soil and the living plant root. Native soil humic substances enhance plant growth both directly and indirectly.
Plants and animals can absorb elements more readily in humic form because the elements have already been assimilated and chelated by the micro-organisms which may have utilized parent organic matter eons ago. Many of the components of humus are heterogeneous, relatively large, stable organic complexes. Physically, they function to give the soil structure, porosity, water-holding capacity, cation and anion exchange. Biologically, they affect the activities of microorganisms. Chemically, they serve as an adsorption and retention complex for inorganic plant micro-nutrients, especially iron. Inorganic iron compounds are very unstable in soil and tend to become insoluble and unavailable, especially in calcareous soils. However, humate compounds can incorporate iron into chelated complexes, maintaining its availability to plants, although still in insoluble form.
Therefore, it can be said that humic substances are high molecular weight compounds that together form the hydrophilic, molecularly flexible, polyelectrolyte components of humus. The elemental analysis of humic substances reveals that they are primarily composed of carbon, oxygen, hydrogen, nitrogen, and sulfur in complex carbon chains. (Petitt) Specifically, humic matter has been shown to increase the uptake of nitrogen by plants, and to increase soil nitrogen utilization efficiency. Nutritionally, humic substances are also sources phosphorus, as well as, enhancers to the up-taking of potassium, calcium, and magnesium, benefiting micro-organisms that benefit the plants in turn. All of these effects increase the productivity of the soil. In addition, chlorosis in plants has been prevented, or corrected by humate application, probably the result of the ability of humate to hold soil iron in an assimilatable form. This phenomenon can be particularly effective in calcareous (alkaline) soils that are usually low in organic matter content, and deficient in available iron.
Humic substances have been shown to contain a wide variety of molecular components. Some typical components are:
aliphatic compounds (carbon chains)
furan-ringed compounds (aromatic) including various combinations
of benzene, acetal, ketal, and lactol.
polysaccharides, and quinones
Applied pesticides substantially interact with humic substances in the soil, but the reactions are complex. Certain pesticides may be immobilized by humates and can be caused to practically disappear from the soil environment. In this case, humic substances can be very effective in removing excess pesticide from sandy soils very low in organic matter thus avoiding toxicity hazards. However, the degradation of a particular pesticide will be determined in part by the rate of release. Amazingly, a number of degraded pesticides can form new groups of compounds that complex with humates, which can then be absorbed safely by plant roots. Degradation or inactivation of other toxic substances such as nicotine, aflatoxins, antibiotics, and shallots, and most organic pesticides is also mediated by humic substances. In the microbial degradation process not all of the carbon contained within these toxins is released as CO2. A portion of these toxic molecules, primarily the aromatic ring compounds are stabilized and integrated within the complex polymers of humic substances. Humic substances have electrically charged sites on their surfaces which function to attract and inactivate many toxic substances. Humates are even recommended nowadays with frequency by the Environmental Protection Agency for the use of for clean up of toxic waste sites. Many bioremediation companies have sprung up that apply humate-based compounds to toxic waste sites as a part of their clean-up program. Growers who farm soils low in humus should include the purchase of humic substances within their fertilizer budgets. Humic substances are extremely cost-effective, as they can be more than offset by reduced costs of other fertilizer ingredients.
In modern agriculture, commercially-available humic substances added to the soil do not directly contribute significant quantities of nutrients to plants at the rates normally applied. However, indirect effects, as catalysts as on fertilizers can be significant as revealed by studies on yield, and enhanced nutrition in plant tissues.
(1) Humic Acid (HA)
Humic acids are colloids and behave somewhat like clays, even though the nomenclature suggests that they are acids and form true salts. When the cation exchange sites on the humic molecule are filled predominantly with hydrogen ions, the material is considered to be an acid and is named accordingly. However, it has no great effect on pH because the acid is insoluble in water. When the predominant cation on the exchange sites is other then hydrogen, the material is called humate. [Senn and Klingman, 1973]
Humic acids or humus can be defined as the end product of the decomposition of organic matter by aerobic organisms. Humic acids readily form mineral salts with inorganic trace elements. An analysis of extracts of naturally-occurring humic acids may reveal the presence of over five dozen different mineralized elements present. These trace elements are bound to humic acid molecules in a form that can be readily utilized by various living organisms. As a result humic acids function as important ion exchange and metal complexing (chelating) systems.
Humic acids comprise a mixture of weak carbon chains and carbon rings. They are organic acids that are not soluble in water under acidic conditions, but are soluble in water, under basic (alkaline) conditions. When the pH is decreased below 2 in water- based solutions, humic acids solidify and fall out of solution, becoming that tiny fraction of concretions (precipitates) visible at the bottom of the test tube. On average 35% of the humic acid molecules are rings of carbon atoms (aromatic) while the remaining components (65%) are molecules in the form of carbon chains (aliphatic).
The molecular weights of humic acids range from approximately 10,000 to 100,000. Humic acid polymers readily bind clay minerals to form stable organic clay complexes. Peripheral pores in the polymer are capable of accommodating (binding) natural and synthetic organic chemicals in lattice-(clathrate)-type arrangements. [Petitt] In the absence of the clay or humic colloids, the cations may be lost either through fixation or leaching, and thereby lost to the plant root.
Humic acids are crucial to life on earth, but they are susceptible to depletion via oxidation, and may become deactivated by sodium or aluminum. This creates soil problems which may impact the food chain all the way up. Humic acids have been shown to stimulate seed germination of several varieties of crops. The plant characteristic that the addition of humic substances has consistently enhanced more than any other is root length, especially on sandy soils. Top growth, vigor, and trunk cross-sectional area are also increased in response to stimulation by humates, but the effect is usually more prominent in the roots. A proliferation in root growth, resulting in an increased efficiency of the root system, is a likely cause of higher plant yields seen in response to humic acid treatment.
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