What are humic substances

Humic substances were first defined in 1761 by Wallerius as a decomposed organic matter . In 1786, Achard extracted a brown substance from soil and peat using a KOH solution and named it humic acid . Humus, a Latin word suggesting a soil-like substance, was first introduced by de Saussure in 1804, referring to dark soil organic matter . In 1837, Sprengel developed several methods for preparing humic acid by pretreating soil with dilute mineral acids before alkaline extraction . Sven Oden (1919) postulated that HS are the light to dark-brown substances of unknown materials, which are formed in nature by the decomposition of organic matter through the actions of microorganisms or in a laboratory by oxidizing chemical reagents. Alternatively, it was suggested that humus is the product of the condensation reaction between carbohydrates and amino acids in a microorganism-free environment . It was also stated that phenol, quinone, and hydroquinone oxidation in an alkaline solution yields compounds similar to humic acids .

Humic substances (HS) are major components of the natural organic matter (NOM) in soil and water as well as in geological organic deposits such as lake sediments, peats, brown coals and shales. They make up much of the characteristic brown color of decaying plant debris and contribute to the brown or black color in surface soils. They are major components of NOM in surface waters and at higher concentrations can impart a dark color, especially in brown fresh water ponds, lakes, and streams. In leaf litter or composts, the color may be yellowish-brown to black, depending on the degree of decay and concentration.

Humic substances are very important components of soil that affect physical and chemical properties and improve soil fertility. In aqueous systems, like rivers, about 50% of the dissolved organic materials are HS that affect pH and alkalinity. In terrestrial and aquatic systems HS affect the chemistry, cycling and bioavailability of chemical elements, as well as transport and degradation of xenobiotic and natural organic chemicals. They affect biological productivity in aquatic ecosystems, as well as the formation of disinfection by-products during water treatment.

Humic substances are complex and heterogeneous mixtures of polydispersed materials formed by biochemical and chemical reactions during the decay and transformation of plant and microbial remains (a process called humification). Plant lignin and its transformation products, as well as polysaccharides, melanin, cutin, proteins, lipids, nucleic acids, fine char particles, etc., are important components taking part in this process.

Humic substances in soils and sediments can be divided into three main fractions: humic acids (HA or HAs), fulvic acids (FA or FAs) and humin. The HA and FA are extracted from soil and other solid phase sources using a strong base (NaOH or KOH). Humic acids are insoluble at low pH, and they are precipitated by adding strong acid (adjust to pH 1 with HCl). Humin cannot be extracted with either a strong base or a strong acid.

Aquatic HS contain only HA and FA and these components are generally removed from water by lowering the pH to 2 and adsorbing both components on a suitable resin column. The HA and FA are extracted from the resin with strong base followed by lowering the pH to 1 to precipitate the HA. The resin column separation is also used to separate FA from the non-humic materials (amino acids, peptides, sugars, etc.) extracted from soils. At low pH the FA adsorbs on the resin, but non-humic materials pass through the column.

Humic substances are highly chemically reactive yet recalcitrant with respect to biodegradation. Most of the data on HA, FA and humin refer to average properties and structure of a large ensemble of components of diverse structure and molecular weight. The precise properties and structure of a given HS sample depends on the water or soil source and the specific conditions of extraction. Nevertheless, the average properties of HA, FA and humin from different sources are remarkably similar.

Although the global population has been increasing at an alarming rate, agricultural land has not expanded significantly . In this circumstance, improving the human ability to grow grains in a limited space, e.g., in small fields, is critical. Farmers depend on inorganic chemical fertilizers to keep the soil fertile for cultivation. However, the overused lands become unfertile and saline with a different pH in the long run. Soil salinity is characterized by high amounts of Na+, Mg+2, Ca+2, Cl–, HCO3–, and SO4–2, affecting plant growth . Moreover, the total carbon content in the soil decreases daily. The organic matter of soil contains the residues of plants and animals and other organic compounds that form during the biomass decomposition processes in the soil. In this case, about 60% of the organic matter of soil is humic substances (HS), which play a vital role in the health of soil for cultivation.