Soil Humus: Reference Information on Agronomically Important Soil Properties (Part 1)

Part 1. Humus

What humus is, what its properties are and its role in farming

Humus is a heterogeneous polydisperse system of high-molecular nitrogenous compounds of acidic nature. By colour and attitude to solvents, humic substances are divided into three groups of compounds: humic acids, fulvic acids and humins.

Translated from Latin, «humus» is nothing other than soil. And that says it all: without humus there is simply no soil. This term denotes a class of substances contained in soils that are similar in chemical characteristics but very diverse in composition and therefore in properties. Humus contains a large amount of mineral nutrients, which become available to plant root systems through exchange reactions and after its breakdown by mineral acids and microorganisms. Since it has no constant chemical composition, its main characteristic is the ratio of humic and fulvic acids. The ratio of the acids and their characteristics are determined by the conditions under which they formed. The most important are: climatic conditions, the type of plant cover from whose biomass these substances formed, and the mineralogical composition and physico-chemical characteristics of the mineral part of the soil. The most valuable part of humus is humic acids. Humic acids contain on average (%): carbon 52–62; hydrogen 2.5–5.8, oxygen 31–39 and nitrogen 2.6–6.1. They form in temperate climates on soils with a reaction close to neutral from the organic matter of the biomass of cereal grasses and the litter of deciduous trees.

At the moment of formation humic acids are soluble in water, and on interaction with hydrogen cations and di- and trivalent elements they precipitate. Their salts, humates: NH₄ and K+ are soluble in water and can be leached into lower horizons, forming true and colloidal solutions. Humic acids and their salts form organo-mineral microaggregates of grayish, grayish-brown and brown colours, the so-called humins. Humates and humic acids, as they accumulate, colour the soil gray, brownish-gray or black. Soil lumps impregnated with them stick together, forming structural elements. Humic acids and their salts are decomposed by bacteria.

Fulvic acids form mostly in a humid cool climate under the predominant action of fungal microflora, that is, in the decomposition of mosses, lichens and forest litter, mainly of coniferous species. Fulvic acids have a brown-yellow colour, a very acidic reaction (pH 2.6–2.8) and are soluble in water. Their elemental composition (%): carbon 40–52, hydrogen 4–6, oxygen 40–48 and nitrogen 2–6. With mono- and divalent cations fulvic acids form water-soluble salts, fulvates. Fulvates of Fe³⁺ and Al³⁺ as a rule form complex compounds insoluble in water but soluble in solutions with acidic and alkaline reactions. With a high content of iron and aluminum ions in the soil, iron and aluminum fulvates form that precipitate, forming colloidal compounds. Fulvic acids are able to react with the mineral part of the soil, as a result of which they destroy secondary minerals, releasing calcium, magnesium, potassium, iron and aluminum. Subsequently part of these cations is leached into deeper layers.

The overall activity of humus acids with respect to the mineral part of the soil depends on the ratio and content of humic acids and fulvic acids in soils. With a humic to fulvic acid ratio up to 0.2, humus accumulation is almost absent and destruction of the mineral part is maximally intense; at 0.2–0.5 humus accumulation is weak and the effect of humus acids on the mineral part is active; at 0.5–0.7 a medium rate of humus accumulation is observed and the action of organic acids on the mineral part of the soil is weak; at a ratio above 1.0 humus accumulation is intense and the mineral part remains almost unchanged.

Humins are now usually called non-hydrolyzable residues. They are the totality of humic and fulvic acids firmly bound to the mineral part of the soil. They also include components of plant residues that are hard for microorganisms to decompose: cellulose, lignin, charcoal. Humins do not dissolve in any solvent, so they are called inert humus.

Besides humus substances, soils contain organo-mineral compounds. Mostly these are salts of non-specific acids (oxalic, citric, acetic, formic), complex salts of non-specific and humus acids, and adsorption organo-mineral compounds. Salts of humic acids are called humates, and of fulvic acids fulvates. Most fulvates are readily soluble compounds.

To understand the quality of this important soil fertility indicator, methods for determining the content and ratio of components have been developed. By humus content soils are divided into groups. The limits of humus content for each group in our country and in many countries of the world are standardized. The grouping of soils by humus content under DSTU 4362:2004 is given in table 1.

Table 1. Grouping of soils by humus content (by the Tyurin method)

The humus content indicator is linked to a considerable extent with human economic activity. In the vast majority of cases this influence is negative, that is, it leads to irreversible humus losses. According to scientific institutions and laboratories of the State Institution Derzhrodiuchist, humus losses over the last 50 years average about 1%. The main «destroyers» of humus are nitrogen fertilizers and fallow. Therefore incorporating by-products, organic fertilizers, composts and the biomass of green manure crops into the soil is a stabilizing factor that slows humus decomposition and ensures higher soil fertility. Incorporating by-products has a noticeable effect on the balance of nutrients, first of all carbon as the main element for forming humus substances, and other chemical elements including microelements (Table 2).

Table 2. Micronutrient balance in the soil when straw is incorporated in a rotation

(60% stubble crops, 40% row crops)

Note: K – without incorporating by-products, B – with incorporating by-products.