Organic Matter of Soil & Humus
All materials of plant, animal, or microbial origin, produced in the soil or added to it are termed as soil organic matter. The soils whose properties are dominated by organic matter are called organic soils.
The primary source of soil organic matter is plant tissue. The fallen branches, twigs, leaves and roots of trees, shrubs, grasses, and other plants supply large quantities of organic residues. Roots and stubbles (remains of harvested crops) also contribute to soil organic matter. The soil organisms decompose and digest these organic materials so that they are incorporated into the soil, i. e., become part of the soil. Animals are secondary source of soil organic matter. They attack the plant tissues, contribute waste products and leave their own bodies after their death. Earthworms, termites and ants help in translocation of soil and plant residues.
The plant residues are high in water contents ranging from 60-90%, the optimum being 75%. On a weight basis, the dry matter is mostly carbon and oxygen, and about 10% each of hydrogen and inorganic elements (ash). Thus, major elements of soil organic matter are carbon, oxygen and hydrogen. However, other elements such as nitrogen, sulphur, phosphorus, potassium, calcium, and magnesium are also present in small quantities. These elements play a vital role in plant nutrition although these are present in small quantities.
The carbohydrates (simple sugars, starches, cellulose) are most prominent organic compounds of plants. Lignin is found in older plant tissue of woody plants.
They are very resistant to decomposition. Fats and oils are primarily found in seeds. Simple and complex proteins are found in plant tissues. These are primary source of essential elements such as carbon, oxygen, hydrogen, nitrogen, sulphur, manganese, copper, and iron.
The decomposition rate of organic compounds contained in the plant residues vary greatly. The carbohydrates are decomposed very rapidly, followed by proteins, hemicelluloses, cellulose, fats and waxes, and lignin being the most slowly decomposed organic compounds. However, all the organic compounds begin to decompose simultaneously when the fresh plant tissue is added. Decomposition involves three general reactions: enzymatic oxidation, release of inorganic ions such as NH4+, NO3–, and SO4–, and formation of humus.
The soil micro-organisms decompose organic material to get food and energy. Less resistant components such as cellulose, peptides, and simple organic compounds are immediately used for energy and synthesis of microbial tissue, whereas the resistant components such as lignin, etc., are decomposed slowly and accumulate in the soil in partially decayed state in the form of a brown to black structure less soil organic polymer called humus. The process of humus formation is called humification.
Humus is a complex and resistant mixture of brown or dark brown amorphous colloidal organic substances that results from microbial decomposition.
Properties of Humus
Humus is a colloidal organic substance. The tiny humus colloidal particles (micelle) are composed of carbon, hydrogen, and oxygen in the form of polyphenols, poly-quinones polyuronides, and polysaccharides. Most of the properties of humus are its colloidal characteristics. Some important properties of humus are as under:
Color: The color of the humus is brown to dark brown. The typical dark of some soils is due to presence of humus in these soils.
Solubility in Water: The humus is insoluble in water due to its association and divalent and trivalent salts. This property help the humus to stay in the soil not lost by leaching. However, isolated humus is partly soluble in water.
Water Retention Capacity: The water holding capacity of humus on mass basis 4-5 times is that of the silicate clays. It can hold water up to 20 times its mass. This property helps prevent drying and shrinking of soil separates. Also, humus improves moisture retention in sandy soils.
Cation Adsorption and Exchange: The surface area of humus particles per unit mass is very high, usually more than silicate clays. The colloidal surfaces of humus are negatively charged. The sources of the charge are hydroxy (-OH), carboxylic (-COOH), or phenolic groups. Therefore, humus micelle like particles of clay carry a swarm of adsorbed cations. These cations are exchanged between humus particles and soil solution as well as roots. The presence of humus increases cation exchange capacity of the soils up to about 20-70%.
Chelation: The humus forms stable complexes with copper, manganese, zinc and other polyvalent cations. This property ensures the availability of micronutrients to higher plants.
Combination with Clay and Clay Minerals: Humus particles combine with clay particles to form clay-humus combinations which attract and hold amino acids, peptides, and proteins. This result in formation of complexes that protect these nitrogen-containing compounds from breakdown by the action of microorganisms. This property of humus helps in N2-conservation.
Combination with Organic Molecules: The humus particles form complexes other organic molecules present in the soils. This property of humus is of immense, agricultural importance as it affects the bioactivity, persistence and biodegradation of pesticides. It helps in modification of the application rate of pesticides for effective control.
pH Relations: Humus acts as buffer and helps in maintaining a uniform pH in the soil in slightly acidic, neutral and alkaline soils.
Mineralization: Humus enhances Mineralization, i.e., breakdown of organic matter into minerals such as CO2, NH4, NO3, PO4 and SO4. This property helps in increasing nutrients in the soils necessary for the growth of plants. Humic acid present in the humus attacks the minerals, decompose them so the cations are released and are adsorbed on the surfaces of humus colloidal particles. In this way, these become available to plants more readily
Composition of Humus
Two groups of compounds make up humus: humic group, and non-humic group.
The humic group comprises of complex materials that are resistant to microbial attack. These are aromatic, ring-type structures that include polyphenols (numerous phenolic compounds tied together) and poly-quinones, which are even more complex. These are formed by decomposition, synthesis and polymerization of plant residues. The humic substances are amorphous, dark in color, and have very high molecular weight. These make up about 60-80% of the soil organic matter. The humic substances are classified on the basis of resistance to degradation and of solubility in acids and alkalis as followings:
Fulvic Acid: These are lightest in color, soluble both in acid and alkali, have lowest molecular weight, and can be decomposed by microbes very easily.
Humic Acid: These are medium in color as well as in molecular weight, soluble in alkali but not in acid, and intermediate in resistance to degradation.
Humin: These are darkest in color, insoluble both in acid and alkali, highest in molecular weight, and most resistant to microbial attack.
The non-humic group forms about 20-30% of humus present in the soil. These substances are less complex and less resistant to microbial attacks than those of humic group. These are composed of specific organic compounds with definite physical and chemical properties. Some of the non-humic substances are synthesized whereas the others are modified by the action of microorganisms. The non-humic group comprises of:
Polysaccharides: These are polymers with a general formula of Cn (H2O) and sugar-like structure. Polysaccharides enhance the stability of soil aggregates.
Polyuronoides: These substances are synthesized by soil microbes, are present in their bodies, and after the death of microbes these are either modified by further action of microbes or interact with other organic material in the soil
Simpler Compounds: In addition to above mentioned compounds some simpler compounds such as organic acids and protein-like materials are also present in the non-humic group, but in smaller quantities. However, these are very important as these affect the availability of some important plant nutrients such as iron and nitrogen.
Formation of Humus
The formation of humus can be studied in a thickly populated deciduous forest very easily.
Litter or 01 Layer
Every year thousands of tons of leaves, twigs. Etc., fall on the ground during autumn and accumulate. This fresh, dead organic matter is called litter or 01 layer. The larger pieces of plant residues are broken by soil fauna such termites and earthworm. Those smaller sized plant residues can be easily attacked by soil decomposers (bacteria, fungi, actinomycetes).
Duff or 02 Layer
This partially decomposed litter is known as duff or 02 layer. Simple sugars, most proteins, amino acids and certain polysaccharides decompose very quickly and consumed within days. Complex organic molecules such as lignin and larger proteins are, first broken into smaller units and then into simpler organic compounds under the influence of enzymes secreted by microbes. Lignin decomposes very slowly but plays a very significant role in the formation of humus. It is degraded to simpler phenolic compounds which interact with amino compounds to form a resistant component of humus. Fresh litter fallen next year covers this litter layer in which decomposition is in progress.
Humification — Humus Formation
After the normal biologic processes of decay decompose litter, the resultant product becomes incorporated into the mineral soil. It is in the form of amorphous organic matter and is called humus and the process leading to its formation is humification.
The organic material degraded to humus is not contributed by aerial plant parts only but some of the material is derived from dead roots and soil organisms as well. Also, products synthesized by soil micro-organisms become part of the humus. Therefore, humus is chiefly composed of those organic residues of the litter that have resisted decay the longest, but it also includes organic wastes synthesized by soil organisms. Finally all these compounds are converted into carbon dioxide, water, and minerals, a process known as mineralization.
Importance of Humus
Some of the important effects of humus are as under:
Source of Mineral Nutrients
All plants take up minerals from the soil and synthesize them into the complex organic compounds of which their tissues are composed. The humus is decomposed to inorganic minerals (Mineralization). These mineral forms of nutrients are converted into organic forms, i.e., plant useable forms such as SO4, NH4 and PO4. This process is called immobilization. Therefore, the humus is an important source of mineral nutrients and makes the soil fertile.
Source of Food for Soil Organisms
The green plants absorb small amounts of soluble minerals from the soil and in turn greater quantities of organic matter such as celluloses, lignin, starches, sugars, fats and proteins as humus. These compounds are source of energy (food) for soil organisms (saprophytes), and in turn of organisms which are parasites of these saprophytes.
Improvement of Physical Conditions of the Soil
Humus helps in formation soil aggregates because it occurs as coating over the particles, therefore provides excessive stickiness to them. Thus, the humus helps maintain soil porosity and allow downward movement of water in the soil. If the soil organic matter decreases below a critical level, soils tend to become hard, compact, and cloddy.
Improvement of Water Holding Capacity
Due to its colloidal nature, the humus has a relatively high water holding capacity. The humus particles can hold water 20 times greater than their mass, a proportion considerably higher than clay particles. The humus prevents drying and shrinkage of soil separates. Similarly, addition of humus to the sandy soil improves moisture retention capacity of sandy soils.
Improvement of Adsorptive and Cation Exchange Capacity of the Soil
Humus, like clay, can hold larger quantities of nutrients in ionic form by adsorption due to their higher surface areas (900 x 103 m2/kg). This results in increase in cation exchange capacity of the soils (1500-3000 mmol/kg).
Because nutrients are retained in exchangeable form, humus buffers soil pH in the slightly acidic, neutral, and alkaline ranges. This helps maintain a uniform reaction (pH) in the soil.
Source of Toxins
Certain crop residues, e. g., those of Melilotus alba (sweet clover) yield water-soluble organic compounds which depress germination and growth of Zea mays (maize).
Interaction with Organic Chemicals
The soil organic matter interacts directly with pesticides and herbicides and influence the behavior of these agrochemicals present in the soil. It effects toxicity, leaching ability, and biodegradation of these chemicals. More agrochemicals are required for the humus rich soils than the mineral soils.
Direct Effects on Plant Growth
Under certain circumstances, the humic compounds can stimulate plant growth. Certain substances are present in the humus which stimulates the growth. Also, humic acids increase permeability of the cell membrane which helps in increase in uptake of water and mineral nutrients. Increase in root length has been noticed in water -medium to which humic acid is added.