Humus

 

Humus 

Humus is dark, organic material that forms in soil when plant and animal matter decays.  Humus is the organic component of soil, left over after plants and animals have undergone a long process of thorough decomposition done by earthworms, bacteria, fungi, and other microorganisms. It is found in the top few inches of soil. It is the portion of organic matter that is mostly resistant to decomposition and remains in the soil. The color of humus is brown or black, and it has a loose, crumbly, and spongy texture.  Humus is composed of small particles, with huge surface area. These particles have a very great capacity to retain and supply nutrients, as well as hold water.  When humus is in soil, the soil will crumble. Air and water move easily through the loose soil, and oxygen can reach the roots of plants.

Humus contains the elements necessary for plant growth: nitrogen, phosphorus, sulfur, calcium, magnesium, and potassium.  Humus has high nutrient content that improve the health of soil. The ratio of carbon to nitrogen (C:N) of humus ranges between eight to fifteen.  Depending on the original plant and animal material, the chemical composition of humus varies.

Humus formation

Humus formation is a biological process.  Humus formation occurs in two steps. Initially, the organic substances and minerals in the soil disintegrate by the action of microbes, nematodes, and earthworms. Next, totally new combinations of these broken-down products develop and the humus is formed.

Humus can be made in two different ways: naturally and by human activity (Composting).  If it is formed naturally, the plant and animal remnants get decayed naturally by microorganisms and if manual intervention is there, collected organic material is left at a place to allow it to be decomposed by microorganisms. Either way, the process of humus formation is the same.

Naturally, humus is made over a longer period of time where plant and animal materials decompose in a wide range of environmental conditions. Since man-made humus is made under controlled aeration with the right amount of moisture and heat in a compost pile or bin, the decomposition process occurs in a faster rate.

In the early stages of decomposition, some of the carbon, hydrogen, oxygen, and nitrogen in the decaying materials are released as water, carbon dioxide, methane, and ammonia and all these results in the typical rotten compost smell. As the decomposition progresses, the organic materials are broken down further into more stable forms of nutrients that plants can use as food. 

Compost is like humus and is made of decomposed organic material. Compost usually refers to material created by people from leftover foods and yard waste and Humus refers to the natural decay of material such as leaves in the soil's top layer.

Humification: - A substantial portion of the organic matter in the soil is decomposed by microorganisms into inorganic minerals.  This is termed as mineralization. Nitrogen and other nutrients in decomposed organic matter are recycled in this process. Sometimes, depending on the conditions of decomposition, a portion of the organic matter does not mineralize and is instead converted into humus. Humus is concatenations of organic polymers. This process of creating humus is called humification.  These organic polymers are stable and are immune to the action of microorganisms. The rate of humus formation and its accumulation are influenced by nature and type of microorganisms in decomposition, temperature, moisture, aeration and pH of soil.

Humus include

Fulvic acid is a yellow to yellow-brown humic substance of low molecular weight that is soluble in water under all pH conditions

Humic acid is a dark-brown humic substance that is soluble in water only at higher soil pH values and is of greater molecular weight than fulvic acid. Humic acid may remain for centuries in undisturbed soil.  Humic acid bind to and help plant roots receive, water and nutrients.  High humic acid levels help to increase crop yields. 

Humin is a black humic substance that is not soluble in water at any pH, has a high molecular weight. Based on humin, humus is classified into mor, mull, and moder.

Mor humus is also known as raw humus condition. It occurs when the soil has very few microorganisms or animals such as earthworms are present. These animals aid the decomposition of the organic matter found under the soil. Mor is characteristic of podsol, a type of soil.  It is formed at a low pH (below 3.8-4.0). Thus, organic matter in acidic conditions does not decompose rapidly due to inhibition of microbial activities. Organic matter thus accumulates as peat or muck. This is a compact, tough, black or brown peaty material in which the individual fragments are easily recognized microscopically as plant material

The second type of humus is the mull humus. This usually occurs in hardwood forests or grassland where the weather is warm and humid and there is a rich population of bacteria, insects, and earthworms in the soil. The humus tends to decompose quickly and mix thoroughly with the minerals in soil. Mull humus is having a high pH level (Above 5). It is microbiologically rich than mor. Since microbial decomposition is rapid, microbial remains are recognizable in mull. It is amorphous and colloidal in nature.

The moder humus lies in between the two extremes of the mor and mull humus. Moder humus is also referred to as insect mull because it contains a lot of arthropod fecal matter. These materials tend to bind the plant and mineral particles to form a netlike structure. The moder humus has more organic material than the mull, but it is unable to mix well with the mineral substances in the soil.

Benefits of Humus

·         The conversion of soil organic matter to humus provides nutrients to microorganisms and other soil fauna. The rate at which soil organic matter is converted to humus promotes (when fast) or inhibits (when slow) plant, animal, and microorganism coexistence in the soil.

·         Humus is a colloidal material and boosts soil's cation exchange potential, so nutrients are stored by chelation. As a result, these nutrient cations are available to plants, they are retained in the soil but are not washed away by rain or irrigation.

·         Humus can retain the moisture equivalent of 80–90% of its weight in water, increasing the soil's ability to survive drought.

·         Humus' biochemical composition allows it to buffer, or moderate excessively acidic or alkaline soil conditions.

·         Humus' dark colour, which is normally brown or black, aids in the warming of cold soils in the spring.

·         Humus sequester carbon and thus can help to mitigate climate change.