The dark, decaying organic matter in the soil, formed from the decomposition of plant and animal remains, is known as:

Humus is the dark brown or black, partly decomposed organic matter in the soil. It is formed when microorganisms such as bacteria and fungi break down dead leaves, roots, animal remains and other organic material over time.

How humus is formed:

• Plant and animal remains fall on or into the soil.
• Soil organisms, including earthworms, insects, bacteria and fungi, break them down.

The more easily decomposed materials are consumed first, leaving behind a complex, stable, dark substance that resists further decay. This is humus.

The process of forming humus is called humification.

Importance of humus:

• Fertility: it is a store of plant nutrients, such as nitrogen, phosphorus and sulphur, which are released slowly as it decomposes further.
• Water retention: humus can hold several times its own weight in water, helping soils retain moisture during dry periods.
• Soil structure: it binds soil particles into crumbs (aggregates), improving aeration, drainage and root growth, and reducing erosion.
• Nutrient holding: it has a high capacity to hold positively charged nutrient ions, such as calcium, magnesium and potassium, preventing them from being washed away.
• It improves biological activity by providing food and habitat for soil organisms.
• It gives soil its dark colour, which helps it absorb heat.

Humus content in Indian soils:

Most Indian soils, especially in hot and dry regions, are low in organic matter, because high temperatures speed up decomposition.

Ways to increase soil organic matter:

• Adding farmyard manure, compost and vermicompost.
• Growing green manure crops.
• Leaving crop residues on the field instead of burning them.
• Mixed farming with livestock.

Why not the other options?

• ✗Loam is a soil texture class, a balanced mixture of sand, silt and clay; it is not organic matter.
• ✗Gravel is made of small rock fragments larger than 2 mm.
• ✗Kankar is a layer of calcium carbonate nodules found in some soils of dry regions.