In the USDA soil classification, the soil order characterised by a very high content of organic matter (such as peat and bog soils) is:

Histosols are soils made up mainly of organic matter, such as peat and muck. They are one of the 12 soil orders of the USDA Soil Taxonomy. The name comes from the Greek "histos", meaning "tissue", referring to the plant tissue that makes up these soils.

Characteristics of Histosols:

• They contain a very high proportion of organic matter, usually more than about 20 to 30 per cent by weight, in a thick surface layer (generally 40 cm or more).
• They form in waterlogged places, such as bogs, fens, marshes, swamps and poorly drained depressions, where lack of oxygen slows the decay of plant material.
• They are dark brown to black, spongy and light in weight when dry.
• They are usually acidic and low in some nutrients.
• They have a very high water-holding capacity.
• They shrink and can subside when drained, as the organic matter oxidises.

Where they are found:

• In cool, wet regions, such as northern Canada, Alaska, Scandinavia, Russia, Ireland and Scotland (peat bogs).
• In tropical swamp forests, such as parts of Indonesia and Malaysia.
• In India, organic (peaty) soils occur in small areas, such as the Kuttanad region of Kerala, the Sundarbans, coastal Odisha and some Himalayan wetlands.

Uses and importance:

Peatlands store huge amounts of carbon, about twice as much as all the world's forests, although they cover only about 3 per cent of the land surface. When drained or burned, they release large amounts of carbon dioxide.

Some other USDA soil orders:

• Aridisols: soils of dry (arid) regions, often with salt or lime layers.
• Entisols: very young soils with little or no horizon development, such as recent alluvium and sand dunes.
• Alfisols: moderately leached soils with a clay-rich subsoil, common under forests.
• Inceptisols: young soils with weakly developed horizons; the most widespread order in India.
• Mollisols: dark, fertile grassland soils.

Why not the other options?

• ✗Aridisols are dry soils, low in organic matter.
• ✗Entisols are young, poorly developed soils.
• ✗Alfisols are moderately leached forest soils.