The heavy, dark soil associated with floodable valley bottoms is a

Hydromorphic soil is the heavy, dark soil associated with floodable valley bottoms. The term "hydromorphic" literally means "water-shaped," indicating soil that has been significantly influenced by the presence of water.

Hydromorphic soils develop in areas where waterlogging is frequent or prolonged, such as river floodplains, valley bottoms, swamps, and deltas. They are characteristically heavy (high clay content) and dark in colour due to the accumulation of organic matter that decomposes slowly under waterlogged, anaerobic conditions. These soils often have a gleyed horizon (grey-blue layer) resulting from the chemical reduction of iron in oxygen-poor conditions.

The other soil types form under different conditions:

• Lateritic soil is a reddish, iron-rich soil formed by intense tropical weathering in well-drained upland areas.
• Ferrallitic soil is a deep, heavily leached tropical soil rich in iron and aluminium oxides, typically red or yellow.
• Podzolic soil forms in cool, humid climates under coniferous forests, with a distinctive leached, ash-grey layer.