Ferralsol

A Ferralsol is defined by extreme weathering: a deep, uniform, oxide-rich subsoil in which most weatherable minerals and soluble nutrients have long since been removed. The dominance of iron and aluminium oxides gives strong, water-stable aggregates and vivid red or yellow colours, but also the low nutrient-holding capacity that limits fertility.

Ferralsols form on old, stable land surfaces in the humid tropics, where high temperatures and abundant rainfall drive intense chemical weathering and leaching over very long periods. This progressively strips out silica and base cations, concentrating resistant iron and aluminium oxides. They are extensive in the humid tropics of South America, notably the Brazilian and Amazonian uplands, and of central Africa, with occurrences in parts of tropical Asia.

Oxide dominance

Iron and aluminium oxides dominate the mineralogy, producing red and yellow colours and stable aggregates.

Low fertility

Low-activity clays give a small cation exchange capacity and poor retention of nutrients, so natural fertility is low.

Phosphorus fixation

The oxides strongly bind phosphorus, so applied phosphate can become largely unavailable to crops.

Good structure and drainage

Strong aggregation gives friable, well-drained, physically stable soil despite a high clay content.

Sustaining crops on Ferralsols depends on overcoming their chemical limitations while conserving the organic matter and structure that hold the system together.

• Manage acidity with lime and address possible aluminium toxicity to allow sensitive crops to root
• Supply phosphorus carefully, recognising that much can be fixed by the oxides, and use efficient placement
• Maintain organic matter and continuous cover to protect the small nutrient reserve and prevent erosion
• Favour perennial and deep-rooting crops and agroforestry, which suit the deep, well-drained profile and recycle nutrients

With appropriate management, Ferralsols support important tropical crops, especially those tolerant of low fertility and acidity or grown as perennials that recycle nutrients. Cassava is well adapted to these soils, and oil palm, coffee, rubber, and pineapple are widely grown on Ferralsols across the humid tropics.