How Does Leaching Create Laterite and Other Weathered Soils?

Leaching is the process where water dissolves soluble minerals and transports them downward through the soil profile. In areas with very high rainfall and high temperatures (like tropical rainforests), this process is extremely intense and is the primary mechanism that creates highly weathered soils like laterites.

The Process of Laterization:

1. High Rainfall: A tropical climate provides a massive and constant flow of water moving down through the soil.
2. Dissolving Soluble Minerals: This warm, percolating water acts as a solvent. It easily dissolves and removes the most soluble minerals from the soil. These include vital plant nutrients like calcium, magnesium, potassium, and sodium.
3. Desilication: The process goes even further. The water is so abundant and chemical weathering is so rapid that it even begins to dissolve and leach away silica (a primary component of many minerals, including clay).
4. Concentration of Insoluble Minerals: The only materials that are *not* soluble and cannot be leached away are iron oxides and aluminum oxides.
5. Formation of Laterite: Over thousands of years, this intense leaching strips the soil of almost everything *except* these iron and aluminum oxides. This leaves behind a soil profile that is:
   • Extremely infertile (all nutrients are gone).
   • Very acidic (all alkaline minerals are gone).
   • Distinctively red or yellow due to the high concentration of iron oxides.
   • Very rich in aluminum (bauxite, the primary ore of aluminum, is a type of laterite).

When this laterite soil is exposed to the sun and dries, it can harden into a brick-like, irreversible crust, making it useless for agriculture. Other leached soils formed in cooler, temperate climates include Spodosols (where iron and humus are leached from an E horizon) and Ultisols (highly leached, acidic forest soils, but not as extreme as laterites).