To elucidate the characteristics of tropical soils, Prof. V. Timmer contrasted young boreal and temperate soils such as forest podzols and prairie chernozems, with the old, highly weathered soils of the tropics, such as oxisols and ultisols, which are high in kaolinite and oxide clays. Whereas acidity and the rate of litter decomposition are low in temperate and boreal regions, decomposition and leaching are rapid in the tropics. As a result, tropical soil nutrient reserves are low. Nutrients are instead concentrated in above ground biomass and necromass. Consequently, the characteristics that limit tropical soil fertility are quite different than those limiting the productivity of younger soils in cool climates.
Tropical soils have low cation exchange and water retention capacities.
Because tropical clays tend to have net positive charges that are often pH-dependent, these soils have an anion exchange capacity often lacking in younger soils. As a result, nitrate is not the limiting factor for fertility in the tropics that it often is in temperate and boreal environments. But since tropical soils do not chemically retain nutrient cations, essential nutrients are rapidly leached from the system. This is the root of tropical fertility problems. Because nutrients are concentrated above ground, with little accumulation of organic matter in soil profiles, forest nutrient cycles are delicate.