Lateritic Soils in the Tropics: The Problems and Management Possibilities
The soil name "laterite" comes from a Latin word "later" meaning brick. It is an appropriate name, as soils under this classification are characterized by forming hard, impenetrable and often irreversible pans when dried (Soils and Soil Fertility 1993). Lateritic soils are also characterized by their low soil fertility. Due to the high rate of weathering, and resulting low charge minerals, the soil is unable to retain the nutrients needed for plant growth (Ibid., Coleman 1989).
Laterite soils have many names. In the United Nations Food and Agriculture Organization they have been named Ferralsols and Acrisols, and in the United States Department of Agriculture Soil Taxonomy, Oxisols and Ultisols. Ferralsols, like Oxisols have high iron and aluminum oxide contents, whereas Acrisols and Ultisols are characterized by extreme stages of weathering (Oades 1989).
Below, we take a look at the formation of lateritic soils, the importance of biota for soil fertility and some suggestions for laterite management.
Both climate and parent material are important in the pedogenic processes which go into the formation of lateritic soils. Laterite soils are formed in moist, well-drained, tropical conditions (usually in areas with a significant dry season) on a variety of different types of rocks with high iron content. (See Appendix 1.) Initial stages of weathering lead to the formation of kaolinite and iron oxyhydroxides. Micro and macro-level movements of iron through soil minerals also begins to occur (Nahon 1986).
Next, mottled clay layers are formed. Iron oxyhydroxides continue to migrate within the soil profile, becoming more crystallized in the top horizons and eventually forming large iron-rich nodes. These nodes are surrounded by yellow or white iron-depleted zones. The formation of the iron nodes occurs in a soil matrix made up of mostly weathered kaolinite and quartz and is called the "mottled clay layer." The soil at this point contains large tubules and alveoles, formed during the creation of the bleached zones (Ibid). (Appendix 2, horizon III.)
As weathering continues, these voids begin to fill with kaolinite and quartz grains. Further weathering leads to the ferruginization of these fill particles, as kaolinite and quartz are replaced with Fe, and purple to red indurated facies are produced. These processes complete the formation of the ferricrete soil horizon (Nahon 1986). (Appendix 2, horizon V.)
Often ferruginous pebbly layers are formed in close association with the ferricrete horizon. (Appendix 2, horizon VI.) They are formed from the break-down and downslope movement of pisolitic iron crusts. The end result is a "reddish matrix" made from kaolinite, goethite and "fragments of the pisolitic iron crust " (Ibid: 180).
The amount of time needed to create a fully developed lateritic soil is still unknown, but guesses range...