| Soils, Crops and Fertilizer Use |
|Chapter 1: Down to earth - Some Important Soil Basics|
Note that "tropical" clays don't necessarily make up the major portion of the clay in all soils of the tropics. In fact, temperate clays are surprisingly common, especially in younger soils or those formed under drier conditions or where drainage isn't good. A true tropical soil (one whose clays are mainly I:1 or hydrous oxides) requires good drainage, centuries of weathering, and lots of rainfall and leaching to form. Iikewise, not all clays in the temperate zone are 2:1 clays, especially in areas that may have once been tropical thousands of years ago. Some soils are mixes of both types.
Spotting "tropical" soils: A distinct red or yellow color, especially in the subsoil may be one indication. Such soils are unlikely to form in depressions but are found on gentle to steep slopes where drainage is good.
The extent of tropical soils in the tropics: Overall, true tropical soils account for about half the soils in the tropics and often exist side by side with "temperate" ones. They're fairly diverse themselves and are grouped into 2 broad categories based on the current USDA (U.S. Dept. of Agric.) soil classification system:
• Ultisols: Their clays are mainly 1:1 types, along with varying amounts of hydrous oxides of iron and aluminum, and their workability is usually goad. They are moderately to very acidic and may have a high capacity to "tie up" added phosphorus, preventing its full use by plants.
• Oxisols: The most strongly weathered and leached of all soils. They're acidic and have high clay contents (mainly of hydrous oxides), but don't tend to be very sticky when wet. Like ultisols, they may tie up added phosphorus readily. One well known member of this order (group) are laterite soils whose subsoils are rich in a clayey material called plinthite that contains red mottles (blotches) and highly weathered oxides of iron and aluminum. Plinthite can harden irreversibly into ironstone (formerly called laterite) when exposed by erosion, as has occured following deforestation. Note that true laterite soils at risk of ironstone fomation are estimated to amount to less than 10% of all tropical soils.