Tropical soils are found in tropical regions between the Tropic of Cancer and the Tropic of Capricorn. These soils are distinctive to their area due to natural factors, such as the ocean, wildlife and decomposing minerals found in the earth around the equator. Understanding the characteristics of tropical soils will help you to identify what you need to grow specific plants in the tropical region.
The kaolinite (clay) soil that is commonly found in tropical regions has very good drainage. Due to the large micelles (phospholipid compartments within the soil) that are tightly bound together, the clay in tropical regions does not expand with the absorption of water. The soils of temperate regions that contain clay also often contain the element montmorillinite. This material absorbs water and expands and contracts as heat is applied. Temperate region clay is sticky when wet, unlike the firm structure of kaolinite clay in the tropics.
Low Base Saturation
A base is the term used to describe the alkaline extreme on the pH balance chart. There are equally spaced numbers, 0 through 14 that represent either an acidic element or an alkaline element within any given compound. You can measure pH within the soil to determine whether your plants will thrive or will need a balance component, such as organic material to provide the best environment possible. Having a low base saturation simply means that tropical soils are more likely to be acidic in nature rather than alkaline. This would give them a reading of 0 through 6 on the pH scale.
Low Cation Exchange
The cation exchange relates to the ability of the soil to hold specific nutrients required for plant life to grow. Often times a low cation exchange capacity, or CEC, will result in a sandy soil that has difficulty holding water as well as minerals. Tropical soils are more likely to have sandy structure; however, Purdue University explains the low CEC within tropical soils is due more to the lack of nutrients since the soil itself is comprised of quality clay. The clay soil must be treated with organic material and other nutrient-enriched fertilizer to provide the best growing environment for native plants.
closeup of dried out brown soil image by Victor B from Fotolia.com