Tropical Forests and Grasslands (Savanna)
Tropical soils are formed in areas with high annual temperature and rainfall. Even though the savanna and the tropical rainforestes are VASTLY different in organisms and extent, they both have a climate that results in deep, highly weathered soils. The intense weathering causes these soils to be nutrient poor and low in organic matter.
The savannas are grasslands that have several months of dryness, followed by a rainy season. A majority of the soils in this area are Alfisols and Ultisols. These soils are very old and low in fertility, but since there is a dry season, more of the nutrients can stay in place.
In the tropical rainforest, however, rainfall is year round, and can be daily. This strips out most of the nutrients. Many of these soils are Oxisols and Ultisols. In an oxisol, even the clays have been leached out of the soil, and replaced with aluminum oxides. Ultisols still contain clays. Both of these types of soils have weathered for thousands of years to create the characteristic red and yellow soils of Africa, Australia, South America, and Southeast Asia. Both types of these soils are low in organic matter.
The savanna biome has very tall grasses with open shrubs and trees. Because of its lush grasses, large herds of zebra, elephants, and wildebeast can be sustained. These herds are large and vast, and can support large predators and scavengers, like hyenas and lions. Termites are also very common, and create large mounds using nothing but their saliva! These mounds can be the height of a three story building!
The tropical rainforest is one of the most biodiverse regions in the world! Even though there is a ton of lush vegetation, very few nutrients are in the soils. Most of the nutrients are trapped in lush vegetation and trees. Because it is so warm, when a creature dies, it is immediatly decomposed and turned into new plant growth. There are some creatures that spend their entire lives never touching the soil! A lot of fruit (bananas, mango, papayas, etc.) come from this part of the world.
Relief and Parent Materials
The topography here varies from flat, swamp like regions to steep mountain cliffs. These soils were formed by bedrock that has weathered in place over a long period of time, or from river sediments.
These soils are very, very old.
Issues in Tropical Soils
Slash and burn (shifting cultivation) agriculture is commonly practiced in this region. A forest is cut for fuel for cooking fires. The remaining vegetation is then burned off. This process releases nutrients from trees that plants need to grow. The area is cropped for a few years until the nutrients run out, and then will be abandoned so vegetation can grow again.
After 20-25 years, the land owners return again. However, because of an increase in people needing food, the farmers are forced to return to lands before they are ready. This can cause soils to become too poor to even grow native vegetation. This is called deforestation. Without vegetation, the soils dry out and become desert like, even though they get plenty of rain. This can cause severe erosion.
Where to Find Tropical Soils
The savannas are commonplace in the Sahel region (sub-Saharan Africa). Tropical rainforests are found in Africa, Central and South America, Southeast Asia, the Pacific Islands, and the northern tip of Australia.
For more information on tropical soils, including downloadable PowerPoints, assessment questions, and educational links, please visit the SCOOP! Teachers Guide.