The tropics are defined as the area of the earth running from the Topic of Cancer in the north to the Tropic of Capricorn in the south. Within this area, there are many types of ecosystems and their associated soils. Soils will vary according to local geological and climate factors, but there are two major ecosystems across the region that have some common soil characteristics. These are tropical rain forests and tropical savannas or grasslands. The climates affecting these ecosystems are generally hot with distinct wet and dry seasons.
In general, the older the land form, the poorer the soil. Many tropical rain forests and savannas are among the oldest ecosystems on earth today. This means that over the millennia, the soils have been leached of nutrients during the rainy seasons. Also in the warm, wet climates of the tropics, aerobic bacteria are active and break down plant material faster than the plants can replace it. This results in a very thin and poor humus layer in the soils. So although rain forests in particular may look rich because of the great diversity of plant life, when the forest is cleared, the soil is so poor that it is rapidly depleted and the ultimate result of clearing is the creation of a desert. Rain forests of Australia and the Amazon are old and the soils are poor. In contrast, the volcanic soils of the Central American rain forests are both young and rich with nutrients.
Soils throughout the tropical rain forests are classified into two basic types. The first are called ultisols and are old, weathered, acidic red clay soils which are deficient in nutrients such as calcium and potassium. The other type are called oxisols. These soils are also acidic, old, red in color and highly weathered and leached The main difference is that they are relatively well-drained compared to ultisols. The clay content of ultisols is high, making it difficult for water to penetrate and flow through. The reddish color of both soils is the result of heavy heat and moisture forming oxides of iron and aluminum.
In both tropical rain forests and savannas the key to plant diversity is not soil fertility but the high rate of nutrient turnover. Because of the heat and moisture, bacteria and other decomposers work quickly to turn dead plant and animal material back into nutrients that plants can then take up and reuse.
In general then, the characteristics of tropical soils are that they are clay based and acidic in their pH. They are poor in nutrients because they are old. They may or may not be well drained depending on the composition of the clay. They are usually reddish in color because of the high iron oxide content. It must be emphasized though that there is a lot of variation in tropical ecosystems so these are just generalisations, and exceptions can always be found.