One look at the myriad of plant life that grows in tropical soil might lead one to believe that it's among the most fertile soil in the world. Sporting lush green forests and flowers of every color, it's easy to think that it must be full of nutrients. But, some of the characteristics typical of tropical soils might be surprising.
Poor Soil Nutrients
In spite of lush, verdant plant life, tropical soils are usually notoriously poor in soil nutrients. In fact, it's because of the plant life that the soil is so poor. At any time, most of the nutrients and minerals that might be contained in other soils is tied up in sustaining the area's vegetation. Matter decays quickly, and the nutrients that are released into the soil are turned around almost immediately and reabsorbed in a process called nutrient recycling.
This is one reason why the destruction of forests in order to create more fields for farming results in reduced fertility, soil deterioration, loss of minerals and nutrients and erosion after only a few years of planting.
Most tropical soils--especially those in the Amazon--are considered senile, or old, soils. What this means for the soils is that they are very nutrient impoverished. With no volcanic activity in most tropical regions, there is no way to bring up a new, fresh supply of nutrients to the surface. What is left is old nutrients that have been recycled through living plants and decaying matter countless times.
Much of the Amazon soil originally came from old river beds and flood plains, making the soil sandy. Because of this sandy quality, what nutrients that are in the soil are quickly leached away by the extreme rainfall that the tropics are subjected to. Without the heavy vegetation to stop nutrients from being leached away, the soil quickly becomes barren and unable to support other life.
Aside from the sandy soils left from dried river beds, much of the other soils of the tropical forests is clay. These clays are not only low in essential nutrients like calcium and phosphorus, but they are also extremely acidic. This acidity results in another phenomenon called chemical weathering. Chemical weathering takes place in a number of different areas in a number of different forms, but that which is particular to the tropical soils is oxidation. The clays are rich in aluminum and iron, which then reacts with oxygen. This causes oxidization, which gives the soil its unique red and yellow coloring (from iron and aluminum respectively). This process is sped up by high rainfall and the extreme temperatures characteristic of the tropical forests.