What makes the soil in tropical rainforests so rich?

Category: Earth Science
Published: July 12, 2013

By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University

Actually, the soil in tropical rainforests is very poor. You would think with all that vegetation, warmth, and moisture that the soil must be very rich. But the truth is otherwise, as people who live in these regions are well aware. According to the textbook "Tropical Rainforests: Latin American Nature and Society in Transition" edited by Susan E. Place, there are several reasons for the poor soil of tropical rainforests:

Public Domain Image, source: NASA.

1. The soil is highly acidic. The roots of plants rely on an acidity difference between the roots and the soil in order to absorb nutrients. When the soil is acidic, there is little difference, and therefore little absorption of nutrients from the soil.
2. The type of clay particles present in tropical rainforest soil has a poor ability to trap nutrients and stop them from washing away. Even if humans artificially add nutrients to the soil, the nutrients mostly wash away and are not absorbed by the plants.
3. The high temperature and moisture of tropical rainforests cause dead organic matter in the soil to decompose more quickly than in other climates, thus releasing and losing its nutrients rapidly.
4. The high volume of rain in tropical rainforests washes nutrients out of the soil more quickly than in other climates.

When farmers cut down tropical rainforests and use its soil to try to grow crops, they find little success because of the poor nature of the soil. The textbook quotes soil authority Robert Pendleton as saying,

"In higher latitudes, and particularly in the United States, a widespread opinion prevails that such humid regions as the enormous Amazon basin, now occupied by luxuriant and apparently limitless tropical high forests, must certainly have rich soils, and hence, great potentialities for the production of food, fiber, and other agricultural crops...on the whole, the soils of the humid equatorial regions have distressingly limited possibilities for plant production... This pessimistic attitude is no longer a result of mere opinion, for in a number of widely scattered regions in the humid low latitudes agricultural scientists have been and still are seriously at work."

If the soil is so poor in tropical rain forests, how does such a dense array of shrubs and trees grow there? The answer lies above the soil. On the ground of the rain forest, there is a thick layer of quickly decaying plants and animals. Nutrients are washed by the heavy rains almost directly from the rotting surface material into the the trees without entering the soil much.

Topics: clay, climate, decomposition, dirt, rainforest, soil, tropical rainforest