When we talk about rainforests we don’t often think about the soil. Why is soil important?
It provides for our food by way of farming, it is the home for a remarkable array of organisms (biodiversity), including many we have yet to discover; it stores and filters water which eventually becomes our drinking water, and last but not least it is an important key to global well-being.
Because of high diversity and large amounts of foliage in tropical rainforests, you might think that rainforest soils are rich in nutrients. However, it’s just the opposite.
Tropical soils are often several meters deep but because of the heavy rains, the soils are often washed out, or strongly leached, with large amounts of nutrients and minerals being removed from the subsoils. Over many millions of years this leaching has left most of the soils lacking many of the fundamental nutrients needed by the above ground vegetation. So, the nutrients are stored in the vast numbers of trees and plants rather than in the soil. The thin top soils are typically a product of decaying vegetation and animal remains.
The rainforest depends on the constant recycling of its enormous biomass for it’s nutrients. Constant warmth and moisture promote rapid decay of organic matter.
For example, when a tree dies and its trunk falls to the forest floor, it decays and the nutrients it contains are recycled. However, if trees are removed from the forest, the nutrients are removed with it, along with the protection provided by the soil binding tree roots and the forest canopy that acts as an umbrella from rain. The unprotected soil is then simply washed away in heavy rains, causing blockages and floods in lowland rivers, while leaving upland rivers dry.
When tropical forests are cut and burned, heavy rains can quickly wash the released nutrients away, leaving the soil even more impoverished.
Cutting down the rainforest disturbs the natural soil-plant cycle and makes the soils extremely vulnerable to soil erosion and loss of this vital topsoil.
Already in the last 50 years there has been increasing evidence of mismanagement of our soils. Soil erosion and desertification is widespread and affecting millions of hectares of land.
Intensive farming, while producing increasing crop yields to feed the growing population, has put increasing pressure on the soil and this has led to soil degradation. It is more and more important that we understand the pressures that we are putting on our soils and take steps to safeguard them as we go into the future.
The topsoil also holds huge amounts of carbon which is now known to have a major potential influence on CO2 levels in the atmosphere and hence a major potential influence on climate change.