Understanding Tropical Soils: Part 2

Continued from Understanding Tropical Soils: Part 1

by Dexter B. Dombro

 

Sandy tropical soil has almost no CEC.

Now that we know the two most serious handicaps faced by tropical foresters, the low cation exchange capacity (CEC) and the acidic pH of the soil, the question is what can we do about it? The answer, surprisingly, can be found in the past. Spanish and Portuguese conquistadors noted that some indigenous communities in the Amazon had highly productive, multi-year food cultivations. This phenomenon continued into the present, at which point some scientists started to wonder why certain Amazon River basin communities had dark, productive soils. The term Terra Preta or black soil was coined from Brazilian Portuguese to describe this phenomenon. Archaeologists and others conducted excavations near indigenous Amazon communities, and discovered that the native people in pre-conquest South America had mixed charcoal and clay pottery shards into the soil they were cultivating. Since then, similar observations have been made in Africa and Asia at ancient sites.

Black soil in the tropics with biochar.

Very quickly scientists realized that those ancestors had known something we have forgotten. Charcoal in the soil can act as a retention agent, stopping nutrients, micro-fauna and fertilizers from being leached from tropical soils during heavy rainfalls. It allows organic material in the soil to build up, thereby providing plants and trees with a much better and more productive natural environment in which to grow. Clay has a similar retaining capacity, which would explain the use of the broken pottery shards. Obviously, the charcoal from fires and broken pottery were added to the soil as a means of creating Terra Preta in ancient times, thereby boosting food production and the quality of cultivations in pre-conquest indigenous communities. This led scientists and modern agriculturists to ask the obvious question: how can modern planters benefit from this knowledge? The list of benefits, as it turns out, is amazing and can be applied anywhere in the world, not just in the tropics.

Charcoal in the soil can act as a retention agent, stopping nutrients, micro-fauna and fertilizers from being leached from tropical soils during heavy rainfalls. It allows organic material in the soil to build up, thereby providing plants and trees with a much better and more productive natural environment in which to grow. 

Charcoal can be made of any organic matter.

    Charcoal when embedded in the soil has a half-life of 1,000 years. Why is this important, you ask? Simply put, this means that charcoal obtained from organic matter and woody biomass can be sequestered in the soil for centuries, making it an extremely effective and potent way of capturing and storing atmospheric carbon on a very long term basis. Done on a worldwide basis this could be an important tool in the struggle against climate change.

Note dark soil near surface - nutrients.

The charcoal can’t be placed in the soil on its own. It first needs to be charged with an organic fertilizer, like compost, cow manure or cow urine (urea). Charcoal is negative and on its own would attract all the nutrients in the soil, taking them away from the plants and trees. However, once charged, it becomes a potent agent for retaining and holding nutrients, organic material and therefore micro-fauna in the soil. A new term has been adopted to describe charged charcoal for agricultural use: biochar.

Worldwide interest in biochar is huge.

The process of making biochar also results in the production of wood ashes. Those ashes, when added to the soil at the same time turn out to be full of essential elements required by trees, such as boron (Chemical: B), phosphorus (P) and potassium (K). But perhaps more importantly, wood ashes are almost 10 times more alkaline than lime, meaning that in controlled applications they can reduce or neutralize soil acidity, thereby greatly enhancing agricultural productivity and the range of species that can be cultivated. 

On the economic side of the equation, biochar can be produced using local resources. In India rice husks are charred, charged with cow urine and then added to the soil. Virtually any organic material can be charred and processed. This means that poor farmers, tree planters and communities in developing countries can not only enhance and improve their own soils, but also fertilize them with local resources, thereby eliminating high and environmentally unfriendly transportation costs for expensive chemical fertilizers and limes that kill the microfauna, leaving dead soil behind.

                                      Biochar and wood ash  can be added to tropical soil before planting.

Clay in soil can prevent drainage too.

Needless to say, this is all very exciting, but still faces some technical challenges. For example, how can biochar be produced on a large scale? How can it be added to existing tree cultivations? Where does the required biomass come from? The answer to these and other questions can be found in the approach being taken by Amazonia Reforestation and CO2Tropical Trees at their La Pedregoza and El Encierro plantations in Vichada, Colombia. Let’s examine some of the solutions being developed in Part 3 of this series of articles.

Understanding Tropical Soils: Part 1

by Dexter B. Dombro

Tropical tree plantation in sandy acidic soil.

Tropical forests are the lungs of our planet. 95% of all tree-based carbon sequestration occurs in the tropics, primarily between both 15° northern and 15° southern latitudes from the equator. The huge forests of Canada, Russia and Scandinavia and other temperate zones only account for 5% of tree-based carbon capture. This means that the decline of tropical forests is a huge cause of climate change. Many studies show that tropical deforestation emits greenhouse gases (GHG) and may well be the leading cause of desertification of the Earth. All trees play important roles in removing CO2 from the atmosphere, but also other dangerous contaminants. They help hold ground water and thanks to the process of transpiration are crucial for cloud seeding and the maintenance of global rainfall patterns. Needless to say, forests also account for 90% of terrestrial biodiversity, making them essential habitats for life on our planet.

Imagine transpiration being like tree sweat.

However, planting trees, especially tropical trees, involves more than just sticking a seedling in the ground. Tropical soils are often poor and acidic, in large part due to millennia of torrential rains that have leached the nutrients and organic material out of the soul, a process called lixiviation. For example, the grasslands of eastern Colombia (llanos orientales) and Venezuela have soils that are mainly composed of sand, ferrous oxide gravels and some clay. Similar conditions exist in large parts of Brazil and in the Amazon basin. This is why Amazon deforestation is such a huge problem: poor people cut down trees to grow subsistence crops, collect one harvest and then find that the soil is depleted, so they repeat the process, cutting down more rainforest. Unfortunately, this is the definition of insanity, doing the same thing over and over again, expecting different results each time.

Tropical deforestation

I don’t want to bore you, but it is important to understand two key issues involving tropical soils. The first issue is acidity, measured in a logarithmic scale from 1 to 14, with a pH below 7 being acidic, while a pH over 7 is alkaline. Logarithmic means that each number on the scale is either 10 times more acidic or 10 times more alkaline than the previous number. A pH of 7 is considered neutral and is the value of pure water. Most plants do very well in neutral soils. However, the majority of tropical soils are acidic, which means that native trees and plants have had to adapt to acidic soil conditions. For example, the soil at La Pedregoza in the Orinoco River basin of Colombia has an average pH of 5.9, meaning it is 90 times more acidic than pure water. The traditional agricultural solution to soil acidity is to dump tons of lime on the soil, in order to achieve a more neutral pH, without regard to the cost or the damage done to micro-fauna.

Cation Exchange Capacity or CEC

The second issue is something called the cation exchange capacity or CEC of the soil. For simplicity’s sake this is best described as the capacity of the soil to retain nutrients, be that organic material, micro-fauna or fertilizers. CEC is measured on a scale of 0 to 50, with 0 being soil that has complete filtration or lixiviation of any nutrients, making for rapid drainage with no retention of any kind. In contrast, soil with a CEC of 50 is solid rock or hard clay, which does not allow for drainage, causing plants and their roots to drown. In the case of tropical soils the majority have a very low CEC. For example, the average effective CEC of soils at La Pedregoza is around 1, so pretty much total filtration of the soil with very little retention of nutrients, organic material or micro-fauna. This explains why rainforest trees are complete recyclers, drawing the majority of their requirements from the atmosphere and from the dead fall of leaves, branches and other organic matter inside the forest. They have very little dependence on the soil for their food requirements. Now you know why rainforest deforestation in the tropics produces such poor agricultural results.

Typical soil sample analysis from the Orinoco River basin. CIC is CEC in Spanish, while CICE means the  effective retention of the soil. Note the pH of the soil meaning it's very acidic in its natural state. Also note how poor the soil is in elements.

In Part 2 of this series of articles, I will address the most promising solution to the nutrient retention and acidity problem in tropical soils. That solution has positive implications for carbon sequestration, plant nutrition, enhanced agricultural and agroforestry production, is financially sustainable and has huge socio-economic development benefits in tropical regions. It is also 100% organic and natural, without any chemical or artificial elements.