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.

Theobroma Cacao – The Need for Hybridisation

by Melissa Cotterell

World demand for chocolate is still growing, with new markets opening up in China and the Far East and demand expected to increase by around 25% within the decade. The cacao industry is unable to cope as it stands at present, with outmoded farming practices and lack of disease and pest-resistant hybrids taking their toll. While for many years small producers have struggled to eke out a living from cacao farming, the industry is just beginning to adapt to the changing face of cocoa farming by the development of sustainable practices.

The Origin of Cacao

Cacao originated in the Americas and spread throughout the globe to Africa and South East Asia. The tree grows only in the tropical and sub-tropical belts of the Equator, although there are regional differences in the cocoa produced due to climate and soil. Venezuelan cacao is perhaps considered the best in the industry, although the majority of the world’s cacao crop is produced in Ghana and the Ivory Coast. Colombia is pushing its cacao industry to become a major export. Cacao has been identified as a viable high cash flow crop to replace illicit drug cultivations in Colombia and elsewhere in South America.

Varieties of Tree

There are four main varieties of cacao: Criollo, Forastero, Trinitario, and Nacional. The Criollo is typically low cropping, but of excellent quality and the beans from this tree are often used in a blend for the production of chocolate. The Forastero is the most common variety in the industry, producing around 80% of the world’s annual output. This variety is favoured due to its high yield and rapid growth, as well as resistance to disease. The Trinitario is a hybrid of the Criollo and Forastero, predominantly cultivated in Central and South America, as well as in Asia. The Nacional is vulnerable to disease and more difficult to cultivate, but has a much sought-after aroma.

Pests and Disease

The cacao industry is subject to a number of different pests and diseases, which has lead to losses of up to 30-40% of global production each year. Heavy research has been devoted to minimizing this damage by extensive study into zoology and identifying animals such as the Capuchin monkey, who like to eat the soft pulp surrounding the cacao seeds from which we produce chocolate. The seeds themselves are poisonous to animals due to their theobromine content and are rejected in favor of the sweet flesh. Theobromine is an alkaloid from the same family as caffeine, contained in the cocoa butter from which chocolate is made. For this reason, commercial crops do not suffer damage from animal foraging in the same way that other crops such as fruit crops might.

It is the damage from the infestation of different varieties of rot and fungi, as well as insects such as the Cocoa Moth, which are responsible for the problems in cacao plantations worldwide. Cocoa mirids bore into cocoa stems, pods and branches, killing the cells they enter, which then produce necrotic lesions. Mirid infestations on shoots frequently cause destruction on the tree as the terminal shoots and leaves die. They prefer trees which are growing in sunlight, although once they have colonized they will spread into trees which are shaded. Insecticide use is common, although interplanting with indigenous plant species, which the pest finds suitable as a host, combined with reduced use of insecticide to allow other insects to prey on the mirids allows for biological control. The Cocoa Pod Borer (or Cocoa Moth) caused widespread losses in the industry during 1890’s and 1900’s. This pest now affects virtually all cocoa producing areas in Indonesia, which has led to decreased production in Malaysia. Although pesticides are effective in controlling this pest at sustainable levels, the high cost of spraying makes this type of control nonviable.

 Witches Broom disease is a fungal infection, which spread throughout the cocoa-producing regions of South America, Panama and the Caribbean. The losses sustained by the industry have perhaps been heaviest in Brazil, where for a decade the Bahia region suffered a 70% loss of production. Although some Trinidad hybrids were developed in the 1950’s which showed resistance to the disease, more aggressive strains of the pathogen from other regions has made these hybrids ineffective. Frosty pod rot is prevalent throughout the Latin American cacao region, causing significant losses and leading to the abandonment of farms. All cacao species appear vulnerable to this disease.  

Black Pod Rot (Phytopthera) has three fungal species of the same genus: P. palmivora, P. megakarya, and P. capsici. P. palmivora and is responsible for global crop loss of around 20-30%; P. megakarya is the most aggressive of these three pathogens, most prevalent in Central and West Africa; while P. capsici is widespread throughout both Central and South America, where it causes significant losses. Vascular-streak die back decimated mature plantations in Papua New Guinea during the 1960’s and has since spread through South East Asia, where it continues to cause major losses.

Cacao Genome Sequenced

At the end of the first decade of this century, an international team led by Claire Lanaud of CIRAD sequenced the DNA of the Criollo variety of Theobroma cacao, identifying a variety of gene families which it is hoped will contribute to the improvement of the cacao tree and fruit – whether by improving growth and crop yield or by enhancing resistance to disease and pests.

Development and Distribution of Resistant Varieties

Through projects such as Cocoa Productivity and Quality Improvement: A Participatory Approach, a number of high-yielding and resistant varieties of cacao have been released to farmers, while research continues into new and improved cocoa planting materials for future release. This global project has been successfully implemented in 13 different countries.

Approximately 2,000 farms across ten different countries were surveyed, with around 2,000 trees identified by the farmers as resistant being of interest due to their high resistance to disease and infestation, or for their high crop yield. Out of these, about 1,500 farm selections had in-situ observation plots established, or had on-farm trial plots set up across 8 countries. The genetic diversity of several thousand farm selections in Africa were analyzed, showing great genetic variation in farm populations, which are of mainly hybrid origin (with important contributions of the Amelonado, Trinitario and Upper Amazon parental genomes).

The project established approximately 240 on-farm selection plots, with varieties selected by breeders being compared with farm selections (clones or seedling progenies). The number of plots has since declined through farmer neglect or drought to around 120, which are still being observed. A number of new varieties have already been selected or confirmed for distribution to farmers, while others have been selected for ongoing trials in other cacao growing regions.  New variety trials have been implemented in: Brazil; Ivory Coast; Ecuador; Colombia, Malaysia; and Papua New Guinea.