Tropical soils are generally more dependent on organic matter for fertility than temperate region soils, plus many, if not most, tropical agricultural soils are low in organic matter. Soils in many parts of the tropics, including Africa, are geologically old, weathered, leached, and are made up of what are known as secondary clays. These clays, which are often reddish from iron oxides, unlike the geologically young, glaciation-derived soils in North America and Europe, generally have a low nutrient holding capacity. Decomposing organic matter, as well as post-decomposition humic substances, provide a critical source of nutrient-holding surfaces, among other benefits.
Warm-wet tropical climates make for high decomposition rates, further exacerbating the soil organic matter deficiency problem. Some countries such as Brazil have developed effective small-holder mulching practices that reduce soil disturbance and keep soil temperatures low, retaining much needed organic matter levels.
The deficit of organic matter in African agriculture can be resolved by intensifying crop production – especially by improving and tightening up crop rotations and polyculture combinations that include legumes and grasses, and by reducing soil disturbance by moving to minimum- and zero-tillage. The former increases the source and the latter reduces the sink for organic matter.
Biochar. There’s a new kid on the organic-matter-and-tropical-soil-fertility block, and he’s big and is going to be a force to contend with – biochar. Biochar is charcoal – pyrolized biomass – mostly pure carbon chains produced by heating (fire temperature) plant biomass in the absence of oxygen, which vaporizes everything except carbon chains and some tar. The biochar as soil amendment phenomenon was in recent years discovered in ancient soils of the Amazon that were biochar-amended and which remain some of the most fertile soils in the region as a result.
Left: Charcoal dust being applied to garden beds.Right: We obtain charcoal dust that is currently a by-product of the local charcoal market points. At right can be seen a few pieces of charcoal.
Biochar is causing a small revolution in tropical soil fertility management. Its surfaces hold nutrients for plant roots to access, it is microbe-friendly, and it doesn’t decompose or leach away. Numerous field trials have shown higher crop yields, sometimes double, in biochar-amended soils.
Currently in Africa biochar as a soil amendment is a new concept, and knowledge of it hasn’t reached more than a tiny fraction of farmers and agricultural development workers. I am currently able to buy charcoal dust – a waste product – very cheaply from charcoal processing areas in the city of Arusha. These are places where bush-dwelling producers, mostly Maasai women walking with heavily-laden donkeys, bring charcoal. I don’t expect the low prices to last a year as agriculturists learn the value of charcoal dust. Charcoal production is unsustainable in much of Africa where forests are being cut for the wood, but dry savannah areas, such as those surrounding Arusha, can support production when shrubs and trees that can regrow are used as the wood source.
Biochar-producing stoves that burn pellets that are waste-derived or sustainably produced are being introduced in Africa. These stoves have separate chambers for pyrolizing some of the pellets and producing biochar for soils. Pioneering efforts are being made by a few NGOs (non-governmental organizations) to quantify the amount of this stove-derived biochar going into soils, a form of long-term carbon sequestration, and to tap into global carbon-offset market funds to pay the stove users. This may become a significant source of income for people in Africa.
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