Why does slash and burn not work?

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This field was created by logging and then setting fire to a patch of forest and then sowing maize seeds. It is a lot of effort and will result in two years worth of cultivation after which time it will be abandoned

Looking at 40 m high trees in the Amazon rain forest it must be counter-intuitive as a farmer to think that these are poor soils. If you can grow giant luxuriant trees then surely maize should be no problem? Well the thing is Amazonian soils are very poor and acidic and don’t support that wonderful forest. What does support these giant forests is a thin strip of root-mat maybe 20 cm deep that lies over the impoverished soils and in which a near miraculous juggling of decomposition and reabsorption keeps nutrients cycling and available through complex food web of insects, fungi, bacteria and roots. Tragically when the farmer burns the cleared vegetation in order to be able to sow a crop most of this incredibly productive layer is lost or killed. Thousands of year of verdant forest are replaced by  a couple of years of maize, bean, rice or cassava and then degraded scrub.

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This field was cleared three years ago, maize then cassava grown on it for two years and then abandoned as the soil became exhausted. It still has a few tree trunks and stumps but it will take probably hundreds of years for the original forest to return,

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