Slash and burn farming is a form of shifting agriculture where the natural vegetation is cut down and burned as a method of clearing the land for cultivation, and then, when the plot becomes infertile, the farmer moves to a new fresh plot and does the same again.
The richness of the rainforest is in the trees. As leaves fall or trees die everything is broken down by the soil’s organisms, nutrients are returned to the soil and the tree roots take them up again. Thus continuous recycling keeps everything fertile and growing. When this no longer happens in a cleared plot it soon becomes infertile. Torrential tropical rains quickly wash nutrients out of the soil when it is left bare after harvest. This necessitates the frequent clearing of new plots from the forest, sometimes as often as annually.
Estimates suggest some 250 million farmers are forced to eke out a living like this on poor rainforest soils. By slashing and then burning the forest, these farmers can usually sustain themselves for 1-2 consecutive years on the same patch of soil.
The soil then loses its fertility and the farmer is faced with either a daily walk of several miles to a new patch or, increasingly as the number of landless farmers grows, they may have to uproot their families to move.
Some farmers they may go on using the same plot for several years by having several plots and leaving them fallow for 4 years+. But they cannot leave them fallow long enough (20 years+) for the plot to fully recover. The result is increasing degradation of the soil until it can become useless for anything but tough weeds.
Often farmers have to cultivate hillsides, as all the land lower down is used up. As they progress ever higher up the slopes they are likely to meet another farmer at the top who has similarly worked his way up from the other side.
When the plots are far from the dwelling place, cash crops cannot be guarded from thieves or wild animals. Nor can the farmer’s family help when they have young children. These distances help to ensure the farmers remain on the poverty line.
Historically, when the population densities were lower, slash and burn worked reasonably well. It was then possible to leave the plots fallow for 15 to 20 years, which allowed considerable regrowth of the forest and good restoration of soil fertility. Now the plots have to be reused too soon, with increasing loss of fertility.
Not only is this devastating the worlds remaining tropical forests but it is also forcing many farmers to abandon the land, and migrate to city slums in the hope of feeding their families. Life in the slums can be very hard indeed with a lack of jobs available.
Recent research (https://news.mongabay.com/2018/09/whats-causing-deforestation-new-study-reveals-global-drivers/) said that shifting agriculture accounts for 24% of forest loss worldwide, but over 90% in Africa. It is common in Cameroon.