Shifting cultivation

Author: Leandro Alegsa Creation date: November 13, 2022

Shifting cultivation is an agricultural system in which areas of land are cultivated for a short time.Shifting cultivation is practised in the thickly forested areas of Amazon basin,tropical Africa,parts of southeast Asia and northeast India. These are areas of heavy rainfall and quick regenerating of vegetation. Then they are left to grow back their natural vegetation, while the farmer moves to another area. It is practised in forested areas of the Amazon basin.It is one of the rudimentary forms of cultivation. After cultivation a plot of land is cleared by felling the trees and burning them.The ashes are then mixed with the soil and crops like maize,yam,potatoes and cassava are grown. After the soil looses its fertility, the land is abandoned and the cultivator moves to a new plot. Because the soil is no longer protected from the large trees in the rainforest system, it can be ruined easily by thick rain and too much sun. One of the main problems with shifting cultivation is that the land is ruined after being used 3 to 4 times, which means that the areas of land are abandoned and not prepared to grow again- and the rainforest becomes smaller with each preparation for a new piece of land. Shifting cultivation is also known as slash and burn agriculture.

Example cycles of a typical shifting cultivation over nine years

Typical "hole" after slash and burn in the forest to create a field of shifting cultivators (here the Jumma in Northeast India)

"Patchwork" landscapes, like this one in southern China, emerge when settlement in traditional shifting cultivation areas becomes too dense

Transitions to other economic forms

Shifting cultivation is - at population densities below 6 inhabitants per km² - an efficient and adapted agricultural strategy on poor and fragile soils, which are common in the tropics. However, shifting cultivation was also practised in temperate latitudes in the past. If, in the past, climatic factors or increasing population pressure led to declining yields, people were forced to migrate or to develop new technologies that allowed intensification of agriculture. The resulting overuse usually leads to such severe degradation of the soil that the reduced soil fertility does not allow further cultivation of the area. This is how the field economy was introduced in Europe or the land rotation economy in the tropics. This was the case, for example, in Central America (milpa agriculture of the Maya) - partly supplemented by irrigation measures - or also among West African field farmers.

The modern "population explosion" and the destruction of ever larger areas of forest also call for solutions in order to be able to continue to sustainably feed the forest dwellers. One promising approach to sustainable intensive use is to supplement the short-term soil improvement of slash-and-burn agriculture (melioration) with the deliberate addition of extra charcoal, human feces, manure, compost, and the like. This insight is not even new, as many indigenous peoples of the Amazon (example Tupí) have produced and used this anthropogenically modified soil, now called terra preta, for centuries.

Nutrient cycle and regeneration phase

The length of the required fallow phase has a direct impact on population density: the longer it lasts, the fewer people can live off shifting cultivation in a given area. The length of the fallow phase depends on the type of use and crops, climatic conditions and soil quality. In tropical soils, which are usually deeply weathered and nutrient-poor, the regeneration phase can last up to 30 years. Due to the tropical conditions such as humidity and temperature and due to the high age of the soils, their minerals can store only few nutrients (low cation exchange capacity, CAC). The nutrients are therefore almost exclusively contained in the biomass (mainly plants, but also animals and microorganisms) and in the humus, i.e. the organic fraction of the soil. Many are in a short-circuited cycle (tropical nutrient cycle).

Slash-and-burn increases solar radiation and thus the soil temperature, which causes increased mineralisation, reducing the humus content to such an extent that only a few nutrients can still be stored there. After slash-and-burn, most of the nutrients are initially in the ash, from which a high proportion is then lost through precipitation: Thus, they are either washed away directly at the surface by heavy rains, or the downward soil water flow washes the nutrients to such great depths that they are no longer reached by the roots of the crop plants. The nutrient content of the topsoil is reduced, making further cultivation no longer worthwhile. However, if sufficiently long fallow periods are observed, deep-rooted fallow trees can take up nutrients and a secondary forest can form. This also leads to the rebuilding of humus in the soil.