The Big Culprit Behind
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Land Deterioration – Agriculture
By Professor Nils Nykvist
The start of agriculture
The burning caused a loss of nitrogen compounds, but other plant nutrients became more available to plants.
However, the loss of nitrogen could be replaced by organisms found in, for example, clover and alfalfa, which are able to fix nitrogen from the air.
On the burnt area that was relatively free of weeds, grain or hill rice was mostly grown at first (Fig. 2). After one or a few years, depending on the fertility of the soil, larger plants were cultivated that could more effectively compete with weeds and provide a good transition to the shadow-tolerant tree species that later colonized the old plantations.
Figure 2. Slash and burn agriculture with mountain rice. While the rice is maturing the crop must be protected from birds, which often is a task for the children. Northern Philippines.
The cultivation led to a loss of plant nutrients through removal of the harvest, increased leaching, and soil erosion.
After a few years the yields began to decrease, and the people were forced to clear, burn, and cultivate a new forest area.
But we were still in harmony with nature
As long as the population remained low and the forest was left to grow, such that a sufficient nutrient capital could be built up by the biomass growth, cultivation caused no great impoverishment of the soil.
The nutrients that were lost through leaching or soil erosion
from the farm mostly increased the fertility in the areas lying below. The nutrients that were removed with the harvest were not lost in the long run, as long as the farmers moved from place to place within a large area, or the harvest was not sold and inorganic nutrients removed from the area.
Slash and burn agriculture has occurred in all forest areas where farming has been carried out. In Sweden, it was a relatively common practice in the forest regions as late as the 1800s. Large tracts of highly productive spruce forests of Värmland and Dalarna have grown up in old slash and burn land.
In tropical rainforests, slash and burn agriculture has been practiced for thousands of years without any indications of forests being destroyed in the long run. The colonization of trees takes place quickly, since the slash and burnt plots usually are not larger than one hectare. The area is thus free of trees only for a few years and is then in various stages of reforestation (Fig. 3). If the population pressure is not so great that the re-growing forests are being cut down prematurely, the forests will mostly return to their original state.
Figure 3. Slash and burn agriculture in a tropical rainforest in the northern Philippines. The picture shows three plantations of mountain rice and forests in various stages of re-growth.
When agriculture becomes problematic
The trees are a big drawback for the farmers when slash and burn cultivation is practised in tropical rainforests. They therefore prefer recently felled areas that foresters have left behind.
When larger numbers of people burn the land more often, the forest does not have enough time to build up a sufficient nutrient capital in its biomass, which results in a lower harvest when the forest is felled and the area cultivated after burning.
Larger and larger areas have to be cultivated by the farmers to support themselves and their families. This effect, combined with the increasing growth of population, has led to a decrease of the forest area and an increase of grasses and shrubs.
See the references
for this article.
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