Saving The Already Depleted Soils
By Professor Nils Nykvist
Several million people suffer from lack of adequate food in many poor countries today. The question of sufficient food has been more and more present in recent years, as interest in bio-energy also contributes to the competition for cultivated lands. Yet there are large areas with degraded land that previous generations have left behind when cultivating their soils.
What are the possibilities for restoring the fertility in these areas? Is land conservation even possible?
Allow me to provide some insights on what can be done to save our already depleted lands.
In cases where the erosion
is so advanced that most of the original soil on the slopes has been eroded away, reclamation is not possible, unless lost soil is brought back to the land.
It would take many thousands of years of chemical weathering of the underlying bedrock
to form a sufficiently thick layer of soil. But if a sufficiently thick layer of fine-grained soil is left on the slope, the possibilities for the improvement of the soil fertility are very large.
The reason that the soil has deteriorated is in most cases that the amount of plant nutrients has been reduced so much through harvest and leaching that farming is no longer profitable.
The loss of plant nutrients can, however, be easily replaced by the supply of inorganic nutrients in one or another form. The situation is worse for soil organic matter that is of great importance for the soil’s capacity to retain water and nutrients. The formation of it can be accelerated by ploughing down crops of nitrogen-fixing plants such as clover or peas, or in the long run by afforestation.
Land improvement through afforestation
For the farmers or countries that cannot afford chemical fertilizers, the soil can be improved by better management of growing vegetation on the land.
If the growing vegetation can be protected from fire
, cattle grazing, or gathering of firewood
, the end result could be a more or less closed forest (which is in turn more resistant to fires).
This is because the plants growing on the land can improve the soil through dead plant debris that contains and forms water-soluble organic substances during its decomposition that accelerate the chemical weathering of minerals, and thus the availability of plant nutrients in the soil
. Moreover, the plant debris contributes raw materials to soil organic matter, which is of great importance for the binding of water and nutrients in the soil.
In the grasslands and the shrub lands in southern Africa known as miombo, the original vegetation disappeared as a result of burnings and was been replaced by a low-productivity plant community adapted to the frequent fires. Even if effective firebreaks can prevent fires, it would take a very long time to restore the original vegetation, as appropriate seed trees are missing.
It is therefore necessary, at least in a transitional stage, to help nature along by planting suitable tree species (Fig. 29). In the selection of trees, it is important to choose species which are not only suitable for the mean annual rainfall of the area but those that also can survive some considerably drier years, which are rather common in the tropics.
Figure 29. Pine plantations on savannah in Mbeya, southern Tanzania.
The best tree species of all are those that also increase the soil fertility by fixing nitrogen from the air, such as many species of acacia. In arid areas one of the best tree species for agrosilviculture is Faidherbia albida (former Acacia albida), with a very deep-penetrating taproot that makes the tree highly resistant to drought.
Another advantage of the tree species is that it sheds its leaves in the rainy season and therefore does not shade agricultural crops growing underneath the tree canopy. Several examples of the much greater crop yields under compared to outside Faidherbia albida canopies are given for unfertilized soils of low fertility .
The wide firebreaks that are necessary to protect the forest plantations can be used for farming or for grazing land for cattle. The main thing is that vegetation is kept low, so that the firebreaks cannot catch fire.
For farmers who cannot afford high doses of fertilizers, a sustainable agriculture could be created in combination with livestock
rearing in a manner similar to that which occurred in many northern countries before the introduction of chemical fertilizers in the early 1900s.
Planting of trees on areas with deteriorated soils would improve the living conditions for the poorest people through more fertile soils and better access to fuel wood
. In addition, tree plantations contribute to the reduction of global warming through large amounts of carbon being stored in vegetation and soil.
See the references
for this article.
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