Page 137 - Soil Degradation, Conservation and Remediation

126 5 Chemical Soil Degradation

Table 5.1 Global extent of soil degradation (M ha) by loss of nutrients, salinization, and acidification
Regions Loss of nutrients Salinization Acidification Total
Africa 65 15 1 62
Asia 15 53 4 72
South America 68 2 − 70
Central America 4 2 − 6
North America − + + +
Europe 3 4 + 26
Oceania + 1 − 1
World 136 77 6 229
Data extracted from Oldeman (1994)

of degraded soils are due to chemical degradation. In North America and Oceania,
chemical degradation of soil is of little concern. Loss of nutrients is the major
type of chemical soil degradation in Africa and South America, while salinization
is the major type of soil chemical degradation in Asia. Chemical soil degradation is
mainly caused by agricultural mismanagement (56 %) and deforestation (28 %)
(Oldeman 1994). Table 5.1 shows the global extent of chemical degradation of soil.

5.2 Loss of Nutrients and/or Organic Matter

Although it is said that the loss of nutrients is a problem with low-input agriculture,
it also occurs in several other situations. It may occur if agriculture is practiced on
poor or moderately fertile soils, without sufficient application of manure and fertilizers.
The rapid loss of organic matter and nutrients of the topsoil occurs due to clearing
of the natural vegetation. Burning of biomass, shifting cultivation, and erosion are
other important causes of loss of nutrients and organic matter. Human- induced soil
nutrient depletion is the process by which the soil nutrient stock is shrinking due to
continuous nutrient mining without sufficient replenishment of nutrients harvested
in agricultural products and of nutrient losses by accelerated soil erosion and leaching
(Tan et al. 2005). Natural erosion and leaching are exacerbated by human actions
for the exploitation of vegetation and soil resources. The general cause of fertility
depletion is a negative balance between output (through harvesting, burning,
leaching, etc.) and input (through manure and fertilizers, returned crop residues,
flooding, etc.) of nutrients and organic matter. The nutrient balance situations in some
African countries are cited as an example below. International Fertilizer Development
Center (IFDC) conducted some studies of soil nutrient balances in some African
countries. The methodological approach to estimate nutrient balances, depletion
rates, and requirements combines information on agricultural production, soil
characteristics, and biophysical constraints with methods and procedures designed
for making such estimates (Henao and Baanante 1999). The information and data
related to agricultural production included land use, population- supporting capacity
of land, crop production, and use of mineral and organic fertilizers. The approach