2. Causes of Soil Degradation

“Agriculture was supposed to help feed the world, but ironically it now threatens us with starvation”
George Monbiot

Agriculture is the single main cause of global soil degradation, either directly via poor land management or indirectly via deforestation, and there are numerous examples of failed societies that can be linked to this. This post investigates how agricultural practices can cause physical, chemical and biological soil degradation...

Fig. 1 A plow team in Egypt’s Old Kingdom. 

Physical

Soil erosion primarily occurs when deforestation or overgrazing exposes the soil to the wind and rain which then blows/washes it away. 

Fig. 2 River erosion due to deforestation is destroying agricultural land in Jaffa Village, Wolayita, Ethiopia (Source: Mike Goldwater, Blue Cube Productions) 

Soil compaction primarily occurs due to the use of heavy machinery. It reduces the porosity of the soil. Pores are crucial for distributing air, water and nutrients within the soil and for allowing plant roots to penetrate and anchor themselves into it. 

Chemical

Nutrient depletion occurs due to harvesting. Crops use nutrients from the soil to grow. When we harvest them, the nutrients now locked up in the plants are removed. (Under natural conditions these nutrients would get mixed back into the soil as dead organic matter/animal faeces).

Salinization is mainly due to over-irrigation (in arid and semi-arid regions). Water contains dissolved salts. Irrigation water is either absorbed by the plants or it evaporates, leaving the salts behind on the soil surface. In the absence of sufficient rainfall to wash these salts back down through the soil, they accumulate in the plant root zone. High salt concentrations are toxic to many plants and make the land unusable.

Fig. 3 Soil salinization in the Grand Valley, Western Colorado (Source: National Geographic Photograph: Jim Richardson) 

Acidification (due to acid rain) and pollution (from toxic chemicals) of soils can also occur as a result of industrial activities (both agricultural and non-agricultural). Soils that contain fewer base cations (such as the granitic upland catchments of North America and Northern Europe) have a lower pH buffering capacity and thus are more susceptible to acidification. 

Biological

The loss of soil organisms mainly occurs as a result of physical and chemical changes that make the soil uninhabitable. Therefore, it is often referred to as a secondary type of degradation. However, its importance should not be underestimated! I shall be discussing the role of the microbiome later in my blog

Fig. 4 An earthworm burrows into the soil

What are the main types?
According to the last global survey, the two main types of soil degradation are erosion and nutrient depletion, accounting for ~84% and ~7% respectively, as shown in the table below.  

Fig 5 Types of degradation as a % of total degradation (data derived from GLASOD 1991) 

Anthropogenic climate change:

Rising temperatures and more variable rainfall exacerbate many of the processes described above and are having an increasing impact on soil quality.