4. Defining Soil Degradation

“We know more about the movement of celestial bodies than about the soil underfoot”

Leonardo Da Vinci

I have established that agriculture is the primary cause of soil degradation and that 1/3 of global soils are now considered degraded. But what does the term 'soil degradation' actually mean?

Degradation of what?

... of the things the soil 'does'. Soil does more than just grow food, as discussed my first post. It provides a wealth of 'ecosystem services', such as carbon sequestration, nutrient cycling and water filtration. Over time our perception of soil degradation has progressed from a reduction in the soil’s ability to grow plants to a reduction in the soil’s ability to provide ‘ecosystem services’. 

1. Soil degradation = a reduction in the soil’s ability to grow plants.

2. Soil degradation = a reduction in the soil’s ability to provide ecosystem services.

Therefore, to assess soil degradation we should measure a soil's overall 'ecosystem service provision' (ESP) not just its productivity. Assessment frameworks that do this have been developed, but they generally require more time, expertise and resources. Therefore, soil quality still tends to be measured in terms of productivity because it is cheaper and easier. For example, the GLASOD mapping project (see my third post) assessed degradation 'in relation to changes in agricultural suitability'. It did not explicitly consider any other ecosystem services. Nevertheless, soil productivity is a relatively good proxy for overall ESP because the two are intrinsically linked, e.g. better productivity often means more soil organic matter, more carbon sequestration, more nutrient cycling and a better habitat for organisms.

However, there is one key exception: the cold soils of the Northern Hemisphere. The agricultural suitability of these soils is naturally low and has remained relatively unchanged. Therefore, the GLASOD project identified them as 'stable' rather than degraded. However, their carbon sequestration capacity is naturally high. In fact, permafrost soils alone contain twice as much carbon as the atmosphere. Increasing temperatures (due to climate change) may cause soil organic matter decomposition rates in these soils to increase, reducing the amount of carbon they sequester (this shall be discussed in a later post). In terms of food production, such changes would not be classified as soil degradation. In terms of overall ESP, they would. 

Degraded compared to what?

Most analyses imply that current soil ESP is being compared to some 'natural baseline level'. However this level tends to remain undefined. Furthermore, it is different for different soils because some soils are just inherently poor. Thus two soils may currently have the same level of ESP but one is considered degraded and one is not, as demonstrated in the diagram below.

Fig. 4 Diagram showing that two soils may have the same level of ESP but one is considered degraded and one is not due to different levels of inherent soil quality. (Diagram made by me!).

   

The capacity of the soil to function is referred to as ‘soil quality’ (SQ). Some recognise the situation described above and argue that SQ should be divided into: dynamic and inherent quality. Inherent quality is assumed to be 'fixed' and is the original state of the soil under natural conditions (i.e. the 'baseline'). Dynamic quality is how the soil has diverged from this original state due to human influence. Using these concepts… soil degradation = a reduction in the soil's dynamic soil quality.

However, the boundary between these two categories is not clear-cut. The assumption that inherent SQ is fixed and that dynamic SQ can only be influenced by humans is an oversimplification of reality.  This is because natural climatic fluctuations mean that SQ is naturally dynamic. Furthermore, distinguishing between ‘natural’ and ‘anthropogenic’ climate change can be challenging. 

At what point does a soil become considered degraded?

SQ may be naturally dynamic but it is also resilient (to varying extents). A short period of low rainfall may temporarily reduce SQ but subsequent rainfall will rapidly restore it. Some argue that this counts as degradation while others argue that degradation should only refer to soils that cannot restore themselves naturally. I agree with the latter therefore… soil degradation = a reduction in SQ that cannot recover unaided.

Final definition:

Clearly the term 'soil degradation' is multifaceted. Yet I haven't found a single paper that provides a holistic definition incorporating all of these aspects. Therefore I have summarized my findings and created my own definition: soil degradation = a reduction in the soil's dynamic ecosystem service provision that cannot recover unaided.

Soil degradation vs land degradation

These terms are often used interchangeably. A recent guardian article stated that 1/3 of global 'soils' degraded and then later stated that 1/3 of global 'land' is degraded, implying that they are the same. However, soil degradation is an aspect of, rather than equivalent to, land degradation. This is because the ‘land’ includes other physical conditions such as mineral deposits, water resources and vegetation cover. The two are intrinsically linked - about 85% of land degradation worldwide is due to soil degradation - but not synonymous.