Distribution Of Soil Lifespans

this graph was created in OurDataWorld:

The honest answer is that we don’t know. Botanist and science communicator, James Wong, tried to trace these claims back to their roots for an article in the New Scientist.1 We know that a senior official at a UN FAO farming conference was quoted with the “60 harvests” figure and that Michael Gove mentioned a 30 to 40 year deadline. But we don’t know what they based their assessments on.

The “100 harvests” figure seems to link back to a study in the UK conducted by researchers at the University of Sheffield.2 I say “seems to” because there appears to be no mention of the 100-year figure in the paper. James Wong failed to find where this number came from; I also spent a lot of time digging and did no better.

In any case, this study looked at the difference in soils properties of city allotments in Leicester, a city in the UK, and soils from some surrounding farms. It concluded that the soils in city allotments had more organic matter, higher nitrogen levels and a better soil density. Not exactly informative for the larger and more urgent question on the state of the world’s soils.
What did this study tell us about the lifespan of our soils?

Soils from the 4285 data points in the study were grouped into three categories.

‘Bare’ soils are plots of land which are deliberately kept free from any crops to determine erosion rates of soils without vegetation. These are used to assess a ‘worst-case scenario’.

Conventionally managed soils are those which are actively farmed, without implementing notable conservation practices. These are used to represent a ‘business-as-usual scenario’.

Conservation management soils were those that had been subject to soil conservation techniques such as land use change (to forests and grasslands) or improved agricultural practices such as intercropping, no-tillage, or contour farming. We will look at the impact of these techniques later.

In the chart here we see how the distribution of estimated soil lifespans in these three categories varied across the global dataset. On the x-axis we have the lifespan in years and on the y-axis we have the cumulative percentage of soils that were found to have that lifespan. Notice that the scale on the lifespan axis is logarithmic and stretches from 10 years to 10 million years. This further demonstrates how citing a single lifespan for the world’s soils is inaccurate and nonsensical.

Let’s focus on the ‘conventionally managed’ soils, shown in blue. These data are relevant for understanding many of the world’s farming practices. We will look at conservation techniques later.

Many of these soils are thinning; some very quickly. 16% have a lifespan of less than 100 years if they continue to erode at their current rates. This is not a local problem: there are examples of soils with lifespans shorter than a century on all continents, including the United States, Australia, Spain, Italy, Brazil and China. The longevity of these soils is concerning and we should be acting quickly to preserve them.