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According to this 2014 Scientific American article by Chris Arsenault

Generating three centimeters of top soil takes 1,000 years, and if current rates of degradation continue all of the world's top soil could be gone within 60 years, a senior UN official said on Friday.

About a third of the world's soil has already been degraded, Maria-Helena Semedo of the Food and Agriculture Organization (FAO) told a forum marking World Soil Day.

I haven't found a transcript of Semedo's speech, or any basis for her statement. I find it curious that such a dire statement supposedly made by a credible authority is not much more widely supported on the web.

Is the world's top soil predicted to be gone in 60 years, according to scientists?

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    This is a media story to generate interest/ fill a newspaper. If taken literallyit would mean the cornbelt in the US that has produced corn for over 200 years in some locations and produces more corn per acre every decade is "fake" news ? – blacksmith37 Jan 23 '19 at 15:55
  • @blacksmith37 Actually Semedo's comment supports your assertion. The increasing demand on soils owing to modern agriculture, particularly the mechanization and use of fertilizers that lead to the increasing yields you speak of, is central to the problem being discussed. – Selene Routley Jan 23 '19 at 23:16
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If she said it in 2014, she wasn't the first.

For example, in a 2012 interview, Professor John Crawford of the University of Sydney said ,

A rough calculation of current rates of soil degradation suggests we have about 60 years of topsoil left. Some 40% of soil used for agriculture around the world is classed as either degraded or seriously degraded – the latter means that 70% of the topsoil, the layer allowing plants to grow, is gone. Because of various farming methods that strip the soil of carbon and make it less robust as well as weaker in nutrients, soil is being lost at between 10 and 40 times the rate at which it can be naturally replenished. Even the well-maintained farming land in Europe, which may look idyllic, is being lost at unsustainable rates.

See also the 2010 article Britain facing food crisis as world's soil 'vanishes in 60 years' which also quotes Crawford as saying:

It could be as little as 60 years and that is a scary figure because it is not obvious that we have time to reverse decline and still meet future demands for food

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    The key phrase may be 'naturally replenished'. Modern farming often adds nitrogen and other compounds to soil under cultivation. – BobT Jan 23 '19 at 16:22
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    @BobT - or plant soy/beans to naturally replenish nitrogen. But then, why pay attention when facts contradict one's apocalyptic theory – user5341 Jan 23 '19 at 20:58
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    @user5341 That we are not paying enough attention to the kinds of measures you speak of is exactly the basis of the "apocolyptic" theory. The concern stems in no small measure from what our present usage and situation is, and why what could be being done is not. – Selene Routley Jan 23 '19 at 23:20
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    @DavePhD This is exactly the kind of information I needed. Many thanks. I'll probably make it the "accepted" answer in a few days. – Selene Routley Jan 23 '19 at 23:22
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    @DavePhD BTW I wouldn't have expected Semedo to be the first. She is the assistant deputy director of the UN FAO, and one would naturally expect her to be citing the common knowledge of her organization rather than being original, which was much of the reason I find it strange that it is rather hard to find good sources for her claim. – Selene Routley Jan 23 '19 at 23:28
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This posted article from May 2019 discusses the claim and its origins.

The idea that there are only 100 harvests left is just a fantasy

Despite dozens of headlines quoting these predictions, surprisingly only one peer-reviewed paper from a scientific journal is ever cited as evidence to back them up. This 2014 study from the University of Sheffield compared the soil quality of a range of sites in the English city, including agricultural, garden and allotment soils.

Now, before we question whether the results of this single, small study can be extrapolated to represent all of England, let alone the whole UK or even the whole world, let us take a look at their findings: basically, some urban soils in Sheffield are higher in carbon and nitrogen than some nearby agricultural ones. OK, but where is the 100-year statistic? It turns out that nowhere in the study was there any calculation, prediction or even passing reference to the claim. None whatsoever. Perhaps not so much shaky evidence to support this assertion as much as non-existent.

The article goes on to say:

Maybe this is the result of a typo and the work is in another research paper? After an 8-hour trawl through the academic journals failed to pull up a single study that even attempted to make this calculation, I contacted six leading soil scientists across the world to ask if they had ever come across such a prediction in either the published literature or their work. Not a single one had.

In fact, the words they used to describe this claim were “bold”, “too Malthusian”, “hardly useful”, “almost insulting” and “I have used this in my soil science lectures to show the students to be wary of headlines!”. Ouch.

Does that mean there aren’t real threats to some agricultural soils around the world? Absolutely not. Indeed, all the scientists I spoke to went to great lengths to point these out, where they exist.

However, they also highlighted how incredibly complex the calculations needed to make such predictions would be, based on myriad factors, only some of which can be predicted with any reliability, with generalisations almost impossible. The boring reality is that while soils in some parts of the world might be in decline, others are not.

Then there is this article, Borrelli, Pasquale et al. “An assessment of the global impact of 21st century land use change on soil erosion.” Nature communications vol. 8,1 2013. 8 Dec. 2017, doi:10.1038/s41467-017-02142-7, whose abstract is:

Human activity and related land use change are the primary cause of accelerated soil erosion, which has substantial implications for nutrient and carbon cycling, land productivity and in turn, worldwide socio-economic conditions. Here we present an unprecedentedly high resolution (250 × 250 m) global potential soil erosion model, using a combination of remote sensing, GIS modelling and census data. We challenge the previous annual soil erosion reference values as our estimate, of 35.9 Pg yr−1 of soil eroded in 2012, is at least two times lower. Moreover, we estimate the spatial and temporal effects of land use change between 2001 and 2012 and the potential offset of the global application of conservation practices. Our findings indicate a potential overall increase in global soil erosion driven by cropland expansion. The greatest increases are predicted to occur in Sub-Saharan Africa, South America and Southeast Asia. The least developed economies have been found to experience the highest estimates of soil erosion rates.

However see as well these articles describing soil degradation due to industrial agricultural practices which is disrupting the soil.

From POLITICO Sep-2017 Can American soil be brought back to life?

Now, some farmers and soil scientists are realizing that for the health of both people and farms, the most important thing you can do is look at soil differently—seeing topsoil as a living thing itself, which can be tended and even improved. Good soil is alive with a host of delicate organisms, many of them microscopic, producing structure and nutrients. As long as they’re thriving, soil can better absorb and retain water and feed plants and control pests. But when they die off, because they’ve been churned up and exposed to the sun and air or smothered with chemicals, the soil gradually becomes little more than powdered minerals.

As well see this Corteva Agriscience article published on the BBC, Why soil is disappearing from farms which describes a number of different ways that soil health is degraded as well as ways that farming practices are changing to reduce those negative effects on soil health.

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