https://www.scientificamerican.com/article/only-60-years-of-...
Such a vague assertion leaves me with more questions than it resolved. Topsoil should properly be measured in volume, not distance. Presumably they mean a given area will gain three centimeters in 1,000 years, but what area and under what conditions? There are some environments with zero topsoil and some environments with quite a lot of topsoil, so is that "three centimeters per 1,000 years" figure a global average for natural topsoil renewal? I can't imagine how that's a valid measure of anything. You'd have massive regions totally devoid of topsoil like the Gobi Desert dragging down global averages.
And what about anthropogenic topsoils? Maybe one of the most famous examples is terra preta in the Amazon, which not only is madmade but reportedly regenerates itself at a rate of 1 centimeter per year (a few hundred times faster than that "3 centimeters per century" figure..)
Maybe this is true under "natural conditions" (however you want to define those). But intentional farming practices can produce this much top soil every year. The problem is that few farmers use those practices. But I do, and I can tell you that planting green fertilizers and nitrogen-fixing cover crops, and then plowing them under, will produce pretty good topsoil very quickly.
Source: Farming & gardening background in the midwest.
Presumably topsoil generation is proportional to land area, and volume per unit area reduces to distance. 3 cm per 1000 years is equivalent to 30 m^3 per km^2 per year.
That said, the number still seems wrong. Table 5.2 in [0] indicates that regeneration rates range from 0.25 mm/year to 2.0 mm/year. 3 cm per 1000 years is equivalent to 0.03 mm per year, almost an order of magnitude lower than the lowest rate in that table.
Should it though?
And regardless of how clear or unclear that is, I gotta say I dislike seeing when the sum of a multitude of reports is expressed in a headline, only for people to nitpick through it without even attempting to check the actual methodology for themselves.
1 cm/year is 33 times faster than 3 cm/100 years, not a few hundred times faster.
AFAIK terra preta is far from fully understood, and farther still from being widely used. Very cool stuff though.
The article is crap, it doesn't answer any of my questions. You think I didn't check it before I commented? It quotes Maria-Helena Semedo throwing out that figure without any clarifying context or explanation. Without saying where it came from or what it's actually a measurement of.
Also, three centimeters per one thousand years is much slower than three centimeters per century.
I don't see volume being very natural reporting unit as farming usually counts by area, so reporting a total volume just forces everyone to convert to length themselves.
You want more details, read the Status of the World’s Soil Resources report: http://www.fao.org/3/a-i5199e.pdf
The problem with that UN official quote is the area being talked about is poorly defined. Is it the global average for all farmland, or the global average of all land area (including Antartica?) The official doesn't clarify, but it matters because if it's the later that number is obviously being suppressed by regions that lack topsoil and were never considered arable in the first place. How long does it take the sand dunes of Gobi desert to create a centimeter of top soil? And I'm sure permafrost regions create new topsoil at an absolute snails pace if at all (although global warming might change this!) It seems obvious that regions like this will make that global average number seem a lot more dire than the situation actually is.
Don't get me wrong, I'm confident that topsoil erosion is a real problem that needs to be addressed. But I think that number; without the official providing any clarification, justification or context for it; qualifies as alarmism. It's obviously not going to take farmers 1,000 years to regenerate three centimeters of top soil on their fields.
It hasn’t been scientifically validated. Anecdotal reports from farmers like Mark Shephard are that it works. [1]
1: https://books.google.com/books/about/Restoration_Agriculture...
Ridiculous claim made by "UN official". Sad state of modern journalism. Are we 6 cm away from discovering Roman artifacts in any of the fields across Europe? Or we have to dig a bit deeper?
Not anyone who ever had a large compost pile in the backyard...