[darkpuma]

> "Generating three centimeters of top soil takes 1,000 years,"

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..)

[GlenTheMachine]

It's also just wrong.

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.

[neolefty]

Yep. You can definitely "grow" topsoil very very quickly. Similar to the ratio between the productivity of land for hunting-gathering and agriculture.

Source: Farming & gardening background in the midwest.

[tfehring]

> Topsoil should properly be measured in volume, not distance.

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.

[0] http://www.fao.org/docrep/009/t0733e/T0733E06.htm

[viivaux]

> Topsoil should properly be measured in volume, not distance

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.

[splendidtube]

Only responding to the one part of your comment, it was 3cm/1000 years, so a few hundred (333) years was accurate.

[sigmaprimus]

You are both right, farmers measure rainfall in inches, (which translates to inch acres) so it is both a mesurement of distance (inches) and acres (how many they own) think of centimeters per year the same way. Yes topsoil can be made much faster than these numbers given but there are consiquesnces, methane is released from the process of composting which is 19 times the greenhouse gas than the CO2 released from burning. Also the volume is affected by soil compaction and the production potential is affected by increased salinity from unused fertilizer, the outlook is scary but humans always adapt.

[darkpuma]

> "ROME (Thomson Reuters Foundation) - 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."

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.

[SiempreViernes]

The statement was about the global reserve, so clearly these are all global averages.

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

[darkpuma]

Farmers know their area, so when they count rainfall in inches it's implicitly volumetric.

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.