On the regeneration of topsoil

I often see this statistic….or at least numbers like it…..and I\’ve always another question I want answered:

Yet all terrestrial life depends on a thin layer of earth, normally just six to 10 inches thick, for survival – and the world loses at least 30 billion tons of it a year, mostly from the drylands that provide nearly half of the world’s food.

OK, yes, topsoil washes away. We know it\’s been doing this forever. Over thousands of years by looking at river deltas, over millions and hundreds of millions by the existence of sedimentary rock.

So we do know that it goes away, off somewhere instead of being in those nice fields. That it has been doing this forever also leads us to the necessary conclusion that there is some process which creates it. Here I\’m thinking of Darwin\’s observation about earthworms and the stony field on the estate. Or it might be humus that collects in what soil is left etc.

What we want to know therefore is the balance: OK, so 30 billion tonnes a year is gently floating downstream each year. How much is being created each year? And I\’ve never actually seen a number for that. P
perhaps my Google Fu isn\’t good enough.

Does anyone know? It wouldn\’t surprise me at all to find out that the balance is negative, that we are eroding that topsoil on balance. But I do think that the net figure is likely to be more useful than the gross one which we\’re continually presented with.

Update, in the comments, so, is 30 billion tonnes a lot or a little?

Hmm, zeros tend to go missing when I do this sort of thing. But cropland (ie, pasture plus arable) is apparently some 4.7 x 107 km 2……. 47 million I make that. Call the topsoil 10 cm, there\’s 100,000 cm to a kilometre, thus there\’s 47million / 10,000 km 3 of topsoil. Am I wrong yet? 4,700km 3 of topsoil.

1 km 3 of water weighs 1,000×1,000×1,000 tonnes, or one billion tonnes. Earth is, to a reasonable approximation, 2x density of water (erm, might be a bit high, mining assumption is that whatever is 1.8x). Call it 9,000 billion tonnes of topsoil among friends.

Gross loss is 30 billion tonnes a year. 0.3% of all topsoil is lost in any one year.

Which leaves us with our final thought. Do we think that the regenerative capacity of topsoil is greater than that? Alternatively, does it take more than three centuries to replace what is washed away?

15 thoughts on “On the regeneration of topsoil”

  1. The other issue is that I doubt anyone has the slightest idea whether 30 billion tonnes of soil is a lot or a little.

    Anyone like to hazard a guess at the total amount of top soil globally ?

  2. I’m a geologist, not a soil scientist, but soil is constantly being created. As existing soil is lost, fresh rock becomes exposed, and weathers to generate the clay minerals (amongst others) which comprise the matrix of the new soil. Add in the biology “soil forming processes”, and the old soil is replaced by new.

    Loss of soil mass is partially compensated by isotatic rebound, so the crust will rise buoyantly and maintain freeboard (height above sea level).

    Material lost to the continents is eventually returned via subduction and volcanism. There’s a good reason why people live on and around volcanos. Fresh volcanic soils are rich in the trace elements needed for fertility. The least fertile soils are found on the ancient cratons, where you are likely to find Al and Fe oxides and little else.

    If that helps(?)

  3. On the slopes of Mt Mahameru (E Java) they grow square cabbages, the soil is that fertile.

    Erosion causes civilisation, of course. (Think Ganges, Nile, Yellow, Euphrates rivers.) Can’t have that.

  4. Tim, the article says topsoil is 6 to 10 inches thick. The average of 8 inches is about 20cm so you can double your estimate.

  5. Topsoil is not destroyed by erosion:it is either diluted and dissolved permanantly in water or it is moved and re-distributed/ re-deposited elsewhere.
    The topsoil washed out to sea is prob irrecoverable. A lot of topsoil is washed into lakes and some could be recovered by dredging same. The amounts would not be vast but it would be high quality stuff.
    Also, we could grind up volcanic rock and mix with clay/diatomaceous (sic) earth/seed” loam and bacterial culture to create more if needed. (This same process is used to help terraform Ganymeade in Robert Heinleins “Farmer in the Sky” novel).

  6. The issue is the rate at which topsoil is being lost. Saying that some has been being lost, forever, tells you nothing useful. I also suspect that looking at the global figure isn’t too useful: if topsoil in the UK, say, is stable; but its being lost at 10% a year in bits of Africa, say, then looking at the average isn’t helpful.

    I’m not sure but I think earthworms are primarily turning over soil, so I doubt that helps your argument.

  7. I think there’s a reasonable science-based argument that primitive modes of production are a kind of environment destruction system. You start off with some good land, plant crops on it until it’s too poor for that, put high yield animals on it until it’s too poor for that, and then it’s scraggy goat time.

    Botswana looks like that kind of disaster in the making; currently doing well economically, but half the population subsist by grazing cattle. Uh oh.

  8. William, the global figure is still useful. We don’t try and use 100% of the surface for farming. Some countries do better at farming, others mining, others services. If Africa is losing but another country is stable or gaining then that country will do more farming and Africa wil find something else to do.

  9. English farmers have strict rules to follow to ensure their soil isn’t lost or degraded. DEFRA published the background evidence for this strategy here –
    http://archive.defra.gov.uk/environment/quality/land/soil/documents/evidence-paper.pdf

    It seems the erosion evidence is from one paper
    Evans, R. (1996): Soil Erosion and its Impact in England and Wales. Friends of the Earth Trust.
    (which I can’t find on line)

    But the estimates are “Typical soil erosion rates are in the order of <1-20 tonnes/hectare/year." which I think tells us they haven't a clue as to what they are.

    Many millions are being spent on a potential problem which hasn't been quantified.

    On my own farm, where I have to keep records of any soil erosion my highest estimate of loss is under 10kg per ha. There may be a bit of soil moving from steep slopes to the bottom of fields but that is about it.

  10. Mr Ecks
    “A lot of topsoil is washed into lakes and some could be recovered by dredging same.”

    I don’t know if anyone has tried doing that on a large scale but it is possible to buy bags of peat that has been recovered from rivers at water treatment plants. Costs a bloody fortune though.

  11. ” Do we think that the regenerative capacity of topsoil is greater than that? Alternatively, does it take more than three centuries to replace what is washed away?”
    Data point;
    There’s a church close to Abridge, Essex. Built C12th. There’s no sign of any subsidence & as it’s likely the foundations rest on the gravel layer, visible by the nearby river, no reason there should have been. The doorstep is approx 6ft below the adjoining field. Very uncommon to see a church built below the general land level for obvious drainage reasons & as the surrounding land is reasonably flat, there’s no particular reason to apportion any raising of the ground level to erosion deposits.

  12. It’s worth mentioning that not all crops need topsoil. Commercial tomato growers in northern countries, for example, don’t use the topsoil much since it spreads diseases and tomatoes grow without it. They lay down an inert material called rockwool on the floor of their greenhouses and the plants grow in that. They are watered, fed nitrogenous fertilizer and a few trace nutrients.

    I’m not an expert in the subject, but I think there are quite a few commercial crops that can be grown directly into subsoil. I’m sure there is an expert around here who can clarify things.

  13. bloke in spain at 13:

    Churches lower than their surroundings are quite common, and has been commented on in passing in Bill Bryson’s book “At home”.
    When you estimate how many burials there have been over the centuries in the average small rural
    churchyard — e.g just 10 a year is 1000 burials per century, for centuries — it’s not surprising the soil level has risen since the church was built.

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