Are We Baking with Volcanic Ash?

wheat-rachis.jpg I was trawling the USGS photo archive for upcoming Friday Rock Blog candidates when I came across this scanning electron micrograph of wheat. It’s from a gargantuan volume published in 1981, full of initial reports about the eruption of Mt. St. Helens.

Why is there a picture of wheat in a book about a volcano?


It turns out people were curious about how quickly material from the ash would be incorporated into the soil nutrient supply, and particularly into crops. So a month after the eruption, USGS scientists sampled wheat and soil from fields that had been ashed on.

The answer? Wheat from sites with 20mm or more of ashfall had slightly elevated sulfur content. I do not know if this means that bread made from Washington wheat in 1980-1981 was more or less delicious than it would have been otherwise. Worrisome things like cadmium and other heavy metals were apparently not affected by the eruption, at least not in the 1980 wheat crop.

wheat-floret.jpg But before they could get to this conclusion, they had to carefully wash all the tiny bits of ash off the wheat. You can see in this picture how the ash tends to hide in little crannies on the wheat, so if you’re not careful, your analysis will just tell you the bulk composition of wheat + ash, which is not useful. So what you do first is to hang the wheat stalk upside-down in a beaker of distilled water, and whack it with some ultrasonic vibration to knock the ash off.

Reference

L.P. Gough, R.C. Severson, F.E. Lichte, J.L. Peard, M.L. Tuttle, C.S.E. Papp, T.F. Harms, and K.S. Smith, Ash-fall effects on the chemistry of wheat and the Ritzville soil series, eastern Washington, in USGS Professional Paper 1250 which is available for purchase or free download from the USGS publications warehouse.

Comments

  1. Cyrano Jones wrote:

    This is so cool.

  2. ScienceWoman wrote:

    But were the commercial wheat harvests hung “upside-down in a beaker of distilled water, and whack[ed] … with some ultrasonic vibration to knock the ash off.” Because otherwise, we were baking with volcanic ash, whether or not it was incorporated into the plant itself.

  3. Ellery wrote:

    The ash could have affected the soil pH too, I’d suspect. The soil pH, in turn, could have altered the solubility of the existing minerals in the soil, causing further changes in trace elements absorbed into crops like wheat.

  4. Maria Brumm wrote:

    My guess is that there wouldn’t be much ash stuck to the actual wheat kernels, and the threshing process would take care of it. The cows or whatever animals get the chaff and straw would be the ones eating the ash. Nom nom nom.

    Ellery, I think they measured soil pH, but without going back to the library to look I can’t remember whether it changed much or not. They did speculate that changes to soil albedo (and therefore soil temperature) might have affected the plants’ development as well.

  5. Ellery wrote:

    Yeah, an albedo change — I wouldn’t have thought of that!

    On a related note, when I teach my “electron microprobe analysis and its use in geology” course, the students are surprised and amused to see that one of the first week’s readings is titled Electron Microprobe Analysis of Minerals in Barley and Malt Tissues. When I first came across that article, I knew that it had to go in my course packet.

  6. Maria Brumm wrote:

    Is that barley and malt as in for beer? That’s awesome.

  7. Ellery wrote:

    Absolutely — the authors analyzed barley as it was steeped, malted, and kilned for beer. It was in the Journal of Food Science (1976, vol 41, pp 1024-1028).

  8. Lab Lemming wrote:

    “But before they could get to this conclusion, they had to carefully wash all the tiny bits of ash off the wheat. ”

    Although this was their method, they probably didn’t have to do it this way. They could have also measured an element known to be biologically incompatable (e.g. Zr), determined the S/Zr ratio in the ash, and then done a correction based on the Zr content of the wheat, which can safely be attributed to the silicate portion.

    That’s how we recommended biologists correct for the glass slide when they were analyzing chopped brain thin sections etc for heavy metal contamintation.

  9. Ellery wrote:

    Lab Lemming wrote: “…They could have also measured an element known to be biologically incompatable (e.g. Zr), determined the S/Zr ratio in the ash, and then done a correction based on the Zr content of the wheat, which can safely be attributed to the silicate portion…”

    That’s a clever trick I’ll have to remember, but it does require two accurate measurements, not just one. It depends if they were more confident in their ability either to clean the wheat thoroughly or to analyze both the ash and the wheat accurately.

  10. Lab Lemming wrote:

    Well, ICP didn’t exist back then, so low level Zr detection might have actually been tricky. Maybe I should do a blog post on the methodology of quantifying ‘clean’…

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