Monday, August 31, 2015

The solution to the spinach fallacy?

Last week I blogged about Spinach and the iron fallacy. I analysed an early set of data by Thomas Richardson (1848), who calculated the amount of iron in combusted ash for various vegetables and fruits, and showed that spinach is not at all unusual in its constituents. The idea that spinach is rich in iron is untrue, and the story about a mis-placed decimal point seems to be nothing more than an urban myth.

In the meantime, Joachim Dagg, at the Natural History Apostilles blog, has reanalysed Richardson's data and revealed that The first source for the spinach-iron myth is likely to have been a somewhat inappropriate attempt to combine his data for the percent iron values in relation to the ash with the percent values of the ashes in relation to the fresh matter.

So, I have recalculated the phylogenetic network using these "adjusted" values. I used the percent values of the chemical constituents in relation to the pure ash (raw ash minus carbonic acid, charcoal and sand), and combined them with the percent values of the ashes. The issue here is that radish roots and leaves have the largest ash values, followed by cherry stems and spinach. This leads to an over-statement of the chemical contents. In particular, the iron content moves spinach from being ranked sixth to second (behind radish foliage, which is not usually eaten).


  1. Is that the long split separating radish roots due, mainly, to its extreme value for phosphoric acid (Phosphorsäure)?

    1. Radish roots are ranked in the top 3 for all components, but phosphoric acid certainly is the extreme outlier, being 5 times greater than the next species (and 15 times greater than radish foliage). /David

    2. I wonder what the network would look like, if you deleted the phosphoric acid data from it? Would that separate spinach by a long split from the rest being due, mainly, to iron phosphate?

    3. The other radish values are still the largest, and since the data are normalized within each column, the network turns out to be very much the same with or without phosphoric acid.