Protea and Iron

Australia is an extremely old continent where erosion and soil leaching
by rain has depleted phosphorus to such an extent that Australian soils
have some of the lowest levels of phosphorus of any soils on Earth. As a
consequence, certain Australian plants, including many Proteaceae, have
evolved the ability to extract and retain phosphorus with extreme
efficiency. One side effect is that, when grown in soils with normal
phosphorus levels or when fertilized with phosphorus, the plants
actually extract and retain so much phosphorus that they suffer toxic
effects.

I have no references readily at hand, but I have rarely read anything on
Australian Proteaceae without the author mentioning the toxicity of
phosphorus. For example, the owner of TransPacific Nursery notes the
following when discussing Australian banksias: "...phosphorus fertilizer
will be very toxic to them--take my word for it." Additionally, Mr.
Fukada alludes to research that phosphorus does cause interveinal
yellowing of the leaves in macadamia nuts: "I recalled that work on
macadamia nuts indicated that they would show this sort of symptomology
as a result of too much phosphorus."

I agree with Jerry Heverly's comments that it is doubtful that
phosphorus *directly* interferes with the absorption of iron. However,
since phosphorus is toxic to many Australian Proteaceae, it may weaken
the plant and its root system such that the plant is not able to
properly absorb nutrients resulting in interveinal yellowing of the
foliage. The situation may be similar to the yellowing of the foliage
that develops when certain plants are grown in excessively wet soils.
Obviously, excess water does not *directly* interfere with the
absorption of iron. However, excess water may damage or weaken the root
system resulting in the poor absorption of nutrients and the development
of iron deficiency symptoms in the leaves.

Of course, I am not saying that excessive phosphorus is the cause of the
yellowing foliage in Mr. Fukada's macadamia nuts. However, in view of
the toxic effect of phosphorus on Australian Proteaceae, it should not
be discounted as a possibility.

R. Xavier Osorio
http://www.wfnirvana.com
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