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With regard to the mechanism of foliar absorption of nutrients,
John Stanley [OGD V7#372 message 3] is right; the wonder is not the
advantages of foliar feeding but that it can taker place at all. As I
recall, it was Tom Sheehan who obtained experimental evidence for its existence,
but I did not read his original papers. As a retired experimentalist,
I can assure you that manpower limitations dictated that it could only have been
examined for a limited number of orchid species and for a limited number of
nutrients.
The waxy outer layer of leaves would limit
the ability of nutrients to be taken into the leaf. Perhaps to some
extent they come into contact with the leaf cell membranes via the
stomata. Once ionic nutrients [potassium, ammonium, nitrate, phosphate,
etc.] come into contact with the leaf cell membranes proper, they would move
into the cell interior via specific carriers within the membrane. Very
small molecules such as water and urea can actually diffuse across the lipid
barriers of cell membranes, but nothing larger, like fish emulsion. Of
course the roots are specialized to promote the uptake of
nutrients.
With regard to Steve Toplitz' "thoughts,"
there are countless cultural advisories based on "thoughts," "gut feelings" and
"revelations" that give rise to cultural myths without value. As a retired
Physical Biochemist, Steve's musings make no sense to me whatsoever, nor can he
cite experimental data or technical expertise to support them. Good orchid
culture still depends on light, water, temperature,
medium, fertilizers and pest control, and there is little point in
seeking cultural advantages
elsewhere. Bert
Pressman
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