On Thu, 6 Feb 2003 18:43:08 +0000 (UTC),
(Dave Sheehy) wrote:

Speaking of chemical species what's the rationale for the preference of
N as ammonia over N as urea?

The theory was that urea had to be broken down by bacteria before it
was available to the plant which wouldn't happen well in an orchid
potting mix. The results of an experiment to test this was published
in the AOS Orchids magazine last year. It showed no difference in
phals grown using ammonia or urea based fertilizers. It would be
interesting to run an experiment to test whether this holds true for
mounted orchids as well.

deg