Thanks Rick for your fast response and thank you also for the other participants at this very interesting discussion.
Here are some data I taked this morning on some of my plants.
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The plants are cultivated in a substrate made with70% bark/30% CHC at which I added 10% of small expanded clay balls. No dolomitic material added. As fertilyser I use a 4/1/1 NPK (not N/P2O5/K2O). The ratio NNO3-/NNH4+ is 85%/15%. I fertilise at 40 ppm N one time per week. At this Nitrogen concentration the one in Ca++ and Mg ++ is 30 ppm and 10 ppm respectively. Water used: rain water always.
Measured EC of the fertilyser solution ~ 350 µS. pH adjusted to 6 with KOH 1M.
Here are my conclusions after two years of culture using these conditions.
Using low or medium level of fertilisation with a fertilyser having a high proportion of Nitrogen in the nitrate form cannot regulate tightly the substrate acidification.
Because the high humidity, the presence of nitrate (an oxidizing agent) and the bacteries developpement, all these factors (and many more other for sure) leads to a situation where the substrate becomes acid and sometimes very acid.
This situation can be attained in the short or medium terms. (6 months to one year)
I have no data about the ideal pH for nitrates assimilation but in culture In-Vitro it is common to use a pH between 5.7 and 6.
My understanding of the reasons for which it is necessary as often to repot are :
The acidification of the substrate below 5.5 do not allow the nitrogen assimilation (and maybe other minor elements) and stop the plant developpement .
The acidification create an environment favourable for the roots rot.
In my opinion (IMHO) this is the raison why Rick has so good results with so low nitrogen distribution and using a substrate made mainly with dolomitic stones mixed with some mousse ... the pH of its substrate is stabilized and the Nitrogen assimilation is thus optimized.
Maybe in the future we shall come back on the necessity of granular dolomite addition in paphiopedilums substrate. Not as a Calcium Magnesium source as it was recommended in former days but because its pH stabilization properties.
Your comments will be highly appreciated.