It's a baby factory Bjorn!!!
Crank them out!!!!
Have you been tracking pH or TDS in your potting mixes?
I just started a little bench experiment today with sphagnum moss and aragonite sand for buffer. It's only 3 hours in, but I'm pretty amazed as to how little aragonite it takes to buffer the moss.
But the aragonite is like a coarse sand (not chips), so a little may go a long way for fast pH control.
For preview I'm getting a pH of 5.8 with only 1 gram of sand added to about 50 ml of tightly compressed moss (saturated in RO water). On a weight to weight basis that may seem like a lot of aragonote, but on a volume basis you can do a lot with 50 ml of compressed moss. Also I went down from the 1 gram of sand, and even with only 0.2gr of sand, I'm getting a pH increase over the control. At the end of the test I'll measure alkalinity.
Another concern I posted about calcium carbonate solids is that they can adsorb a lot of phosphate. So you might want to check to see if P is coming up short in some of these systems where plants may be going slow.
Thanks Rick, you do touch something I have been speculating on. Because even if I like to use marble chips, I am not always using it. Some species seem to like it, other not. You see it on their root growth. On the other hand, I do like the gravel due to the packing, or lack of, leaves a lot of air, does not degrade or accumulate salts (leca can), ease of potting due to its free flowing characteristics, imparts stability to the pot due to weight, etc.
Ok, that was my reasons for using marble chips. You are entirely right concerning the effect on pH, if you mix water at pH 5 with one chip, it does increase to approximately 7 at a surprising speed. Whether or not that is a problem is dependant on the species I believe. You are indicating that calcium carbonate can strip the fertiliser solution of phosphate producing highly insoluble calcium phosphate. This made me curious so I checked it up and found that the Ksp of Ca3(PO4)2 is indeed very low, in the ten in the power of -30 range, while aragonite has one of 6x10^-6 only. So based on this, one could believe that aragonite is much more soluble than calcium phosphate. That is not the case, both are soluble to a Ca level of some 100ppm if you do the calculation. It is not that straight forward though it all depends on the amount of calcium and pH in your feed. More about this in this paper:
http://www.jbc.org/content/143/3/703.full.pdf
If you are interested, you may repeat the following exercise with me:
If you use hard water with an alkalinity of 250mg/l and baCk mix 1:10 with RO water then you should have a Ca concentration in your irrigation water of approximately 0.625mmol/l . Using a Ksp for calciumphosphate of 2x10^-33 and a Ca concentration of 0.625 mmol/l yields a phosphate (PO4) solubility limit of approximately 1x10^-9 g/l (1 nanogram!!) expressed as calcium phosphate dissolved. So in other word, an effective way of stripping phosphorous is to use well water.
oke:
That is probably not the case due to complex formation, pH influence etc.
Back to the limestone. It is definitely possible that limestone strips phosphorous. But so would any fertiliser mix with calcium. Based on that, it seems probable that the plants have mechanisms in which the uptake of P is regulated. In the following paper this is described to some extent.
http://www.plantstress.com/articles/min_deficiency_i/P uptake.pdf
The uptake seems to be via organic acids expelled by the roots, so it may definitely be possible that the plant can absorb precipitated phosphate.
Rick, you are right stating that marble chips influence the compost pH. And that not all orchids like it. So far my experience is that micranthum and vietnamense inparticular seems to like it, or at least tolerate it. Rothschildianum is more questionable, guess better to use more standard mixes.
