Rick
Well-Known Member
Also, now according to Rick's findings, a little calcium in the mix helps as well as increased Magnesium (epsom salts). I've known this about Brachy's and certain Phrags (especially besseae). When they are prone to rot, if you add some limestone screenings or feed with a high calcium fertilizer and suplement with epsom salts, rot becomes much less of an issue. What I didn't know is that it's the ratio of Calcium and Magnesium to Potassium that is important, not the total Ca and Mg alone. Too little Ca and Mg and too much K will encourage rot. Reverse this and rot is discouraged.
To modify this partially, the ratio is important, but reducing the overall amount of K is probably more so.
I have another paper on tropical epiphytes (based on bromiliads but applied to all epiphytes, orchids ferns...) that indicates that these plants are extremely efficient at pulling K from the environment. Actually all the papers I've dug up indicate that all plants selectively pick up K efficiently, but the epiphyte paper was the most detailed for something including orchids on that level of physiology.
So If you give lots of K they will selectively pull it out of the environment almost regardless of how much Ca and Mg is laying around.
A paper I have on Wild rice physiology indicates a similar effect. Even with excess Ca and Mg in the environment, the more K offered, the more K picked up by the plant, and blocks uptake of Ca/Mg. When environmental K gets high enough (sequestered in old potting mix for hobby orchids), the blocking effect on Ca/Mg also inhibits the uptake of P.
All natural surface and ground waters have a little potassium in them, and any organic potting material has K in it too. Given the efficiency of K uptake of orchids there really is no need to add any K by fertilizer.