Questions on Correct Fertilization

[Ray]

Originally Posted by Orchid Whisperer

From your second-to-last paragraph, it appears that you agree with what I said in my second paragraph - that a weakly acid pH is best. Nutrients are most available to plants in that pH range. That is essentially what the graph is showing; avoid the extreme pH ranges, and you should be OK with respect to nutrient availability.

Yep. I agree completely. What concerns me about that graph is the fact that in solution, even at realtively "extreme" pH levels, some of those cations are still available to a plant, while they would be sequestered by the CEC of a soil at that pour-through pH.

I have not analyzed orchid media for CEC. I would imagine that the CEC of orchid media would depend on a lot of things. For example, partially decomposed fir bark might potentially have a higher CEC than fresh fir bark. I have read that certain orchid media (especially coconut husk chips-well rinsed) have moderate CEC, but this not from peer-reviewed research. If some orchid media do have moderate CEC, it would still make sense to keep pH in that weakly acid range.

I don't think there has been any published research in that area. My "gut feel" however, is that the CEC in orchid media is relatively insignificant, compared to that in soils.

Regarding potassium and potential interference with uptake of calcium and magnesium, I have not seen any peer-reviewed, published research on the matter (scientific backup, as you say). If you know of any articles on the matter (even if it's only one or two), can you post a citation? I can only say what I've observed in my own plants - I typically use fertilizers that provide NPK and micronutrients, as well as supplemental Ca and Mg (from gypsum and Epsom salts), and haven't observed signs of Ca and Mg deficiency.

I have found Marschner's book on Mineral Nutrition in Higher Plants to be thorough, if not overwhelming, although again, it relates primarily to terrestrial plants, not orchids. Benzing's book on epiphytes (for which I can't recall the title) has been a great help in making that informational translation.

As to fertilizers, I thing Bill Argo and the group at MSU probably did the most for making the importance of calcium and magnedium known to the orchid world.