Rick
Well-Known Member
bjorn, about your plant being insubstantial(?) and more prone to rot - maybe the light is a bit too low so it stretches. this could lead to thin cell walls. the word is that too much ammonium type fertilizer can make plants grow too lushly, which can lead to possible disease. also if a plant can normally grow too quickly, it can be deficient in both calcium and phosphorus. calcium moves at one speed and sometimes if it isn't available all the time in a small amount a fast-growing plant can outpace the movement from the roots to the rest of the plant. with things like mums that can grow very quickly, have large floral structures (lots of branching and flowers), a lack can cause brittleness, less branching and flowers. not exactly sure how this would translate to orchids though I noticed when I was trying a too-low phosphorus fertilizer things that grew quickly would have problems. actually also for larger plants a breeze can help toughen up plants. movement causes some plant cells to react and toughen cell walls and slow upwards growth. air jets over the top of easter lilies have been used in lieu of growth regulators to keep them shorter. I think that you can use too much nitrate fertilizer for orchids (my opinion based on work, reading and seeing discussions here, not experimental), but I believe a balance of different types could be the most benefitial, and from Rick's experiments, one with less potassium and more calcium.
Lots of soft growth (often with lots of blooming), poor substance, susceptability to rots is very much in line with too much N and K with inadequate Ca/Mg. Ca (and Si) needs to get incorporated into cells walls for better substance (and disease resistance). Excess K blocks uptake of divalent cations (Ca/Mg), which in turn reduces uptake of PO4.
In general for all bio (ion pumps) and substrate ion exchange (in bark and CHC) the K will always win out for adsorption if Ca or Mg is not present at higher concentrations than K. A moderately soluble top dress of something with Ca/Mg in the potting mix will offset standard fertilizer NPK imbalances, but you get faster (and I think more controlable) results if your fertilizer and irrigation water always contains more Ca than the K present in fertilizer.
Keep in mind that unless you are using very soft water or RO water there is probably at least 5-20 ppm of Ca and 3-7 ppm of Mg present in standard surface waters. I could guestimate these values better if I at least know the hardness of the water used.
Noticing the sanderianum thread that Wendy just posted, even though she uses a heavy K fertilizer and rain water, the potting mix contains a substantial amount of "limestone screenings".
After the large multi growth multis I've lost over the years that grew and bloomed like crazy for 5 years going into slow burn demise, I'm now to scared about pushing that much K over extended periods of time with only potting mix sources of Ca/Mg for backup.