Good information, thank you!
Ok...seeing that I'm several states away, I think the short answer has to be: we won't know for sure unless we get a leaf tissue nutrient analysis.
However...based on the information we have, we can still try to logically deduce which factors are more likely to be responsible for the symptoms:
- As the substrate pH increases from 6 to 8, the availability of magnesium (Mg), calcium (Ca), and molybdenum (Mo) will also increase.
- As the substrate pH increases from 6 to 8, the availability of iron (Fe) decreases dramatically.
Possibility #1: High pH nutrient lock-out
If there was adequate Mg and Mo in the soil solution, then the plant should have no problem taking it up when the pH of the substrate is 8.
Even when there is adequate iron in the soil solution, the plant will have a difficult time acquiring it when the substrate pH is 8.
It's not always the case with other species, but with poinsettias, I have seen situations where an iron deficiency looks very similar to magnesium deficiency.
Therefore, it's completely possible that we're looking at an iron deficiency...at least to some extent.
Possibility #2: low fertility
Experts disagree on whether it's best to cut off fertilizer in poinsettias for any length of time before shipping. Poinsettias do use less fertilizer as they progress through the flowering stage. As a consequence, excess fertilizer salts can build up in the substrate -- especially if they're not irrigated to leaching.
Some recommend using clear water 2-3 weeks before shipping to avoid salt build-up and help prepare the plant for life on a retail shelf.
Others suggest that using clear water increases the risk of nutrient deficiencies that appear right before shipping time and reduces the shelf life of the plant (and sell-through).
It's equally possible that 3-4 weeks of clear water may have been a little too much.
Other related considerations are water alkalinity, lime type, and the amount of lime in the mix. If your irrigation water is naturally low in Mg and Ca, then the nutrition program should account for that with a little extra of both. Dolomitic lime is absolutely preferred in this situation because it contains magnesium. Poinsettias take time to grow. More time = more irrigations = more opportunity for the lime to leach out the bottom.
I recognize that the 14-0-14 is probably a Cal-Mag formulation. And considering that the pH drifted up to 8, I'm also assuming that you have water with naturally high alkalinity. Correct me if I'm wrong. =)
Possibility #3: Environmentally-induced deficiency / product phytotoxicity
Some nutrients, such as calcium, rely on transpiration to move throughout the plant. If the relative humidity around the stomata is high, then transpiration slows down. I usually see this in seed houses early in January and February when everything is warm and humid, but it could conceivably happen any time of year.
If the foggers are running quite a bit, then it might increase the humidity along the rows to a point where transpiration slows down. I haven't heard of this being a problem with magnesium.
Alternatively, there could a phytotoxic response to the products coming from the fogger.
Another consideration is that the edges are drying out faster and are experiencing an extended series of minor drought-induced nutrient deficient moments.
Possibility #4: Compromised root system
I forgot to ask you about the root systems. I'm assuming the roots look fine, yes? Perhaps you could post a photo? Anyway, root rots can certainly cause nutrient deficiencies by reducing the ability of the roots to take up water and nutrients.
Possibility #3 takes a lot of imagination and seems like a stretch for me.
You should be able to evaluate whether #4 is a reasonable possibility.
I think that #1 and #2 are both possibilities with a reasonable amount of likelihood. It could a single cause or a combination of the two.
If you're keen on getting to the root cause (pun intended), then I highly recommend getting a leaf tissue analysis. If the roots look sketchy, it might be a good idea to send it in to a diagnostics lab to see if you've got a disease problem on your hands.
I like that you're working on lowering the pH. A pH of 6 would be right in the sweet spot.
If you're into experimenting, consider something like this:
- Apply an iron drench to 3-5 symptomatic plants (make sure to rinse off the foliage)
- Apply a magnesium sulfate (Epsom salts) drench to another 3-5 symptomatic plants (if you have high alkalinity, use 6-8 oz Epsom salts / 100 gallons water. Low alkalinity use12-16 oz Epsom salts / 100 gallons water).
- Wait a few days and observe the results.
You might even try to apply both treatments to another 3-5 plants and see what happens. I'm not sure if it's a good idea to tank mix the Epsom salts and the iron drench, though.
Feel free to contact me directly if you like to chat about this some more.