Answer: I just answered your first question, but if you have determined that the problem is nitrogen (which I would be a bit surprised by since the other plants are doing fine) then a foliar feed would provide the quickest supply of nitrogen. If they are brittle then they are not in good condition and fertilizing would add more stress rather than be helpful. The best thing to do is make sure you are watering correctly so the soil stays evenly moist or damp like a wrung out sponge, not sopping wet and not dried out. If you are using a watering system, make sure it is functioning correctly and applying the water evenly throughout the planting. Unfortunately, this type of symptom is nearly impossible to diagnose long distance. Good luck with your arborvitaes.
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