Hey Gardenking, We can learn from each other.

Here's what I can tell you about yellowing....

If it is a Nitrogen deficiency, then the yellowing starts between the veins on the leaves (older ones first) and then stems and veins under leaf may turn purple. The worse it is, the more leaves that will fall.

Too much Nitrogen can be toxic too. Leaves will turn brown and then fall. Leaves would have been excessively green beforehand.

Could be zinc deficiency. The most common micro nutrient deficient. Growing tips develop thinner leaves and contort as they yellow. Leaf tips will discolor and burn.

Those are the most common causes. Pictures Help to identify. Do you have a digital?