Your hps was on for 3 out of 4 pictures, it totally makes plants look yellow. I know, I've got one also.

In the future it would be best to take pictures with natural light or at least something close to it.

But yeah if it was me I would flush the soil them up the feeding.

Edit: 4 of 4, one is hard to tell.

HPS was on for all the pictures. I know photographs taken in natural light would be more helpful, but the plant is too heavy to move around just to take photos. Grity mix + a thick ceramic pot adds up to what feels like 75 pounds.

You can see in the first photo the bottom right of the plant is uniformly green. If I turned off the HPS, that's what most of the lemon would look like. However, there are a handful of leaves that are splotchy yellow. These are the ones that I'm thinking are showing N deficiency.

I guess I'm just surprised that there could be any N deficiency at all with the amount of fertilizer I'm applying. The amount I'm applying, 1.2 mL per quart, is equal to the recommended production rate of 1 tsp per gallon.

Have you tried a vinegar + water flushing? Perhaps pH is playing a role here.

Josh

Don't forget that individual leaves don't last for ever.
There is a perfectly normal life-cycle for each leaf as the whole plant grows. At times, a few leaves will turn yellow and drop naturally however much you feed.