Hoping I have understood your question correctly, here's my answer.

You should provide as much liquid water-fertiliser solution for each plant as it's watering requirements are. You should calculate the fertiliser dosage by the solution rate and the frequency of application, NOT by the amount of liquid solution you provide in any one application.

Regarding the frequency of application, the ideal would be to maintain a correct nutrient content in the soil solution all the time when fertigating (usually expressed as parts per million or billion). Thus, ideally, fertilising should occur every time you water. This is what, for example, hydroponic growing aims to achieve.

In this way you have only one factor to consider; solution rate and fertiliser contents which may differ due to seasonal plant requirements and plant maturity.

PS. This is a case of do as I say and not as I do, as I find liquid fertilisation too demanding on my pocket and my time.. I use granular fertilisers for all my pots and in-ground plants. In the case of pots I rely on watering and on the kind of fertiliser to not dissolve all at once but fairly slowly through a sequence of waterings. This is NOT the ideal as you really have no way of knowing exactly the actual amount of nutrients present at any time in the soil solution. But it works for me and my amateur growing requirements, probably due to my experience.

Whenever I use Foliage Pro or any other water soluble product , I pour it on until I see it leaking out of the pot. This way I know the whole pot is saturated with it. In answer to the question, my 20" pots usually are still damp so I really don't need the full gallon, but I still check to make sure whatever applied is leaking out of the bottom.

I don't use Foliage Pro on a regular basis because it feels like it takes forever to mix it up for 20 trees. I use an organic granular citrus product, Osmocote Plus and have just started using Alaska Fish Emulsion. Between all the products listed, I believe I have every necessary nutrient covered. lol.

What nikthegreek said. If you want to be pointed to something in black-and-white your question is answered by the instructions on the bottle, but it isn't obvious without prior knowledge. The section which is relevant for growing in containers using 5-1-1 mix is "Hydroponic Production", specifically the last point which says "1tsp/gal for non-recirculating systems". Non-recirculating is otherwise known as 'drain to waste' or 'a container filled with soilless medium (aka: 5-1-1)".

"Production" in this context, based on a thread here from several years ago which I did not bookmark, is for when plants have ideal conditions and you are trying to get them to produce as much as possible. For example, a greenhouse.

Now regarding nutrient deficiencies: these may not be because you are feeding them too little, but sometimes can be caused by an imbalance within the soil that prevents nutrient uptake. Too much of X makes Y not be absorbed, PH above Z means nutrient X can't be taken up. This means adding more fertilizer may make the problem worse.

Do you happen to know the PH of your water? If you're on a municipal supply system they frequently make this information available on a website.

Thanks for the comments. My municipal water is usually pH 7 to 7.3 based on their water quality reports, which I believe. It’s basically all snowmelt from the cascade mountains to which they buffer the pH a bit to make it neutral or ever so slightly alkaline.

It’s only the lemons showing the deficiency, but they’re on different rootstocks (2 on sour orange and one on trifoliate).

I did add some iron chelate as a once-yearly supplementation but I fear if it’s not the cool temps I may have over applied, as iron is antagonistic to zinc and manganese.

What can happen is the various alkaline minerals (calcium and the like) in the soil can build up. Citrus, like most plants, prefer slightly acidic soil, it's possible that over time your soil has become alkaline. Evaporation and the like, such as with a dehumidifier filter.

If you have PH test strips (like those sold for pools) you can measure the runoff water and see how alkaline/acidic it is

Assuming the run-off water is alkaline, this is making the assuming that minerals *have* built up in your soil to make it such, you might need to neutralize the alkaline material or 'flush' it out. Watering with distilled water (or rain water, or really any water low in mineral content) could help too, so long as the minerals are able to redissolve back into the water.

My citrus has had problems with some salt build-up in its pot, started dropping leaves. Pictures looked like others who said they had nutrient deficiencies and I had salt build up on the trunk too. I do *not* have rainwater available (balcony has a roof, I use saucers to catch the run-off because: balcony) so I did the next best thing: ran several gallons of water through the pot which I made slightly acidic using a wee bit of vinegar. I then ran several more gallons of regular plain tap water through the pot to make sure I had not made the soil too acidic. When I measured the run-off PH it was quite a bit below 6. Previously when I had issues with leaf drop and nutrient deficiencies earlier in the year I used slightly acidic water for a couple weeks when watering and it sprung forth with new growth. Perhaps this might give you an idea of what you can do (assuming it *is* PH and salt build up related!).

Of course please post pictures so those who know more than I can help you with the micronutrient deficiency issue, I choose not speak to that. I don't feel I know enough. All this will do will allow nutrient uptake assuming either the PH of the medium got out of whack or you had salt/mineral build up which prevented nutrient uptake.

Edit: Added caution about making sure the soil medium was not too acidic. Slightly reworded the 3rd and 4th paragraph for clarity. Clarified what the procedure could correct.

I am very doubtful that my soil is too alkaline. The trees in question were potted up with a calamondin, a sour orange, a mandarin, and a Sudachi all at the same time and none of the others are showing a deficiency. The trees were potted around April. We get rain occasionally (with low pH) until June / July, our water is nearly neutral, and foliage pro itself lowers the pH of irrigation water. Your points are good ones though for those with more basic irrigation water.

We’re getting very warm temperatures this week, I’ll just keep up with the usual irrigation or maybe slightly increase it and see if things green up.

Thank you for clarify. I doubted it would be relevant, but depending on watering habits (ie: municipal sources, saucers, not watering until water runs out the bottom) can introduce issues.

Anyhow, pics so others can help!

Here’s a fairly representative photo of one of the leaves. This started on the newer leaves, which is part of why I think it’s zinc and / or manganese and not iron (doubly so because this happened after I applied iron chelate).

In my opinion this is pretty normal for younger lemon leaves (lemons not Meyers..). Just provide good balanced fertilisation and they will green up when they are older.