Meyers in containers have a reputation of being finicky... too much water, they drop leaves; let them get too dry and then water them, they drop leaves; significantly change the light conditions, they drop leaves. For a plant your size with 4 fruits, blossoms, and trying to grow new leaves, it is common for Meyers to suck the nutrients out of old leaves to make new leaves/flowers/fruits; but your plant looks happy and well fed; so I would look at one of the first 3 causes.

I wouldn't trust a soil meter for a sec..Been there done that and lost a many trees dependent on them..

Use a wooden dowel or a wooden chop stick and see if the mix is really wet or dry towards the center and bottom of your root ball and pot...

As John said, many things can cause leaf drop, but usually mine suddenly fall off green when left to go too dry just once.

Mike

I really fell in love with citrus last year. Bought one Meyer lemon and have already eaten three from the tree. So. . . bought another Meyer this spring and a Key Lime. I fertilized them all (very lightly, not even a handful of 'citrus tone') the oldest Meyer Lemon' leaves are turning yellow and are dropping off (this tree has tiny fertilized lemons on it and flowers still in bloom ). The second Meyer Lemon the youngest just has its leaves turning yellow and dropping off. The Key Lime tree is holding its own. (It arrived as a skinny topiary shape) I've only seen them grow in the Florida Keys as a bush. Was the tiny handful of fertilizer the culprit?? All three live outside during the summer. I am in Newport, RI and we have been getting tons of rain. They are all potted in Al's Gritty mix. Any help would be appreciated. Thanks Mrs. G

Citrus Tone is a good product; but it is organic, which usually means you need a lot of it. I think the package has instructions for container trees; since it is a Meyer, I would probably use more than recommended, up to 1.5 times. In a porous mix with lots of rain, the fertilizer likely gets leached out.
It is a common characteristic of Meyers with fruit, flowers, and new growth that the plant sucks the nutrients out of older leaves to use for the fruit/flowers/new growth. Give it more food and it should stop dropping the older leaves; but don't expect the yellowing leaves to become green again.

Johnmerr, thanks for the answer. Does the yellowing of the leaves mean the trees are dying or they are just temporarily not happy? My next acquisition is going to be a 'Moro' orange. Can't wait!

Not dying... surviving; more food will make them happier.

On that branch, at the point where it appears to be resting on a metal ring, do those light brown spots rub off? If so, perhaps scale.