The only way to accurately judge if the plant needs water is hands-on - if it's dry to touch several inches down and the soil drains well, you need to water.

Definitely mulch (you can mulch up to the trunk as long as there's not a big mulch volcano to encourage trunk dampness and rot), and be very sparing with the fertilizer/iron supplements/etc. until the tree has leafed out and is growing well. Use weekly/biweekly very dilute fertilizer solutions instead of a big full-strength hit all at once.

Do not kill with kindness. :)

Ah yes, but this has been my problem -- killing with kindness! So what type of fertilizer would work best for a biweekly dilute application? Something like GrowMore chelated minerals? Or maybe something more Phosphorus heavy, like Big Bloom? And very dilute - would that be a 50/50 dilution? Or more like 90/10? Would I still do my quarterly fertilizer application with the citrus food, or would it be more prudent to wait until, say, August, or even September?

Or maybe fertilize with liquid kelp?

What kind of soil do you have? Straight clay, gritty clay, sandy loam or what? That will make a big difference in how often you need to water.
Is your tree in full sun, or less. If less than full sun, how many hours of full sun does it get over the course of the day?

And even though I sell those water meters at work, they are not the highest of quality and get out of calibration very easily, no way to re-calibrate them either. So it could just not be reading correctly.
I like using a thin wooden dowel instead. I can poke it down further than my finger, and it shows very well if the soil is wet, damp, or dry.

Your tree seems to be suffering from lack of air in the root area, either from too much water, or soil compaction or both.
Did those leaves that dropped before seem soft and yellow? And did they drop off easily, like if you just lightly touched them they fell off? If so, that's a classic symptom of the soil staying too wet, which drives out the air, and invites root rot.

How do you apply your water; automatic irrigation with spray heads, bubbler heads, drip irrigation, or hand watering?
That will also make a difference on what fertilizer to use. EB Stone Citrus is a great food for in-the-ground citrus, but it's a powder/granular that's dissolved slowly over time by irrigation. So you need to place it where it gets wet when the tree is watered.
If you fertilize according to the EB Stone label, you may not need to add extra food. But if you wish to, do dilute it at least 50%
I don't think citrus need a bloom type fertilizer, they always need the nitrogen, the ratio recommended for citrus is 5-1-3 NPK.

Most of California's soil is pretty decent with phosphorus and potassium. Even most micro nutrients are in fairly good supply naturally, it's just that if the pH gets too high, they can be locked up in a form the tree cannot absorb. So keeping the pH at a neutral or slightly acid (slightly!) state helps the tree absorb the nutrients in the soil, even those you add by fertilizing. That why most good fertilizers out here add Sulfur. Sulfur is a great soil acidifier, and it lasts quite a while too.

What Barb said. I would use a good granular citrus fertilizer. EB Stone's product is very good. So is Grow More Citrus & Avocado Food. Even Vigoro Citrus & Avocodo fertilizer is a decent product. Citrus require large amounts of Nitrogen, so a fertilizer formulated for "blooms" would not be appropriate. The best NPK ratio is around 5-1-3, so try to find a formulation close to that, if possible, along with micronutrients. Water first, then fertilize and water in.Apply fertilizer at and slightly beyond the drip line where the feeder roots reside. Fertilize 4 times a year, from February through about October. Check your soil moisture using a plant stake. I just use one of those cheap green plastic covered stakes and push it into the ground about 18-24" or so. If I can push it in, soil is moist enough (and it will come out a little moist). We have plenty of Calcium as well out here. We tend to lack in Manganese and in some places (not all), iron. If you're on clay, you'll have plenty of micronutrients, but possibly have water drainage issues, so mind your watering. If you tree has another big decline, especially in late winter/very early spring, it is most likely due to too wet of soil causing an overgrowth of Phytothphora. Treat with AgriFos 2 Tbsp. per 5 gals of water. Treat immediately, then in 4 to 6 weeks, then every 6 months. You can mulch, but take care if you're on clay, as it can cause too much moisture retention, especially if you haven't planted your tree on a mound to facilitate drainage.

Patty S.

Yes my soil is alkaline and some sort of weird mix of sand and clay. I think the previous owner of the house tried to amend the bed with sand to try and fix the clay situation. Patty, per your suggestion, I did treat with Agrifos about 3 months ago or so, but I haven't retreated. Is now a good time to retreat, or will I risk damaging all of that new growth? Back in March-ish, I was going to treat with the Agrifos, then see how things corrected. If nothing changed, I was considering digging the tree up and replanting on a mound.

Ah! This is why your tree is rebounding. Treat, again, and continue to treat every 6 months for the next couple of years.. You will always want to treat in late winter/early spring, when our air and soil temps, as well as precipitation are exactly perfect for Phytothphora to proliferate AgriFos will not damage new growth. Don't waste time trying to amend a tree hole, especially with clay soil. You'll just create a bathtub and drown your citrus. The current line of thinking is to simply plant the tree in your native soil, on mounds if you have clay, and then topdress with amendments if you wish (compost, composted manure, mulch). If you know you have clay, you should always plant your citrus trees on a mound. You can use mulch, just keep it away from the tree trunk. With new trees, I would always treat in late winter/early spring, then again in 6 weeks, then in 6 months. Good job, nika.

Patty S.

Well Patty it's your great advice! We're somewhat neighbors -- I'm in San Marcos/Vista corner!

We sure are! My backyard fence is the San Marcos line. Live north of Buena Creek Road and up Emma, off of Emma and Salem thereabouts. Very small world. And you most likely have a combination of DG and some small amount of clay or caliche. It turns to absolute concrete when dry, needing a jackhammer to dig, but when it rains, it is as soft as butter. The strangest soil I've ever dealt with here in S. California. And, we have many different species of Phytophthora in our soil - P. nicotianae (kills lavender and Pittosporums), possibly P. palmivora,which can also affect lavenders and Pittosporums, P. cinnamomi (Pittosporums, azaleas, avocados), P. spp for citrus. We have "fungus among us" out here with regard to Phytophthora, and when we over-water, especially during our cooler months, we cause it to just explode. And kill our plants.

Patty S.