u/Sweet-Trees-Hawaii

I look at fruit trees almost daily on Oahu, and yellowing leaves is a common reason I get called for. Nine times out of ten somebody already fertilized before I arrive, and if they're calling me it's because it actually made things worse.

Roots drink, they don't chew. Every nutrient has to be dissolved in soil water as a charged ion before a root can absorb it, and soil pH decides what dissolves. Get the pH wrong and the nutrients are still in your soil and they've changed into chemical forms that won't dissolve, so the tree can't drink them.

What that looks like in the two directions:

• Alkaline soil (a lot of the leeward side sits on old coral reef are basically limestone, constantly liming itself): iron, manganese, and zinc react into oxides and hydroxides. Iron hydroxide is more or less rust. The iron is still there; it's just rust now, and rust doesn't dissolve. Dumping more fertilizer on it doesn't help — the new iron hits the same chemistry and locks up too, while the salts it came with stay behind in the root zone and start burning roots. That's how "I fertilized it" becomes "the leaves are dropping."
• Acidic soil (wet valleys): this is the opposite chemistry with the same result. Phosphorus gets grabbed by aluminum and iron into compounds that won't dissolve. Calcium and magnesium wash out with the rain. If pH drops far enough, aluminum itself starts dissolving, and dissolved aluminum is toxic to root tips... "and it was all yellow" - Coldplay, probably

Reading the yellow.
Leaf discoloration is really difficult to describe without pictures, but I'll give it a shot.
Yellowing on the NEW growth with veins still green = iron/micronutrients, and that's your pH suspect: iron can't be moved around inside the tree, so new leaves starve first. Even yellowing on the OLDEST, innermost leaves with new growth looking fine = nitrogen (or just the tree retiring old leaves). Tropical trees don't do fall, but they still recycle; a few old leaves at a time is the normal course of dieback, especially in the understory if there's not enough sunlight passing through.

Why start with pH? a full soil panel is genuinely hard to come by here, but a pH test is easy, and it rules out the majority of issues on its own.

• Leaves show deficiency AND pH is off → correct the pH gradually (agricultural lime brings acidic soil up; sulfur and organic matter take alkaline down, slowly) and feed through the LEAF in the meantime. A foliar spray bypasses the soil chemistry entirely — the leaf absorbs directly, so the tree greens up while the soil catches up.
• Leaves show deficiency AND pH is fine → feed the soil, and look at micronutrients specifically, not just more of the same N-P-K. Foliar works here too when you want a fast correction; it's just a more expensive way to deliver the same nutrients.
• Leaves look fine → test anyway before you feed. The test's whole job at that point is making sure you don't create a problem that didn't exist.

Last year I made this exact mistake. Tree was looking good, some slight discoloration on the leaves and obviously the fix was adding fertilizer. I know better. Guess what!? The leaves dropped off of more than half the tree and I nearly slapped myself: pH test revealed that I was just adding salt to the roots (and none other than a salty fish ferment fertilizer from my local fisherman).

"There has to be a better way" Well yeah, if you read this far get yourself a solid pH test at the base of every tree.

Two more things (okay 3) that masquerade as deficiency:

1. waterlogged roots (uptake is active work that burns oxygen and too many of our Hawaii root systems are sitting in clay. Drowning roots in soggy clay can't run the system, so the tree yellows exactly as if it were starving, and adding fertilizer + water makes both problems worse) and
2. weed-and-feed on the lawn (feeder roots run way past the canopy edge; if the yellowing dates to a lawn application, there's your timeline).
3. Bonus number 3 that hopefully you know better if you're in this channel: DONT SPRAY WEED KILLER WITHIN 30FT+ OF YOUR TREES (or ideally, at all)

One trick: a foliar spray doubles as a test. If the leaves green up within a week or two of feeding through the leaf, you've just proven the nutrients in your soil are there but unavailable: that's a pH problem, confirmed without a lab.

Happy to look at photos — tell me whether it's the new growth or the old leaves going yellow, and what's under the tree (lawn, mulch, bare dirt).

I'll follow up with our citrus growing guide shortly. It has example photos for every type of leaf deficiency (I just need to add a link to download =D )