Yellowing leaves on Cotoneaster microphyllus?

This little guy is trying to turn yellow.

Lots of the very yellow leaves are falling off with a touch.
My guesses as to why
Heat wave of 110F/43C since Tuesday.
Iron or magnesium deficiency.
Overwatering, partly because of the heat wave.

Anyone have any other thoughts? How would I treat a mineral deficiency?

Advice appreciated.

What’s the water pH you have been watering with? I see similar issues when I have to use the garden hose ( avg pH 8.3) for more than two consecutive weeks. I assume this is the beginning of nutrient lock out.

Lowering the water pH is the easiest and most direct solution. I am far from an expert but this has been my experience.

I recall a few live streams where Ryan stated that yellowing leaves is almost always a sign of calcium deficiency ( but avoid calmag as you will never be able to get the mag out of out thr system)

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Thanks. I’ll see if I can check the pH in the next few days. I suspect the city water here is more basic than is ideal. A rain barrel isn’t hugely practical here. We get about 8" precipitation a year, mostly October through April.

I just use a barrel (25-30 gallon plastic pot without drainage holes) that I fill up with the hose let the chemicals in the water off gas for 24-48 hours gas, test ph add a couple drops of “PH Down” (product link below no affiliation) until I get the water at 7.3-7.5…when I dont do this I see the issues you are describing, good luck

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I just tested at 8.0 for the outdoor faucet, which supposedly does not go through the water softener like the indoor ones do. Inside tested at 8.1…
So it seems like a barrel and some pH down stuff might be a worthwhile investment, especially if i try to get down to 6.7 to 6.9.

Add the pH down stuff slowly and give it time to come down…sneak up on the number you need is what I try to remind myself…first couple times I went down to 4😭

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