Tomato leaves turning yellow

My tomatoes are dying. I've been gardening for over 20 years and this one has me stumped.
The new leaves are green, but within a couple of days they begin to turn yellow, with green veins. The older leaves go completely yellow and fall off. All I'm left with are a few new leaves trying to grow on just a stem. It looks like Chlorosis or Nitrogen deficiency..
The plants are all VFNT, so it's not likely disease, but could be. The yellowing isn't spotted, so I don't think it's a blight.
My usual garden is heavy, clay soil, with at most 1/8 inch of top soil, high alkalinity--PH of 8--even with adding lots of organic matter. The garden is at altitude--4,500 ft., temps are going between 102 in the day and 52 at night, 1/2 in. of rain in two months. It's a hard row to hoe!
However, I have planted in my regular garden with and had some success with tomatoes and especially, bell peppers. I finally figured out that to get peppers to grow, I had to add superphosphate. That really helped loosen things up and bring down the PH.. I tried gypsum one year, hoping to break up the soil and release the bound calcium, but it just turned everything to concrete.
This year I decided to make things easier by growing my peppers and tomatoes in a small, dugout 5' x 10' bed. The bed is filled with compost and soil (clay), 50/50 mix to a depth of 12 inches, with landscape cloth in the bottom and on the sides to keep out weeds.
Because of the heat, all the plants in the bed are wilted by mid-afternoon & I have to water them twice or even three times a day, or loose them. Even my Cosmos, that are in another part of the garden that's a little cooler are wilting badly--I've lost three of those plants just this week. Its been too hot/dry, even for them. .
When I set out the tomatoes and peppers, they were young, healthy, green and hardened off. I added a balanced plant food around the plants, and a handful of superphosphate in the bottom of the planting holes.
With the yellowing and dying of the tomato leaves, I thought maybe I'm washing away all the nutriments (N) or locking up the iron. It really does look like Chlorosis. But since my peppers, although stunted, are only 2 feet away and are doing fine, green wise, I am at a loss.
I can't cut back on the watering. It's just too hot. Should I add chelated iron? Ammonium Sulphate? Both?
Or just call it a bad year and walk away? --at this rate, everything will be dead by the end of the week, unless I can stop it.
Thanks, Bob T.
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