To be honest, google the plant symptoms. A lot of yellowing is over watering. But it can be other things too. And each plant can react differently to water, stress, and bugs. I've got a powder fungus issue on my bush pickle cucumbers right now. Too much water, low ground cover plant, probably not enough full sun. So I'm treating them a couple times a week until it's gone or they are dead.
Eggplant hates water, I planted my after some rain then it rained more. They are yellow, bugs are starting on them, I don't have hope but we are trying to save them. But they are awfully weak. They would do better in a raised bed.
Tomatoes need consistent water when they start bearing fruit. Too little calcium will lead to blossom end rot, you loose fruit. But too little water during production leads to split fruit. The dry makes the skin tough, then the rain makes the plant boost production, skin splits open. Then the bugs get it.
I've gone away from 10-10-10 fertilizer. I use lime and bone meal on most plants. Lime can go in the hole. Bone meal goes in a hole about 6" away from the roots. For Tomatoes I add Calcium Nitrate in another hole about 6" from the roots. The Calcium will burn them up you plant the roots in it. I over fertilized the cucumbers with 10-10-10 and nearly killed all of them this spring. That's when I switched. Added a good dose of chicken litter in too and working on getting my composting up to speed and usable. I'll get some pics later, things are going gangbusters in the raised beds.
