Because I do not live in your area, I really suppose I should not be answering your post on GardenWeb, but since nobody else has ventured to do so, here goes. IF you purchase your plants in the Spring, why not ask the person from whom you buy the plants? Other than that, geraniums do NOT like it as hot as you mention. I would try some in pots, so you can move them around to try to locate a favorable spot or two, and see if you can find and try a spot that has dappled shade. Early morning and late afternoon sun might be preferable to searing hot sun, although up here on coastline S.E. Massachusetts geraniums like full sun.
Any water-soluable fertilizer--NOT a dry granular fertilizer--possibly Miracle-Gro should do. I would mix a gallon at a time, and add one(1) tablespoonful of Epsom Salts--buy anywhere. Epsom Salts is Epsom Salts. I would water this solution once a week, BUT NEVER APPLY FERTILIZER TO A DRY PLANT. Water plants the night before, and fertilize with the solution the next morning. Soils are so different I really can not give you much help. If you can find Promix, buy that, add some of your garden soil, and mix in some Perlite---Try a mix of 6 parts Promix, 2 parts your garden soil(sift it through a sifter made up with some 2 x 2 netting) and 2 parts perlite. Sure, it is a lot of work, but anything worth doing is worth doing right the FIRST time.
If you have any further questions write me. If I receive your inquiry I will respond. PLEASE add the word "GardenWebber" in the subject area to keep your e-mail out of the junk pile. Thank you.

judith,i live in south louisiana,same hot humid weather.i don't fertilize but maybe once a year if they're lucky,most of them on southwest side(sun)hardly ever water.i was told not fert often.of course my soil is remarkable.when i first started with them(especially potted ones)i killed a few from overwatering.by the way most of mine are red..wish i could offer more....good luke.
patricia

I live in central Texas.I grow my geraniums in pots and planters. My geraniums always look great in the spring but in the summer they turn yellow looking and ugly with hardly any blooms. I keep them watered and when cooler weather comes they turn dark green and bloom well again. You just have to bear with them during the hot weather. In the fall I move them to a covered patio and cover them with a mattress pad during really cold freezing days and uncover them when it is not freezing. They revitalize when the days get warm. shirley