Hello David,
Okay, you said that you would be planting it on a sloping hill right? How big is the hill and how close to the top would you be planting. I would think if it were up closer to the top, the water would flow away from the tree. If it were closer to the bottom, I would worry about water that would pool around the roots. If you have clay that far down, I wonder if it would affect the roots. I know that clay will help hold the water. I honestly don't know if that would hurt it or not. I would think that the roots would grow that far down. Well someone here should be able to help you out better with that problem. Good luck with your lime tree.
Andrew

I'm wondering if you'll be able to get away with a citrus in-ground where you are.... I'm a big advocate of less frequent and more thorough watering. The clay shouldn't be a problem as long as you refill the hole with natural soil and not a different substance that would cause it to fill with water.

You said that you live in Aiken. How far away are you from the coast? What zone are you in? I would think you would be able to get away with it if you were in zone 8,and if you live closer to the coast, you would be warmer. What variety of lime is it? I know that keylime is ultra sensitive, but I doubt that you have that. Lemons and limes are the most sensitive to cold temperatures. My keylime is potted and I left it outdoors last year even when nightime lows were in the 40's. I would ask around. Do you know of anyone else that is growing lime trees in the ground? I don't like the idea of having to use the hose either but I live in western NY and we don't have water problems. I have been having a hell of a time today. Though I just ran outside and had to pick up my 6ft Meyer lemon off the ground today. We are experiencing record high heat and tonight we are having a cold front come in. The past 3 days have been in the mid to high 80's! We have broken the record high's for the past 3 days. Only problem has been the wind that is rolling thru with the cold front. My Oro blanco was half out of the pot because the wind caught it. Only issues I have will the light soil mixes. Unless the tree is established in the pot, the wind catches it and pulls it out. Oh well you have to take the good with the bad!
Andrew