Try spreading the load. Find out which direction the joists go and place
three or four 2 by 4's on edge, the other direction. then nail some
short decking planks across, for a couple or three platforms to place
large pots/tubs on. For soil, I'd try a peat base, with about 30% leaf
mold and 15% perlite, or dare I suggest styrafoam (polystyrene) balls
added. If you can't water every day in summer, this mix will dry out
quickly, so I would add silicon cristals (hint - those environmentally
un-friendly disposable diapers are full of them) to slow release water
into the soil.
On this subject, these same crystals are sold as a soil amendment for
potting plants, has anyone in Texas/Arizona tried using these crystals
in the sandy soils there? Has anyone else ever thought of using diapers
to make desert soil fertile? It's an idea I came up with ten years ago,
when I was thinkng of buying some land in Arizona, but it fell through,
and I ended up with half a secluded valley in south central Kentucky
instead. I still wonder if it would work.....