Morning Glory Blooms

Last year I built a raised stone bed with good soil, primarily for perennials. The bed has a double trunked maple tree, and I built a twine trellis for Morning Glories. I've always read that if the soil is rich and moist the plant will grow like crazy but won't bloom much. I've found that to be true.
For next year, I was thinking of burying a large ceramic pot w/ poor soil (high clay content) at the base of the trellis. I know from last year's experience that my clay soil is perfect for MGs and they bloomed like crazy. Does anyone think this plan will work, or should I just plop a big chunk of clay at the base of the tree (reverse soil amending ?)? If not, I may just go w/ Clematis, as I think it's an ideal site for that.
Also, regarding the various color varieties of MG - is one color more vigorous in terms of blooming than the others? For me, the reds seem to bloom more than the light blues and I planted equal amounts of each seed.
I also found that morning Glories like to climb up the vertical parts of the trellis as opposed to the horizontal parts ( I was going for height) so next year I'll redo it with that in mind. Is the same true for Clematis?
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