It's difficult to explain why the medium has to be sand. Ideally I would like to avoid having to explain it. Suffice it to say the plan is to transplant some B. Caapi to a soil that is best described as sandy. To some extent even 'gravely'.

This is a "wild" area that is not it's native habitat but I've found it grows quite well where I am so long as it has water but this is when in soil that I can control.

So to narrow down variables for those that might answer this question... all conditions; light, temperature, humidity and water are good. The only question is this soil.

I will take some pictures of the soil tomorrow.

I don't see why it wouldn't grow in such soil. Haven't look up what kind of soil it naturally grows in, but I'm sure it grows in several. Perhaps sandy, gravely soil would be good even, where I live the soil is alluvial, right along the lower missippippi River, so it's all deposited sentiment from the river, there's no sand or rocks of any sort, and cappi grows well, but not very fast.

I am in very very very sandy soil and my caapi grows very fast.

Wonderful. Feeling more excited / confident about my little project. Will post pictures before the years end.

I love growing most plants in sandy soil (even pure sand).

As long as you can provide the moisture and nutrients everything will be fine.
If it's PURE sand, it's essentially like growing hydroponically

I too have basically bare sand for soil. I don't fertilize and the caapis don't seem to care.

I went ahead and took some of the sandy soil from the spot I intend to transplant to and now have my new rooted clone in said soil. Starting out indoors. In a couple weeks to a month I'm going to move it outside to adjust to the intense sun where I am. Once it's used to the sun / weather here I'm going to then transplant to the final location.
In the mean time I've just watered in this sandy soil for the first time and I noticed that the water really just pools up.
My solution was to mix in a little bit of perlite. I also used chopsticks and just made a few holes going down through the pot to get some water down deeper. So I wonder if I'm on the right track? Thanks in advance for any and all help / suggestions.

If this plan works out then in 5 years or so there is going to be a lot of B. Caapi growing in the center of an extremely populated area of the US. If the first clone goes well over the next year then I intend to transplant a few more. So in 10 -15 years could be quite a lot around.

One random thought.
Gravelly soils are often very basic.
You might want to check, some bark or something may be required to help offset the ph.

So my first attempt has been a failure but I'm confident this will work out. The water source in question is a man made river and I underestimated how high the river actually rises on occasion and while I was out of town apparently it rose high enough to submerge the little guy. Plant debri / material of all sorts was in the flowing water so it stripped the plant of all it's leaves (only 9 leaves when I last saw it).
The good news is that it was growing well and thriving. I now have a better strategy when it comes to locations. Going to start cloning my main plant. Assuming I get three that do well I'll be heading back out to try three new locations.
I hope to have pictures inside of a year of it doing very well on it's own. At which point I'll have to share some clues as to their general location.