Your description of the sand sounds like "rock flour", silt size particles of rock, ground by movement of glaciers. It collects as sediment in alpine lakes and areas periodically flooded by glacial melt water. Makes a crappy soil, it has most of the properties of silt, rather than sand. Particles are not as fine as clay.
When I collect trees, I try to get close as practical to bare rooting. No natural soil is any good as a potting media. The less natural soil you bring along with your collected tree the better. However, there is the issue of preserving fine roots on the collected tree, so you end up having to keep some native soil. Remove too much, mortality becomes more likely. Remove too little, (bring along too much native dirt) and you will run into the problems of the native soil being crappy for use in a container. Again, mortality will become an issue.
I would consider putting a heavy layer of long fiber sphagnum over the surface of the soil. Long fiber, so that it is easily lifted if needed to inspect the soil underneath. Keep the sphagnum moist to wet. It should help keep the hydrophobic dirt wet once you finally get it wet. I have use wetter-sticker for pesticides in my water when trying to wet peat mixes that got too dry. Usually no problem. Key is only a drop or two is all that is needed in 5 gallon bucket of water.
I have also used a Yucca sourced product. I forget which one, it was foamy as heck. It did wet peat based media fairly quickly.
