Some of the soil will float up, you just need to be VERY careful with how you fill the tank.
Put your soil down, add your cap and then put a plastic bag or shallow bowl/plate and pour water in very slowly and gently or it WILL be a mess (learned that the hard way lol) then fill and drain the tank several times (this will help with saturating the soil)
Some people add their plants before, some after they fill with water. I don't really know the best method of doing it, what I did resulted in dirt everywhere and a massive headache... so... trial and error, I'd say make sure your soil is completely wet before planting and then drain the tank at least half-way to plant and it should be okay. Granted, it may be messy either way you slice the issue, but it's worth it in the long haul. (my tank is very close if not to the point that it's self sustaining now)