Hosing and troweling is going a bit far, I'd just use a bucket and a broom or rake handle. Fuck it.
Dunno if you'd need a ton of sand, you could use a cement calculator to work out how much shit you need.
( brb Learn Ukrainian and prepare for a referendum, Sun 13 Oct 2013, 18:39, archived)

A cubic metre of concrete is a tonne and a half of ballast, give or take
So he'd get a 2 x 2 metre base six inches thick. If he needs a bigger greenhouse than that, he's the most almighty cunt there ever was.
( Grrrmachine the indifference engine, Sun 13 Oct 2013, 18:45, archived)