My wife and I have just declared that gardening is our hobby from now on.
Having made this tumultuous decision, we are looking at planting a hedge of
Pittosporum Eugenioides. We've dug a 60cm wide and deep trench in the
backyard. But we have found that our topsoil repels water like it's coated
with Scotchguard and our subsoil is quite clayey. I know that gypsum can
correct clay soil, but I also read that it doesn't work for all soil types.

Before we drop these babies in the ground, what should we do to the soil?

Will this hedge tolerate the clay soil untreated?

Any other suggestions?

Cheers