I would dig as big a hole as possible. Amend with gypsum. Take the soil you removed and mix in per 4 cu ft of native soil:
1 cu ft. organic worm castings
1.25lbs or 20 ounces steamed bone meal
1.25lbs or 20 ounces bloom bat guano
1.25lbs or 20 ounces blood meal
4 cups glacial rock dust
12 ounces rock phosphate
8 ounces kelp meal
8 ounces alfalfa meal
4 tablespoons Epsom Salts
2 table spoon feather meal
1/8 cup or 2 tablespoons sweet lime (dolomite)
1/8 cup or 2 tablespoons azomite (trace elements)
1 tablespoon insect frass
1 tablespoon powdered humic acid
1 tablespoon powdered fulvic acid
1 tablespoon recharge
1 teaspoon mychorizae powder
10-32 oz cups coarse perlite
This is a play on super soil.
If all that is too much, dig hole, fill with fox farms ocean forest, and a handful of earthworms. Cover with wheat straw.