I've been growing in the same region for awhile now and suggest taking a pair of posthole diggers and digging down at least half the length of the diggers. Hollow out sides of hole to open it up a little. I like to use potting soil enriched with chicken shit and homemade compost. If u use sand make sure its course sand so it will drain, fine grained sand will pack tightly and slow drainage. Then dig out top of hole
3' across and 18" deep and fill top of hole with soil of choice,then plant a nice sativa,preferably a type of haze which does well in this area. GOOD LUCK AND GOOD GROWING!!