Decks, Patios, Porches and Docks - Elevated Garden Bed

I'm looking to build an elevated garden bed with an open bottom so wheelchairs can fit snug up to the bed.

I intend to build this with 4x4 posts and joists (similar to a deck). The bed will be about 17' by 3'

I have a few questions.

1-what do you recommend for the 4x4 spacing?
2-since the joists will also double as the walls to hold in the soil (i'm thinking) would 2x10 be sufficient?
3-how thick should the "floor" to support the weight of the soil? be

Will this bed be setting on the ground? If so, why install a bottom? I think you are talking about a toe kick space at the bottom similar to kitchen base cabinets, only larger.

Not sure, Ken, with wheelchairs it may have to be 32" off the ground to accommodate the lap portion of the chair. My question would be the holding power of the fastener holding the 5/4 upside down into the joist. It may take two 3" #9's to hold at each rim joist.
I would space the verticals at 8' and make it 16' long, use the 2x10's for rim joists and 5/4 x 6 decking for the bottom. Place your 5/4 boards across the 3' dimension. So you would get 4 of them out of a 5/4x6x12. Don't forget to use angular supports to keep it stable.

I think WP missed the wheelchair thing...I understand what you are talking about, emo. Almost like an elevated rack at a nursery?

Soil, esp wet soil can be pretty heavy. But I would think something like deck boards (1") would work pretty well, and allow some drainage as well. Line with some screen or landscaping cloth. I'd suggest screwing a 2 x 2 to the inside of the outer members and just laying the boards in, like a bed frame and slats. It would be hard to attach them solidly and not have a risk of splitting.

I stand corrected. The bottom of the bed needs to be high enough to roll under, as Larry said. Thanks, Larry.