Untreated pine will rot if in a constantly moist environment. I use a stain containing copper sulfate, which is a great pesticide/fungicide/preservative. I bought some from Loew's a few years back but can't remember the name. Read the ingredient portion on the label.

I know it will rot but I heard it could last up to 5 years. I am looking for another way besides cinder blocks and that is economical and organic. I don't mind changing the board out every 4 or 5 years.

We used untreated pine to build a raised herb garden. I think it lasted 3 years in a pretty moist environment. The frame was 2X4 and sides were 3/4 inch plywood.
Now that I think back, the 2X4s were not replaced when we rebuilt it, just the plywood.

Pam

We used pine 2x10's for one of the gardens and it's starting to deteriorate in about 5 years. In the kitchen garden we used cedar pickets that have held up quite well for the last 7 years. We got the dog-eared type and cut them to size. It was cheap and has easily outlasted the pine.

check with fencing companies for culled material. There is a place in Austin that advertises on Craigs List. They have cedar posts and pickets that they can't use for sale.