Im building a raised garden box and have question

[adnewr]

I am building a raised garden box about 9'x4' and 2.25' tall. It is only going to have peppers in it.  I am going to make pepper specific dirt but I don't know how much of the soil should be pepper specific and how much can be plain garden soil (on the bottom to fill in the box a bit). How deep will the roots go and how much pepper soil do I need to have? Thanks 

 

[Roguejim]

That's a tall bed. Most, I think, are only 12", but then, some folks like to mound up their rows. So, the actual depth ends up being deeper, maybe 16-18".