Disclaimer: This post was originally posted by Hero Member, mjb8743. This user is located in Zone 7, South NJ, Garden State. This was originally posted on February 5, 2009.
I use Pro-Mix BX in my 60 boxes, and will be my 4th year coming up. I disregard any of the additives in the mix, and just follow the Earthbox instructions, which state 2 cups of dolomite every replanting. They also say 5-7 years with the same mix, although the 2-season tomato farm at the EB Research Center has the same mix for longer than that. The mix gets more acidic with time, so its critical to add the dolomite.
Since I'm in NJ I get a full-blown winter. I found that if I leave the boxes sit over winter, in early spring when I'm ready to refurbish, most, if not all the roots have decomposed. I don't dump the contents... I simply discard the top inch or two with the fert. strip, loosen the media a bit, add new dolomite then add new mix, stirring up as I go. I make the mound, add my fertilizer strip, cover it with a little mix, and put on the shower cap.
I don't have the time or patience to make it more complicated than necessary. If it gets to the point that I'm splitting hairs, buying 50 different ingredients, spending hours with my calculator to get just the right micro-balance, and measuring/weighing bits of this and that like a mad scientist, then it will no longer be enjoyable or worth doing. I don't care if things aren't absolutely perfect... all I want is enough harvest to feed my family of two and maybe share a little. I will go to bed satisfied.
Mickie