I have always bought the cheapest potting soil. Recently the knowledgeable gal at Lowes talked me into reading the difference in ingredients between the cheapo stuff and the stuff she was suggesting, Kellogg's. Wow! The cheap stuff is made from bark, sand and vermiculite, which holds water. The good stuff is made from bark, peat, which holds water, chicken manure, bat guano, worm castings, and more. Real organic vitamins and minerals, so to speak. Just like we thrive better when we eat real food instead of assorted flavored byproducts, surely plants must do better with food instead of just something to support the roots. I think especially for food crops, the good stuff is the way to go. Many people over the years have pointed out that Miracle Gro is more like a fortified energy drink, lots of imitation additives that build up over time and actually harm plants, especially in salts laden soil like we have here in the valley. I do not know this by personal observation since I have never spent the money for it, but it would be interesting to check out.