oraganic hydroponics just means you are using natural
nutrients.. This tends to be a more interesting subject, and a bit more chalenging than using normal hydroponic nutrients. I am thinking I might play around with organics later when I have the land to let me do it without having to buy all the "organics". If you get into all of the organic hydroponics from the ground up it is much more sustainable than regular hydroponics. It allows you to provide your own nutrients from compost piles, worm castings, rabbit castings. There are a ton of ways people have been able to grow organic hydroponics from what I have read.
On the main subject of the topic. I saw a 2lb tub of epsom salts at sam's club today in the health care section for about 2-4 dollars (can't remember off the top of my head since I didnt really want to buy it.) Interestingly enough though it was in the health care section it had more writting on it about gardening than the "health" uses.