I couldn't find anything on GW web site. I did a quick search here's the recipe I found:
Here is a simple way to make your own nutrient rich fertilizer that won't alter things too badly in the event that it comes out of the substrate and into the water.
1. Get a couple of ounces of Plantex CSM+b from Greg Watson
2. Get a handful of pure clay cat litter. Any pure clay will work just no dyes or other chemicals.
3. I mix a couple grams of plantex (maybe 1/4 teaspoon) and enough clay to hold it all together, and set it on wax paper to dry. You can also speed the process by using a warm oven (125-150 degrees). Don't set them in direct sunlight as I have heard plantex trace elements are photosensitive once they are mixed with the clay.
once the fertilizer balls are hard enough, push them into the gravel near the base of your rooted plants. I add them once a month to each cryptocoryne and a couple per sword plant. If they happen to break up just bury what you can and don't worry as the plantex in the water column is a very good trace element mix for the stem plants as well.