I came across the same realization with my soil a few days ago when i was thinking about putting grass in mine.
When i head back to the store, I was going to see if any of the organic soils have any additives.
Let me know what you find out and which bag you are using. I may see if they carry it at another store by me.
I recently fertilized my lawn, so i can't transplant any of my grass into the coop and my guess is that if i were to buy grass squares, it will probably have been grown with fertilizer...
When i was thinking of buying grass seed a few days ago, I checked the brands they sell at Lowes and it looks like most of them have a fungicide and also something else to promote quicker growth which i am sure is some kind of fertilizer.