The proof is in the results. See the results here! Two pansy plants in 9cm pots (both watered and fed normally). The pot on the left doesn’t have Root Booster added to the compost, but the pot on the right does. Just look at the difference that Root Booster has made.
I also grew some Geranium Black Velvet in 20cm/8in pots in a sunny position on my patio. This particular geranium develops handsome zones in the middle of its leaves when it is in peak condition. The plant on the left has Root Booster added to the compost, the plant on the right hasn’t. Again, the plant with Root Booster is so much healthier (what’s more, it produced around 50% more flowers during the season).