That would be a raised bed. It's probably not straight fertilizer, as that would most likely burn the plant roots and they would start fast then die fast, if they sprouted at all. I filled mine with a mix of 1/3 compost, 1/3 peat moss, and 1/3 vermiculite, with a few coffecans of home mixed fertilizer (bone meal, blood meal, wood ash, and a few other organic things). Chances are they filled it with something similar to this. The reason stuff grew so well in yours was probably because of a combination of the richness of the soil and the great drainage a raised bed gives you.
Did it look something like this? This is not my garden, just a pic I found online.