Compost would be the most suitable product to add on top of an established garden. That and/or a shredded mulch of some sort. The SOIL product would smother the root system your existing plants rather than enriching and improving the established soil.

Return the miracle gro product to retailer for a refund. It isn't necessary to use it and put more of whatever is in that in the ground.

Well I've used the Miracle Gro soil. All it has in it is some slow release fertilizer. And it's better quality than the regular top soil.

But compost is best.

Thanks, everyone, for your guidance--it was much appreciated and I'll act on it immediately. Compost it is!