We have a large (I would say 20X30) 'rock garden'. The soil on our property seems to grow ANYTHING - *(prior to us purchasing the property, on some part of the 8 acres, there were many sheep roaming, and I have heard that has increased the richness of the dirt!). Last year, I dug two small flower beds next to our deck and was told to toss a bag of Miracle Grow *(I think top soil or potting soil?)* on the dirt and blend it with the top 2 inches. The flowers there are INSANELY beautiful, full and rich in color and have multiplied. Our garden soil grows vegetables at the same rate, so I have no way of knowing if this additional soil helped. My question involves the large rock garden. Along one side, we have hostas and lilies growing and doing beautifully. These are also sprinkled along the outer corners. After having nothing but weeds grow in the center of this rock garden for a couple of years, we have tilled it a few times this season and enlisted some help. We had a couple who used to own a landscape company help us by replanting the hostas and dividing them (they did this project last year) and they have recommended a ground cover to fill in the center, along with 3 Elderberry bushes and a plum tree that I have been nurturing in a pot for 2 years - they recommended Pachysandra and I bought Vinca (the beautiful periwinkle blue version) because I want more spread and I don't care for Pachysandra - my other half and I planted the tree and the Elderberries last week and I am thinking I will plant the Vinca this week - I have heard conflicting recommendations for planting the Vinca. This couple told me they would plant a plug about 4 inches away from the next plug, yet the owner of the nursery we frequent told us each plant needs to be a foot apart. I was told this same thing by another nursery grower - that they need to be about 1 foot apart. I have 3 flats of this plant and if they are going to be right next to each other, I need a lot more flats! What is the true story with these groundcover plants? How close is too close?
The other question: I bought one of those soil testing kits and the nitrogen showed as deficient to depleted. The plants that are in this bed are doing very well. I was wondering if adding a few bags of that Miracle Grow soil would be the proper thing to do in order to create a more hospitable home for the new groundcover and trees.

**I am going to retest today, but I think the soil is deficient!

Sorry for the long post and over information! I have been outside in the sun way too long! :)

Thank you for any advice!