That's a great video link, Tiffany, Thanks!
PLoni, the rocks are added under the shrubs to be sure they don't just subside again before their roots develop and fill the area under the plant. If you add too much organic amendment under there, it will break down and also compact as the plant gets heavier, and may subside again. IF you don't have or want rocks, shovel straight soil from the surrounding area under the plant to raise it, then do mix some amendment with the native soil around the plant. Just 'build' soil by keeping on adding organic stuff around the shrubs as they grow.
It depends upon the plant whether it's ok to dig the stuff in around it, some plants have feeder roots very near the surface so it's not a great practice to till the soil surface around them. Safer to just top dress with a generous amount of organic stuff, which acts as mulch to quell weeds for a while, then breaks down to work its way down into the soil. Don't put it too near the plant's base or trunk(s).
I use a lot of bagged compost/manure in my garden, just because it's so hard for me to make compost here in the summer months when there's lots of materials. It's just too hot to be turning a compost pile, and I don't have any grass clippings to make it really 'work' well.
"Topsoil" should not have rocks or pebbles in it, in my opinion. If you buy from that nursery again, buy what they call "compost". But the stuff in bags will probably have more dependable proportions of good stuff in it. Of course it will cost more, too, but my experience with buying bulk topsoil, compost or mulch is that you get a lot of air for what you pay, because both when they load it for you, either by shovel or a front-end loader, and when it is unloaded, it gets fluffed up.
Check with your local landfill! Here, our landfill makes excellent compost that is 100% organic stuff and well composted. It is free for the taking, so if you have a friend with a pickup truck and a couple of teenagers . . . strong backs to shovel it into your truck . . . buy them lunch or pay them $10 for a half hour's hard work! It's a bargain! (use a cheap plastic tarp to line the truck bed, and it helps with the unloading process, too) In Salt Lake City where my kids live, they charge $25 for a pickup truck load of good compost, and they load it for you with a front-end loader.
