Simple Grow Soil Builder is nothing but 100% pure worm castings.
You might think that's kind of a given when buying earthworm castings, but I'll let you in on a little secret...it isn't...
In fact, it's pretty rare to find a product that gives you nothing but the good stuff.
Some brands on the market advertise themselves as worm castings when in reality, they're 20% only worm castings. The rest of the bag might be made up of compost, dirt, sand, rocks, and stale muffin tops. Gross.
OK maybe not that last one, but you get the picture.
Many companies go out of their way to dilute the contents of the bag down so that they can spread out their limited supply of worm castings over more bags. It helps them make a little bit more money that way, but it doesn't do you any good.