As a  horticulturist I disagree with what you read on the internet. (I especially disagree with using just phosphate) Spiraea is a very tough shrub and not picky about soil. I have grown many in large containers as they look good three seasons of the year. I use just one capful of Osmocote once a year in the spring. Do not overfeed or the shrub will grow too fast and not stay happily in the container.  Just use potting soil and make sure the half barrels have drainage holes and add about one fourth of the volume of the pot in compost to hold moisture.  You can also add compost on top of the potting soil each year. Spiraea 'Magic Carpet' is drought resistant and hard to kill. You can prune this spiraea in early spring if after a few years it needs to be renewed or to control the size. Some years I forget to fertilize at all and my Magic Carpet has been happy for a decade in the same container so don't worry about transplanting to a new pot. The half wine barrels you are using is a great choice for years of carefree color from this spiraea. Keep growing, Marianne Binetti