“You’ve probably heard your grandmother or aunt say it a dozen times during those awkward adolescent days , ‘Honey, it’s what’s on the inside that counts.’ As with so many aspects of growing, what is true in human life is also true in the garden. When you start planning your container gardens, it is critically important to use a good quality container mix. Properties and ingredients that make up a good quality container mix are: the soil should be light and fluffy for good aeration (products like vermiculite or perlite), have moisture retention (products like coir or peat moss), compost (for nutrient retention, minerals, and soil structure), slow release fertilizer, and some type of forest (wood) product. Some also come with soil retaining pellets which is beneficial. The primary things you want to stay away from are using bags labeled simply as topsoil, garden soil, or compost.
Remember that while you may be paying a little more in a quality container mix, your plants will have a much greater opportunity to thrive over longer periods of time – saving you money from having multiple replantings in a season.
Fill your planters completely with your good quality container mix, even in large planters. Over the next two years, in your larger planters remove the top half of the soil in the planter when you need to seasonally replant. On top of this leftover soil add a sprinkling of granular all-purpose fertilizer, then add new container mix for your new plantings. Only completely redo soil every couple of years.