Answer: Drainage is one of the most important characteristics of a planting container, as most plants dislike wet feet. You won't need gravel or stones in the bottom of your plastic window box if it can drain freely on its own. In shallow containers you want to provide the roots with as much soil mass as possible, as well as water, nutrients and air to keep them happy.
Organic matter does break down over time, so it's wise to use fresh potting soil each season. Or you can mix your old potting soil with new mix if you think it is depleted, or needs a little more body. But don't add garden soil. It has a tendency to really dry out and be very difficult to re-wet. Commercial potting soils are usually a combination of sand, peat (or other loose, light organic matter) and perlite. Sand helps plants drain quickly, organic matter helps retain the right amount of moisture and perlite adds air space and keeps the mixture light. Wetting agents are added to peat moss, because when it dries out it resists moisture.
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