For your new urban planter boxes, it's strongly recommended to avoid using regular garden soil alone. While it seems convenient, garden soil is typically too dense for containers, leading to poor drainage, compaction, and ultimately, root rot and unhappy plants. Container gardens have unique needs; they require a growing medium that retains moisture yet drains excess water efficiently and provides adequate aeration for roots.
The ideal solution is a special "potting mix" or "container mix." These are typically soilless blends containing ingredients like peat moss or coconut coir for moisture retention, perlite or vermiculite for aeration and drainage, and often compost or slow-release fertilizers for nutrients. You can create your own perfect blend by mixing one-part compost for nutrients, one-part coarse sand or perlite for drainage, and one-part peat moss or coir for water retention. This creates a light, fluffy, and fertile environment that will support robust plant growth in the confined space of a planter box, ensuring your urban garden thrives from season to season.