Rain usually washes soil out for three reasons: bare, fine soil gets hit by drops; water cannot drain fast enough; and there is no overflow control near the top of the box. Fix those and you will keep most of your soil where it belongs.
Start with drainage. Check that every drainage hole is open. If the holes are large, cover them with a piece of window screen, weed fabric, or geotextile. That lets water through but holds back particles. Do not use garden soil in a planter; it compacts and turns to mud. Use a coarse potting mix with compost, coco coir, perlite, and bark fines. This mix drains quickly and resists crusting.
Next, leave a reservoir rim. Keep the soil surface 2–3 inches below the top edge of the planter. During heavy rain, water can gather in that space and flow out through an overflow hole or drainage opening instead of running across the soil and carrying it away. If your planter has no overflow, drill a hole just below the rim and cover it with mesh. For balcony boxes, make sure the overflow points away from the building or into a drip tray.
Mulch is your best rain armor. Add 1–2 inches of coarse bark mulch, straw, pine needles, coco coir, or small gravel on top of the soil. Mulch breaks the force of raindrops and slows runoff. For a modern urban look, use a thin layer of decorative pebbles around plants, but leave space for stems. A living mulch also works: plant sedum, moss, creeping thyme, or sweet alyssum to cover bare soil.
If rain is intense, add a temporary shield. A clear plastic clamshell, a piece of acrylic, or even a removable wooden cover angled over the box can deflect rain during storms. Remove it after the storm so plants get light and air. You can also redirect a downspout or roof drip that pours into the planter.
Check the slope. The planter should be level or tilted very slightly toward the overflow, not toward the front where soil spills onto the sidewalk. If water enters from one side, create a small berm of gravel or a coir log to slow it down. For larger raised beds, an erosion control blanket or coir mat on the surface can stabilize soil until plants fill in.
Finally, maintain it. Top up mulch after heavy storms, replace compacted soil, and avoid overwatering. If you see soil escaping, inspect the drainage holes, overflow, and mulch layer. In most urban planter boxes, the winning combination is coarse potting mix, mesh-covered drainage, a 2–3 inch rim, and a mulch layer. That keeps rain from turning your planter into a mudslide and your balcony into a mess.