Raised Bed Soil Depth & Mix for a 450-sq-ft Tiered Garden

Building a 450-sq-ft, three-tier raised bed on a concrete base with drainage rock and filter fabric. The question: is 12″ of topsoil enough?

Don’t Use Straight Topsoil

Native topsoil is often low in organic matter and nutrients. Instead, build a living soil blend:

• 50% screened topsoil (mineral base)
• 30% compost (nutrients & biology)
• 20% peat moss or coconut coir (moisture retention)

Depth Verdict

12″ (1 ft) is sufficient for most vegetables—roots concentrate in the top 8–10″. Deeper (18″) benefits tomatoes, carrots, and perennials, but adds cost. For a 450-sq-ft bed, 12″ of the 50/30/20 mix = ~167 cubic yards. Source locally to save freight.

Installation Tips

1. Lay filter fabric over drain rock.
2. Fill in 6″ lifts; lightly tamp each layer.
3. Water thoroughly to settle before planting.
4. Top with 2″ straw mulch after planting.

This mix gives you a fertile, well-drained, long-lasting growing medium worth the investment.