A 10 m2 bed needs 9 bags of bark mulch at 7 cm - but only 13 bags of topsoil at 10 cm. Three garden ground cover materials, three price tags, one calculator each. The numbers look similar until you realize that mulch sits on top and soil goes underneath, that bark lasts 2-3 years before decomposing while compost keeps producing indefinitely, and that buying the wrong depth wastes 30-40% of your budget.
This is not a guide about which material is "best." All three do different jobs. The question is how much of each you actually need - in bags, in liters, in dollars - so you buy once and get it right.
The comparison nobody makes before driving to the garden center
Most people buy soil or mulch by feel. "That looks like enough bags." The result: a second trip, or half a pallet sitting in the driveway for six months.
Here is what the numbers say for a 10 m2 bed - a typical flower border or small vegetable patch:
| Material | Depth | Volume | Bags (80L) | Cost (avg) | Lasts |
|---|---|---|---|---|---|
| Topsoil | 10 cm | 1,000 L | 13 bags | $78 | Permanent |
| Topsoil | 20 cm | 2,000 L | 25 bags | $150 | Permanent |
| Pine bark mulch | 7 cm | 700 L | 9 bags | $72 | 2-3 years |
| Pine bark mulch | 5 cm | 500 L | 7 bags | $56 | 1-2 years |
| Compost (homemade) | 2 cm top dress | 200 L | 0 bags | $0 | Renewed yearly |
The depth column is where all the money hides. A vegetable bed at 20 cm costs twice as much as a flower bed at 10 cm - same area, same material, double the bags. And mulch at 5 cm instead of 7 cm saves $16 per 10 m2 but cuts weed suppression in half.
Soil - the foundation layer
Topsoil goes in first. It is the growing medium - the layer where roots live, water drains and nutrients cycle. Without enough depth, roots hit subsoil (clay, sand, rubble) and growth stalls.
The formula is dead simple: area (m2) x depth (m) x 1000 = liters. A 10 m2 bed at 15 cm = 10 x 0.15 x 1000 = 1,500 liters = 19 bags of 80L.
But here is the part people miss: bagged soil settles 20-30% after spreading and watering. Air pockets collapse, organic matter compacts, and your 15 cm layer becomes 10-12 cm. The fix? Buy 15-20% extra, or add 2-3 cm to your target depth.
Quick reference - topsoil bags by area
- Topsoil for 5 m2 - 500 L, 7 bags of 80L
- Topsoil for 10 m2 - 1,000 L, 13 bags
- Topsoil for 15 m2 - 1,500 L, 19 bags
- Topsoil for 20 m2 - 2,000 L, 25 bags
- Topsoil for 25 m2 - 2,500 L, 32 bags
- Topsoil for 30 m2 - 3,000 L, 38 bags
- Topsoil for 50 m2 - 5,000 L, 63 bags
- Topsoil for 75 m2 - 7,500 L, 94 bags
- Topsoil for 100 m2 - 10,000 L, 125 bags
- Topsoil for 200 m2 - 20,000 L, 250 bags
All values at 10 cm depth with 80L bags. For 20 cm, double the bag count. Above 3 m3 (about 38 bags), bulk delivery by dump truck is cheaper.
Bags vs bulk - the break-even point
Bagged topsoil costs roughly $75 per m3. Bulk delivery runs $30-50 per m3 plus $80-120 for the truck. The break-even sits around 3 m3 - anything above that, call the truck. Below that, bags are simpler and you control the quality.
Mulch - the protective blanket
Mulch sits on top of the soil. It suppresses weeds, retains moisture, insulates roots from heat and frost, and looks tidy. But only if the layer is thick enough.
5 cm is the minimum. Anything thinner and weeds push through within weeks. 7 cm is the sweet spot - effective weed barrier without blocking water infiltration. 10 cm is the maximum. Thicker layers trap moisture against stems (crown rot) and can become hydrophobic - water beads on top instead of soaking through.
The three mistakes everyone makes: too thin (false economy), piled against stems (rot), and forgetting to refresh every 2-3 years (it decomposes).
Quick reference - mulch bags by area
- Mulch for 5 m2 - 350 L, 5 bags of 80L
- Mulch for 10 m2 - 700 L, 9 bags
- Mulch for 15 m2 - 1,050 L, 14 bags
- Mulch for 20 m2 - 1,400 L, 18 bags
- Mulch for 25 m2 - 1,750 L, 22 bags
- Mulch for 30 m2 - 2,100 L, 27 bags
- Mulch for 50 m2 - 3,500 L, 44 bags
- Mulch for 75 m2 - 5,250 L, 66 bags
- Mulch for 100 m2 - 7,000 L, 88 bags
All values at 7 cm depth with 80L pine bark bags. Wood chips in 50L bags cost less per bag but more per liter. Decorative coloured mulch fades after 12-18 months.
Pine bark vs wood chips vs decorative
| Type | Price/bag | Bag size | Lasts | Best for |
|---|---|---|---|---|
| Pine bark | $7-9 | 80 L | 2-3 years | Borders, trees, shrubs |
| Wood chips | $4-6 | 50 L | 1-2 years | Pathways, play areas |
| Decorative | $7-10 | 50 L | 1-2 years | Front garden, curb appeal |
Pine bark is the all-rounder. Wood chips are the budget pick for utility areas. Decorative mulch looks sharp but costs 40% more per liter and the colour fades - you are paying for aesthetics, not function.
Compost - the free soil factory
Compost is the only ground cover material that costs nothing after the initial bin purchase. A 400-liter bin ($80-150) turns kitchen scraps and garden trimmings into dark, crumbly humus in 6-12 months. A family of four with a 300 m2 garden generates about 720 kg of compostable waste per year and gets 250 liters of finished compost back.
That is enough to top-dress 25 m2 of beds at 1 cm depth every year. For free. Indefinitely.
The catch: composting requires the right ratio. 75% brown (dry leaves, shredded cardboard, sawdust) plus 25% green (kitchen peels, grass clippings dried 24 hours, coffee grounds). Get this wrong and the pile smells. Get it right and it smells like forest floor.
Which bin type for which gardener?
| Type | Capacity | Price | Ready in | Who is it for? |
|---|---|---|---|---|
| Open wooden | 400-800 L | $30-80 | 6-12 months | Large gardens, patient composters |
| Plastic enclosed | 200-600 L | $60-150 | 4-8 months | Most home gardens |
| Tumbler | 100-300 L | $120-350 | 6-8 weeks | Impatient gardeners, small batches |
| Worm bin | 30-100 L | $40-120 | 2-3 months | Apartments, balconies, kitchen scraps only |
The tumbler is 5x faster but handles smaller batches. For a typical family garden, a plastic enclosed 400L bin hits the sweet spot between speed, price and capacity.
The verdict - soil, mulch or compost?
They are not competitors. They are layers.
Soil is the base - you need it once to build the bed. Mulch is the top layer - you reapply every 2-3 years to keep weeds down and moisture in. Compost is the amendment - you produce it yourself and spread a thin layer annually to feed the soil biology.
A new 10 m2 vegetable bed from scratch:
1. Topsoil: 20 cm depth = 25 bags of 80L = $150
2. Mulch: 7 cm pine bark = 9 bags of 80L = $72 (reapply in year 3)
3. Compost: 2 cm top dressing from your 400L bin = $0/year after $100 bin purchase
Total year 1: $322. Year 2 onward: $0 (compost only). Year 3: $72 (mulch refresh).
The bed pays for itself if you grow vegetables worth more than $322 in the first season. At typical yields of $5-10 per m2, a 10 m2 bed produces $50-100 of produce per year. Break-even: roughly 4-5 years. But the real return is soil quality - composted beds improve every year while unfed beds decline.
Tools discussed in this article
Soil Calculator - calculate topsoil volume and bag count for garden beds. Four soil types (topsoil, peat, compost, universal), five layer depths from 5 to 30 cm, cost estimate with settling warning.
Mulch Calculator - calculate bark mulch and wood chip coverage per m2. Pine bark 80L or 50L, decorative mulch, wood chips. Three depths (5/7/10 cm), cost comparison.
Compost Calculator - size your compost bin by household and garden area. Annual waste breakdown, compost yield, brown-to-green ratio, four composter types compared.
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