Topsoil vs Compost: Which to Buy for Garden Beds?

Updated 2026-09-21 · Yardly

Buy both, blended — not one or the other: topsoil is the mineral rooting medium your plants stand in, and compost is the decomposed organic matter that feeds it and loosens it, so a garden bed wants roughly equal parts of each (or up to 70% topsoil / 30% compost) rather than a truckload of either alone. Treating them as interchangeable is the single most common reason a new bed ends up half-empty and starving by midsummer.

What each one actually is

Topsoil is the upper mineral layer of the earth — sand, silt, and clay in some local proportion, plus whatever organic matter has built up in it. It gives a bed its structure: it holds the plant up, holds water without drowning the roots, and gives roots something to push through. Screened topsoil from a landscape yard is that mineral soil minus the rocks and clumps.

Compost is organic material — leaves, food scraps, manure, yard waste — that has fully broken down into a dark, crumbly, earthy-smelling humus. It carries almost no sand/silt/clay, so it contributes fertility and texture, not structure. It teems with microbes, holds nutrients, and loosens heavy clay or tightens sandy soil. But on its own it collapses.

The mistake is thinking “compost is the good stuff, so more is better.” It isn’t a substitute for soil; it’s an additive to soil.

The blend extensions actually recommend

University extension services — Iowa State and University of Maryland among them — consistently land on a 1:1 topsoil-to-compost mix as the dependable baseline for a new raised or in-ground bed, with a 70% topsoil / 30% compost mix at the firmer end. Oregon State’s garden guidance makes the same point from the other direction: don’t fill a bed with compost alone, and don’t exceed about a third compost in a fresh mix.

A reliable starting mix: 60–70% screened topsoil + 30–40% finished compost, by volume.

Why not more compost? Because pure compost:

• Settles 15–20% as the soft organic fraction compacts, leaving the bed noticeably lower within a season.
• Holds too much water in a deep layer, which suffocates roots that need air pockets.
• Costs more per yard than topsoil, so you pay a premium for a material that shouldn’t stand alone.

For an existing in-ground bed you’re not rebuilding, you skip the full blend: lay 2–3 inches of compost on top and fork it into the top 6–8 inches. After year one, a 1–2 inch top-dress each spring keeps the organic level and nutrients topped up without re-digging.

How many yards to order

Both materials use the same volume math as mulch or soil:

Square feet × depth in inches ÷ 324 = cubic yards of mix Then split that total by your blend ratio for topsoil vs compost.

One cubic yard of mix covers about 108 sq ft at 3 inches (162 at 2 inches). Worked example — a 4×8 ft bed (32 sq ft) at 10 inches deep for a deep raised bed:

• 32 × 10 ÷ 324 = 0.99 yd³ of mix
• At 70/30: ≈ 0.69 yd³ topsoil + 0.30 yd³ compost
• Add 10–15% for settling → order about 0.8 yd³ topsoil, 0.35 yd³ compost

For a larger 100 sq ft in-ground bed at 6 inches:

• 100 × 6 ÷ 324 = 1.85 yd³ of mix
• At 1:1: 0.93 yd³ each, +15% → roughly 1.05 yd³ of each

The soil calculator and the raised-bed soil calculator do this split from your exact dimensions so you don’t round-trip the arithmetic.

Cost per yard, and why blending saves money

Materials-only retail runs roughly $25–40 per cubic yard for screened topsoil and $40–70 per cubic yard for finished compost. A 100 sq ft bed filled 6 inches deep at 1:1 therefore costs roughly $40–$75 for topsoil plus $40–$75 for compost — call it $80–$150 of material before delivery.

The savings from blending show up when you resist the urge to go 100% compost: the same bed in straight compost would cost $75–$130 and settle into a shallow, waterlogged layer. Blending spends a little less and performs far better.

Where each one clearly wins

Reach for topsoil when:

• You’re building structure — filling a new bed, grading a low spot, or levelling a yard.
• You need a mineral backbone that won’t collapse over the season.
• You’re seeding or sodding a lawn and need a workable rooting layer.

Reach for compost when:

• You’re feeding an established bed with a spring top-dress.
• Your native soil is dead clay or pure sand and needs organic matter to function.
• You’re mixing a custom potting or raised-bed blend and want biology and nutrients.

Most gardeners buy a bulk topsoil load and a smaller compost load, then blend on a tarp. The topsoil-for-raised-beds guide covers sizing the topsoil side for edible beds; the fill-dirt vs topsoil piece explains where structural fill (not topsoil, not compost) belongs underneath.

Mistakes that waste the load

1. Filling a bed with straight compost. It settles and sours; blend at most a third.
2. Buying bagged “garden soil” blind. Some are mostly bark fines and peat with little real mineral topsoil — check the label for a listed topsoil or sand component.
3. Forgetting the settling allowance. Order 10–15% over the raw number or you’ll be short after the first watering.
4. Skipping the spring top-dress. Compost keeps breaking down; a bed left alone loses both level and fertility within a year.
5. Importing unknown topsoil without a test. If the source is a mystery, a $20–$30 extension soil test catches lead or contaminants before you plant food in it.

Keep reading

• Split your exact bed volume by ratio in the soil calculator or the raised-bed soil calculator.
• Filling edible beds specifically? The topsoil-for-raised-beds guide walks the depths and refresh timing.
• Not sure where topsoil stops and structural fill begins? Fill dirt vs topsoil draws the line.

Frequently asked questions