Soil Mix for Raised Garden Beds

The exact blends, ratios, and step-by-step method we use to fill raised beds once—and keep them thriving for seasons.

Shovel mixing compost and soil in a raised garden bed

What makes a great raised bed mix?

Raised beds do best with a blend that drains well, holds steady moisture, and feeds the soil food web. Think of it as structure (minerals), nutrition (compost), and moisture management (coir/peat + aeration). Below are three reliable mixes—pick the one that fits your climate and budget.

Lucky Cat 9b Raised-Bed Mix (Products & Ratios)

For our hot, dry zone 9b summers, this blend stays airy, holds moisture, and feeds steadily without going soggy:

Component	Product	By Volume	Notes
Base Mix	Kellogg Raised Bed & Potting Mix	70%	Ready-to-use raised-bed blend with good porosity and moisture holding. Check bag size (often 2 cu ft; some G&B bales are 3 cu ft).
Compost	Garden Time Mushroom Compost	30%	Rich organic matter; typical bag is 1 cu ft. Don’t overdo—mushroom compost can be salty; 30% keeps texture balanced in hot zones.
Mulch (top)	Sta-Green Premium 2-cu ft Brown Mulch	2″ layer	Critical in heat: reduces evaporation and evens soil temps. (Popular choice — 5k+ bought last week.)
Aeration / Moisture (optional)	Sta-Green Perlite, 8 qt (Organic) • Happy Trees Coco Coir	Add 5–10% as needed	Perlite improves drainage and root oxygen; coco coir steadies moisture in hot/dry beds. Blend lightly into top 8–10″ or through the mix.

Why 70/30? In 9b heat, we want moisture retention without compaction. Kellogg’s raised-bed mix gives structure & airflow; ~30% mushroom compost boosts organic matter while keeping salts reasonable.

Example for a 4′×8′×12″ bed (32 cu ft)

Kellogg Raised Bed & Potting Mix (70%): 22.4 cu ft → ~12 bags at 2 cu ft or ~8 bales at 3 cu ft.
Garden Time Mushroom Compost (30%): 9.6 cu ft → ~10 bags at 1 cu ft.
Top-off for settling: add ~10% extra if you’re filling a brand-new bed (≈ 3 cu ft).

Open Soil Mix Calculator

Fertilizer Plan (Espoma)

At Planting/Transplant — Bio-tone Starter Plus

Bed prep (economical): mix ~4 lb per 100 sq ft into the top 4–6″ (for a 4×8 bed ≈ 1.3 lb).
Potting-mix method (optional): if blending into a custom potting mix, use ~1 cup per cu ft.

Ongoing Feeding — Garden-tone

Soil prep rate: about 3–3.5 lb per 50 sq ft (≈ 2 lb for a 4×8 bed); work into top 4–5″.
Established plants: up to ~⅓ cup per plant or along rows; re-feed every 4–6 weeks during active growth.

Zone 9b tips: Plant slightly high, water in deeply, and keep that 2″ mulch refreshed through summer. If heat is extreme, add afternoon shade and tighten irrigation intervals.

How Much Soil Do I Need?

Measure the inside of your bed in feet. Volume (cubic feet) = Length × Width × Height. 1 cubic yard = 27 cubic feet. Bags are often 1.5 cu ft each.

Example (4′ × 8′ × 1′ bed):

Volume = 4 × 8 × 1 = 32 cu ft
In cubic yards: 32 ÷ 27 ≈ 1.19 yd³
1.5 cu ft bags: 32 ÷ 1.5 ≈ 22 bags

Breakdown (Balanced Mix):

Topsoil (50%): 16 cu ft
Compost (30%): 9.6 cu ft
Aeration (20%): 6.4 cu ft

Settling is normal: After the first few weeks, the mix may drop 10–20%. Top off with compost and re-mulch.

Step-by-Step: Filling a Raised Bed

Prep the site: Level the bed; lay weed barrier only if necessary (cardboard is fine under invasive weeds).
Measure & calculate volume: Use the formula above to plan bulk or bagged materials.
Stage ingredients: Pile compost, topsoil, and aeration materials nearby. Lightly moisten dusty ingredients.
Blend thoroughly: On a tarp or in the bed, layer and mix until uniform. Avoid thick “lasagna” layers if you plan to plant right away.
Charge & amend: Mix in a gentle organic fertilizer per label; if using peat, follow product directions for dolomitic lime to balance pH.
Fill & settle: Fill to the rim; water deeply to settle. Top off if needed and finish with 1–2″ mulch (wood chips or straw) to reduce evaporation.
Plant & water: Plant transplants slightly high; for seeds, keep the top inch evenly moist. Consider a simple drip system for consistent results.

Dial It In (Climate & Crops)

Hot/dry climates: Favor coir over peat; add extra compost, and keep a 2″ mulch layer. Biochar (pre-charged) can smooth moisture swings.
Cool/wet climates: Increase mineral/topsoil and aeration (pumice/perlite); reduce coir/peat slightly for faster drainage.
Heavy feeders (tomatoes, squash): Work in extra finished compost at planting and side-dress mid-season.
Root crops: Sift out large chunks; avoid fresh manure; keep texture loose and even.
pH & nutrients: Aim for pH ~6.3–7.0. When in doubt, follow product labels or do a simple soil test kit mid-season.

Common Mistakes (and Easy Fixes)

Using only compost: Too rich/compact. Fix with topsoil + aeration.
Unmixed layers: Roots stall at boundaries. Blend ingredients thoroughly.
No mulch: Surface dries out fast. Add 1–2″ organic mulch after planting.
Over-fertilizing early: Tender roots can burn. Start gentle; feed as plants establish.

Quick FAQ

Can I reuse last year’s soil?

Yes. Pull roots, top-dress 2–3″ of compost, mix lightly, and refresh mulch. Add aeration if the bed feels dense.

Coir or peat—what’s better?

Both improve moisture retention; coir is a renewable option. If using peat, follow label directions to buffer pH (often with dolomitic lime).

Do I need vermiculite and perlite?

You can use either. Vermiculite holds more water; perlite/pumice improve drainage. In hot/dry regions, vermiculite or coir can help steady moisture.

Want a printable checklist version of this guide?

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