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10.5 Soil for raised beds

Filling a framed bed that stands above the ground, with a mix of topsoil and compost.

Cross-section of a raised bed: a frame standing on the ground, filled 150–300 mm or deeper with a mix of one half to two thirds topsoil and one third to one half compost, with vegetables growing in it. 1 Ground below left in place 2 Bed mix ½–⅔ topsoil, ⅓–½ compost 3 Frame and depth 150–600 mm deep
Cross-section of a raised bed.

At a glance

Difficulty
2 of 4
Time
½ day per bed of 1.2 × 2.4 m, once the frame is built
Tools
Wheelbarrow, shovel, tarp for mixing, rake, tape measure, hose
Parts
  1. 1  Ground below
  2. 2  Bed mix
  3. 3  Frame, 150–600 mm deep

What you need per m² of bed, 300 mm deep

MaterialKindTypeAmount
Topsoil Loam or sandy loam Screened, weed-free; ½ to ⅔ of the mix 0.15–0.2 m³
Compost Plant-based compost Well rotted; ⅓ to ½ of the mix 0.1–0.15 m³
Not Bagged potting soil Dries out too fast in a big bed

Why topsoil plus compost?

Topsoil

Gives the bed its body: the mineral part that holds nutrients and gives texture. Every source keeps it at half the mix or more.

⅓–½ compost

Sources put compost at 30–50 % of the mix. More is not better: organic matter and phosphorus build up (10.4).

No potting soil

Potting soil is made for pots. In a raised bed it dries out too quickly.

Depth

At least 150–200 mm for salad and beans. Tomatoes, peppers and squash want 300–600 mm.

Step by step

  1. 1

    Choose the depth.

    Pick the depth from what you will grow: at least 150–200 mm for leafy greens, beans and cucumbers, 300–600 mm for tomatoes, peppers and squash.

    Tape measure on the frame
  2. 2

    Work out the volume.

    Multiply length × width × depth. A bed of 1.2 × 2.4 m filled 300 mm deep holds about 0.9 m³.

    Measuring the inside of the frame
  3. 3

    Mix topsoil and compost.

    Mix ½ to ⅔ topsoil with ⅓ to ½ compost on a tarp or in the barrow. Equal parts is the simplest rule.

    Topsoil and compost heaps being mixed
  4. 4

    Fill and water.

    Fill the bed to the top in layers, mixing as you go. Water it well; the mix settles.

    Bed filled to the top
  5. 5

    Top up every year.

    Raised beds always sink. Each year add a few centimetres of compost or good topsoil to bring the level back up.

    Compost topping up a settled bed
Specifications
PartKindValueNote
DepthSalad, beansmin 150–200 mmK-State; Maryland 200 mm for leafy greens, beans, cucumbers.
DepthTomato, pepper, squash300–600 mmMaryland.
MixTopsoil50–70 %Minnesota ½–⅔; Iowa State 70 %; Illinois 50 %.
MixCompost30–50 %Minnesota ⅓–½; Wisconsin ¼–½; Iowa State 30 %; Illinois 50 %.
Top-upCompost or topsoilyearlyIllinois: a few inches each year.

Other mixes

Iowa State also gives equal parts topsoil, organic matter and coarse sand. NC State Extension uses 40 % compost, 40 % topsoil and 20 % coarse material for air. All keep compost well under the whole bed.

Settling

A new bed sinks as it settles. Fill to the top and top up after the first season.

Sources

  1. Kansas State University Research and Extension MF2134, Raised bed gardening: at least 150–200 mm (6–8 in) bed depth
  2. University of Maryland Extension, Raised bed gardening: at least 200 mm (8 in) for leafy greens, beans and cucumbers; 300–600 mm (12–24 in) for peppers, tomatoes and squash
  3. University of Minnesota Extension, Raised bed gardens: ½ to ⅔ topsoil and ½ to ⅓ plant-based compost; potting soil dries out too quickly; avoid old CCA-treated wood and railroad ties
  4. University of Illinois Extension, Raised bed gardening info sheet (2023): 1 : 1 compost and topsoil; volume = length × width × height; soil and compost need adding each year
  5. University of Illinois Extension, Refreshing raised beds: settling is a fact of raised bed gardening; top up with a few inches of compost or good topsoil
  6. Iowa State University Extension, A good soil mix for a raised bed: about 70 % topsoil and 30 % compost, or equal parts topsoil, organic matter and coarse sand
  7. NC State Extension, Clay County, Getting started with raised bed gardening: 40 % compost, 40 % topsoil, 20 % aeration material; avoid pure potting soil
  8. University of Wisconsin Extension, Marinette County, Building raised beds: compost about ¼ to ½ of the total mix
  9. Oregon State University Extension, Raised beds: at least one third native soil in the bed
  10. Clemson University HGIC, Raised beds: use treated lumber rated UC4A (ground contact) or higher for horticultural use

Amounts: 1 m² × 300 mm = 0.3 m³ of mix. Inches converted at 1 in = 25.4 mm and rounded.

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