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10.4 Soil for flower beds

Getting the soil in a border, a flower bed or a vegetable plot ready, by digging it over and working compost into the top.

Cross-section of a flower bed: subsoil left in place, the top 150–200 mm of soil dug over with 50–75 mm of compost mixed in, flowers growing on top. 50–75 mm 150–200 mm 1 Subsoil left in place 2 Dug-over topsoil compost mixed in 3 Compost spread, dug in
Cross-section of a bed with compost worked into the top 150–200 mm.

At a glance

Difficulty
1 of 4
Time
1–2 hours per 10 m²
Tools
Spade, garden fork, rake, wheelbarrow, bucket
Parts
  1. 1  Subsoil, left in place
  2. 2  Dug-over topsoil
  3. 3  Compost mixed in

What you need per m², first year

MaterialKindTypeAmount
Compost Garden or green-waste compost Well rotted, crumbly 0.05–0.075 m³
or Manure or manure compost Only 25 mm: it is salty and rich in phosphorus 0.025 m³
Later years Compost Top-up each year 0.025–0.05 m³

Why 50–75 mm of compost?

50–75 mm

Enough to lift the soil towards 4–5 % organic matter in the first three years. After that, 25–50 mm a year keeps it there.

Not more

More is not better. Once the soil has 3–5 % organic matter, stop adding: phosphorus builds up, especially from manure.

Top 150–200 mm

Most roots of flowers and vegetables are in the top layer. Mix the compost through it, not just on top or deep down.

pH 6.5

A good general target for most garden plants. A lab test (10.6) tells you if you are far off it.

Step by step

  1. 1

    Test the soil first.

    Send a sample to a lab (10.6) for pH and organic matter. If organic matter is already 3–5 %, add little or no compost.

    Soil sample in a clean bucket
  2. 2

    Check it is not too wet.

    Squeeze a handful. If it crumbles, you can dig. If it stays in a sticky lump, wait: digging wet soil ruins its structure and it dries into hard clods.

    Squeezed handful of soil crumbling
  3. 3

    Spread the compost.

    Spread 50–75 mm of compost evenly over the bed. Use only 25 mm if it is manure or manure-based.

    Compost spread over the bed
  4. 4

    Dig it in.

    Dig or fork the compost into the top 150–200 mm. One spade's depth, about 250 mm, is enough for most beds.

    Spade turning compost into the soil
  5. 5

    Rake level and plant.

    Break up clods and rake the surface level. Plant or sow, then water.

    Raked bed with young plants
  6. 6

    Top up each year.

    From the fourth year on, add 25–50 mm of compost each year, or use it as a mulch on the surface.

    Thin compost layer on an established bed
Specifications
PartKindAmountNote
Dig depth–1 spade, about 250 mmRHS single digging.
Mixing depth–top 150–200 mmColorado State 7.235. Maryland: top 100 mm.
Compost, years 1–3Plant-based50–75 mmColorado State. Maryland: 50–100 mm.
Compost, laterPlant-based25–50 mm a yearColorado State.
ManureManure, manure compostmax 25 mmHigh in salts (Colorado State).
Organic matterTarget4–5 %Colorado State. Oregon State: hold off above 3–5 %.
pHTarget6.5RHS general target. UConn for flowers: 5.5–7.0, target 6.5.

Amount

1 m² × 50 mm = 0.05 m³ of compost. Colorado State puts it the other way round: 3 cubic yards cover 1,000 ft² about 25 mm deep.

Double digging

On compacted ground the RHS describes double digging: loosening to twice the depth of a spade. It is hard work and rarely needed in beds that are not walked on.

Sources

  1. Royal Horticultural Society, Double digging: single digging is one spade deep (25 cm/10 in), double digging twice that; about a bucketful of compost per square metre
  2. Royal Horticultural Society, Soil cultivation: rotavating wet soil is extremely damaging; wait for drier conditions
  3. Colorado State University Extension Fact Sheet 7.235, Choosing a soil amendment: 50–75 mm (2–3 in) of compost for the first three years, then 25–50 mm (1–2 in); manure and other high-salt materials only 25 mm (1 in); mix into the top 150–200 mm (6–8 in); target 4–5 % organic matter; 3 yd³ covers 1,000 ft² about 25 mm deep
  4. University of Maryland Extension, Soil amendments: 50–100 mm (2–4 in) of compost into the top 100 mm (4 in) of soil; pH 6.2–6.8
  5. Oregon State University Extension, Too much of a good thing: gardeners may overdo compost: at 3–5 % organic matter, hold off adding more; phosphorus builds up, especially with manure-based composts
  6. Royal Horticultural Society, Soil pH and testing: pH 6.5 is the best general-purpose pH for gardens
  7. University of Connecticut Soil Nutrient Analysis Laboratory, Flowers: pH 5.5–7.0 with a target of 6.5
  8. University of Illinois Extension, Spring gardening tips: squeeze a handful before digging; if it does not crumble it is too wet; wet-worked soil dries into concrete-like clods

Inches converted at 1 in = 25.4 mm and rounded. Imperial amounts are per 10 ft².

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