10 Garden soil
Soil for the garden side of a project: filling and leveling, lawns, trees and shrubs, flower beds, raised beds and leveling an old lawn. Topsoil goes where roots grow; subsoil does the bulk.
- 10.1Filling and leveling with soilRaising or leveling garden ground. Subsoil in thin firmed layers, topsoil on top, falling away from the house.
- 10.2Soil for lawns150 mm of loam or sandy loam, the first 50 mm mixed into the ground. Topdress at most 12 mm at a time.
- 10.3Soil for trees and shrubsA wide, shallow hole backfilled with the soil that came out of it. No compost in the hole; mulch on top.
- 10.4Soil for flower beds50–75 mm of compost dug into the top 150–200 mm. Never dig wet soil.
- 10.5Soil for raised beds½–⅔ topsoil and ⅓–½ compost, 150–600 mm deep. Top up every year.
- 10.6Testing your soilJar, ribbon and drainage tests at home; a lab for pH and nutrients.
- 10.7Leveling an existing lawnSoil that matches the lawn's own, 6–12 mm per round. Lift the sod over deep hollows.
Sand, silt and clay grain size
| Part | Grain size | Feels |
|---|---|---|
| Gravel | over 2 mm | Stones: chapters 1–9 |
| Sand | 0.05–2 mm | Gritty between the fingers |
| Silt | 0.002–0.05 mm | Smooth, not gritty |
| Clay | under 0.002 mm | Squeezes into a long ribbon (10.6) |
Limits as in the USDA Soil Survey Manual. FAO and ISO put the line between silt and sand at 0.063 mm, and the traditional UK system at 0.06 mm (Natural England). Clay is under 0.002 mm in all of them. A loam is a balanced mix: 7–27 % clay, 28–50 % silt and no more than 52 % sand.
The layers in this chapter
| Job | Material | Depth |
|---|---|---|
| Fill under a garden (10.1) | Subsoil, thin firmed layers | as needed |
| Lawn (10.2) | Topsoil, loam | 150 mm (min. 100) |
| Shrubs and trees (10.1, 10.3) | Topsoil, native soil in the hole | up to 400 mm |
| Flower bed (10.4) | Compost dug in | 50–75 mm into 150–200 mm |
| Raised bed (10.5) | Topsoil + compost | 150–600 mm |
| Mulch (10.3) | Wood chips, bark | 50–100 mm |
Soil is not aggregate
Topsoil is the living top layer, with about 2–10 % organic matter (UConn). That is what makes plants grow, and also why it does not belong under loads: organic matter rots and the ground settles. Use topsoil only as deep as roots need it, and make anything deeper with subsoil (Defra). Under a patio, a shed or a slab, use crushed stone (chapters 2 and 3). Every page cites its sources at the bottom; all are universities, extension services, public bodies or horticultural societies.
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