Earthworks
Soil improvement with compost
Structure, humus and working it into the growing medium
Good soil is the prerequisite for both planting and grass. This guide shows how to assess the soil's needs and improve structure and humus content with compost — either as a layer or mixed into the top growing layer — without compacting the soil. Humus targets follow NfA; doses are guidelines (Bolius).
Assess the needs of the soil
Assess what the soil is lacking: heavy clay soil needs structure and air, while light sandy soil needs humus that retains water and nutrients. For grass, the target humus content after establishment is 3–5 % of dry weight (NfA); generally at least 2 % by weight (NfA 2023).
Topsoil, lawn3–5 %Topsoil, general≥ 2 % by weightChoose a suitable soil improver
Choose compost, sphagnum or another soil improver according to the soil type and purpose. Use products on Plantedirektoratet's register. Compost adds both structure and nutrients; sphagnum mainly improves the structure and water-holding capacity of heavy soil.
Choose the method: layer or mixing in
For new beds, compost can be laid as a layer of a guideline 5–10 cm on top of the soil, or it can be mixed into the top layer of soil at a guideline 20–30 % compost in the mixed layer (Bolius). Choose mixing in where you are cultivating anyway; choose a layer where you are only top-dressing.
Layer method5–10 cm (guideline)Mixing in20–30 % (guideline)Calculate the quantity
Calculate the amount of compost from your own measurements: layer method = area × layer thickness; incorporation = area × working depth × mixing proportion. Use the »Compost / soil improvement« calculator, where you enter the bulk density from the supplier's data sheet yourself (the function never makes up a density).
Work into the depth of the growing layer
Work the compost in by cultivating to the thickness of the topsoil layer, but no deeper than 25 cm (NfA 1992). Mix compost and soil well, so that the humus is spread evenly rather than lying as an isolated pad the roots cannot get through.
Max. cultivation depth25 cmAvoid compacting the improved soil
Do not work on wet soil, and do not drive heavy machinery over the improved layer — the whole point is an airy, uncompacted soil. For planting areas the dry bulk density must be no more than 1.4 g/cm³ (clayey) / 1.5 g/cm³ (sandy) so that the roots can grow freely (NfA 1992).
Dry bulk density, plant≤ 1.4–1.5 g/cm³Allow for nitrogen fixation
Fresh, immature compost and woody material can lock up nitrogen while decomposing and temporarily starve the plants. Use well-matured compost, or compensate with a nitrogen fertiliser when planting/sowing in freshly improved soil.
Let the compost settle and break down before you plant in it — it halves the risk of yellow, nitrogen-starved plants.
Source
Based on Normer for anlægsgartnerarbejde 1992 (humus content, dry bulk density, cultivation depth) and indicative soil improvement practice (Bolius); doses are user input/indicative.