Using compost and digestate - topsoil

Compost has many benefits in preparing quality top soil.

Before any soil preparation procedures ensue, a soil analysis shall be completed by a reputable laboratory to determine any nutritional requirements, and any pH and organic matter adjustments necessary. Once determined, the soil shall be appropriately amended to a range suitable for the turf species to be established.

Quality compost should be applied over the entire treatment area at an average depth of 25 to 50 mm, then incorporate it to a minimum depth of 150 mm using a rotovator or other appropriate equipment.

Fertiliser and pH adjusting agents (e.g. lime and sulphur) may also be applied if necessary.

The addition of compost may reduce the required frequency of watering and  the addition of pre-plant fertiliser can be reduced and sometimes eliminated when quality compost is applied at recommended rates.

Further reading:

Compost inclusion rates should be specified depending upon soil conditions and quality, plant tolerances, and manufacturer’s recommendations.

For most projects, uniformly apply a 25 - 50 mm layer of compost over the area to be treated, (2.5 – 5.0 m3 per 100 m2). However, soils rich in organic matter may not require any compost, while even higher compost application rates could be used on soils that are very low in organic matter, or droughty (primarily sand and gravel based).

WRAP's recommendations are suitable for most top soil applications. However, they may require modification, depending on soil types.

Compost specification for top soil:

The specification below sets out further requirements, in addition to BSI PAS 100, for the use of compost in this specific application:

  • pH units (1:5 water extract) of between 6.5– 8.7
  • Electrical conductivity (1:5 water extract) of 3000 µS/cm / 300 mS/m max
  • Moisture Content (% m/m of fresh weight) of between 35 – 55
  • Organic Matter Content (% dry weight basis) of more than 25
  • Particle Sizing (% m/m of air-dried sample passing the selected mesh aperture size) - 95% pass through 25mm screen and 90% pass through 10mm screen 
  • C:N Ratio   20:1 maximum 

How to use:

Analyse existing soil characteristics to determine the appropriate ratio of existing soil and imported materials (e.g., compost, soil) necessary to manufacture an acceptable soil. Identify a site location to complete on-site stockpiling of materials and blending activities.

Step 1

Begin the process by cultivating the soil using a one or three tine ripper, or equivalent, to a minimum depth of 300 mm (300 mm to 900 mm) in two directions obliquely, when ground conditions are reasonably dry. Strip this existing, loosened, soil to a depth adequate to obtain the appropriate volume and quality of subsoil needed for the project.

Step 2

Stockpile stripped (and imported) soils, and compost, at the identified location on the site, preferably, at or near the location for blending. Using a 360 degree excavator, front-end loader or equivalent, or customised blending plant, uniformly blend existing soil with compost (on a volume to volume basis [or equivalent weight basis].

Step 3

All materials must be thoroughly blended, but not ‘over mixed’ (not blended to the extent that soil structure is destroyed). Compost inclusion rates will be based on current soil conditions, expected use of the treated site (e.g., vegetation/plants to be established, intensity of activity on site, and the basic BS 3882 topsoil standard you are trying to meet). Also, blend in any imported soils necessary to meet the required soil grade or the textural requirements of the ‘finished’ manufactured topsoil.

Step 4

Subsoil by ripping the area, if required, and then spread the full quantity of manufactured topsoil and gently firm (do not compact manufactured topsoil with machinery). Manufactured topsoil should be applied to a minimum depth of 150 mm for areas to be seeded or turfed, and 300 mm for areas to be planted. Complete a minimum of two weeks prior to seeding, turfing or planting. Analyse manufactured soil.