Compost vs soil

Compost and soil look superficially similar in the bag or on the ground. They serve some overlapping purposes. They are sold by the same suppliers. And yet they are fundamentally different materials with distinct properties, and treating them as interchangeable produces predictable failures across landscaping projects.

The most common mistake is using compost where soil is required – filling raised beds with pure compost, planting trees in pure compost, or replacing existing soil with compost in ground-level borders. The opposite mistake is rarer but equally problematic: using soil where compost is required, typically in containers where soil’s structural characteristics produce drainage and aeration problems that compost would avoid.

This guide explains the compositional differences between compost and soil, the practical consequences of those differences for landscaping projects across Kingston, Surbiton and Surrey, and the specific situations where each material is the correct choice. For broader context see our soil and compost hub, complete compost guide, and topsoil complete guide.

The fundamental compositional difference

Compost is decomposed organic matter – plant material, wood fibre, peat, coir, bark, or green waste broken down by microorganisms into a growing medium. Composition is approximately 90–98% organic matter with minimal mineral content.

Soil is a natural composite of mineral particles (sand, silt, clay) and organic matter. Composition varies by soil type but typically falls within 85–95% mineral particles and 3–8% organic matter. The mineral component provides permanent structure that does not decompose; the organic matter component is dynamic, decomposing over time and being replenished through natural processes.

This difference is not subtle. It determines the materials’ physical behaviour, longevity, drainage characteristics, water retention, nutrient supply, and biological activity. Treating them as equivalent because they are both “dirt-like” produces the failures that account for a significant proportion of disappointing landscaping outcomes.

How structure differs over time

The behaviour over time is the most consequential difference for landscaping. Soil maintains its structure indefinitely. The mineral particle skeleton – sand, silt, clay – does not break down at any meaningful timescale. Organic matter within soil cycles through decomposition and replenishment, but the underlying mineral structure persists.

Compost decomposes progressively. The organic matter that constitutes 90%+ of compost continues to break down after the compost is placed into use. Within 12 months, multi-purpose compost has lost a significant proportion of its structure. Within 24 months, structural collapse is typically advanced. After 36 months, what was originally compost has reduced to a dense, compacted material with poor drainage and limited pore space – the conditions in which most plants fail despite adequate feeding.

This decomposition trajectory has direct practical consequences. A raised bed filled with pure compost will require complete refilling within 3–4 years. A container of pure compost will require repotting within 12–24 months. A ground-level planting in pure compost will sink, compact, and require additional material annually to maintain levels.

Soil-based growing mediums avoid this trajectory. A raised bed filled with topsoil mixed 50:50 with compost retains its structure for decades, requiring only annual organic matter top-up to replace the decomposing compost component. A container filled with John Innes compost (which contains loam – sterilised topsoil) maintains structure for years rather than months. A ground-level planting in improved existing soil performs indefinitely.

How drainage and water retention differ

Compost provides excellent drainage and aeration when fresh, but these characteristics degrade as structure collapses. After 12–18 months, drainage in pure compost is typically poor – water sits on the surface, roots suffocate in anaerobic conditions, and plants decline despite apparent adequate watering.

Soil provides moderate drainage that varies by soil type. Sandy soils drain quickly and dry rapidly. Clay soils drain slowly and retain moisture. Loam (the ideal balanced soil) drains adequately while retaining sufficient moisture for plant requirements. Crucially, soil drainage characteristics remain stable over time. A loam soil that drains well today will continue to drain well in five years.

Water retention follows the same pattern. Compost retains substantial water initially but loses water-holding capacity as structure collapses. Soil retains water at levels appropriate to its composition (clay retains more than sand) and maintains this capacity over time.

For applications where drainage and water retention must be predictable over multiple years – raised beds, lawn establishment, levelling work – soil-based materials deliver reliable performance. For applications where short-term high performance matters and material replacement is acceptable – containers, seasonal bedding – compost works well.

How nutrient supply differs

Compost provides high initial nutrient charge that depletes over 6–8 weeks. Plants in fresh compost benefit from immediately available nutrients, but supplementary feeding is required from 6–8 weeks onwards. The nutrients in compost are largely in immediately available forms that wash through during watering – convenient for short-term high performance but unsustainable for permanent applications.

Soil provides moderate ongoing nutrient supply through cation exchange capacity (CEC), the soil’s ability to hold and release nutrients. Clay soils have particularly high CEC because clay particles carry negative electrical charges that hold positively charged nutrient ions (potassium, calcium, magnesium, ammonium). This nutrient buffer means soil-grown plants typically require less supplementary feeding than container-grown plants in compost.

For long-term plantings where minimal maintenance is preferred, soil-based growing mediums require less ongoing feeding than compost-based alternatives. For seasonal plantings where vigorous initial growth matters, compost’s immediate nutrient availability is the appropriate choice.

When to use compost

Compost is the correct material for several specific applications.

Containers and pots require compost because soil’s drainage characteristics produce poor performance in confined containers. The reduced volume in containers means soil-based mixes drain too slowly, retain too much water around roots, and suffocate plants. Compost (specifically containers compost like multi-purpose, or John Innes for permanent containers) is formulated for this restricted root environment.

Seasonal bedding benefits from compost’s immediate nutrient charge and good initial structure. The temporary nature of the planting matches compost’s temporary structural integrity. Replacing bedding compost annually is normal maintenance.

Seed sowing and propagation requires specialist compost formulations (seed compost, cutting compost) that provide fine texture and appropriate fertility for delicate young plants. Soil is too coarse and inconsistent for reliable seed germination.

Raised beds mixed 50:50 with topsoil use compost for its fertility and structure improvement contribution while topsoil provides permanent mineral structure. This combination delivers both immediate performance and long-term stability. See our compost vs topsoil guide for detailed raised bed specification.

When to use soil

Soil is the correct material for several specific applications.

Raised beds require topsoil for the permanent mineral structure that prevents settling and maintains drainage over years. Pure compost in raised beds settles 100–150mm within 2–3 years. Topsoil mixed 50:50 with compost maintains structure indefinitely.

Lawn establishment requires topsoil for the stable rooting medium that grass requires. Compost settles excessively under turf, creating uneven surfaces. Topsoil provides the consistent structure that produces level, durable lawns.

Ground-level borders benefit from improving existing soil rather than importing compost. Improving the existing soil with organic matter incorporation is more cost-effective and environmentally sound than importing growing media. The existing soil’s permanent structure, biological activity, and connection to the wider garden ecosystem outperforms imported compost in ground-level applications.

Tree planting requires existing soil or topsoil, not pure compost. Trees planted in pure compost establish poorly because the compost decomposes around the root system, leaving the tree unstable. Trees should be planted into prepared existing soil or topsoil-based blends, with compost incorporated as organic matter improvement rather than substitution.

Levelling and grading requires topsoil for the stability that prevents settling. Compost settles 20–40% as organic matter decomposes – unsuitable wherever final levels matter (path edges, patio borders, lawn boundaries).

The 50:50 blend that resolves most questions

For most raised bed and significant border applications, the question is not “compost or soil” but “what proportion of each.” The standard professional specification is 50:50 topsoil and compost.

This blend combines benefits of both materials. Topsoil provides permanent mineral structure that prevents settling and maintains drainage. Compost provides organic matter that supplies fertility, improves topsoil structure, and supports biological activity. The organic matter component decomposes gradually requiring periodic top-up; the mineral skeleton from topsoil remains functional indefinitely.

Initial costs are similar to pure compost but the blend avoids the expensive complete replacement that pure compost requires every 3–4 years. For raised beds particularly, the 50:50 specification is the difference between beds that perform for decades and beds that require rebuilding within 3–4 years.

Common compost-soil confusion failures

Three failures repeat across Surrey gardens with depressing frequency.

Filling raised beds with pure compost. Results in settling, drainage deterioration, and complete refilling within 3–4 years. Solution: use 50:50 topsoil and compost blend.

Improving ground-level borders with imported compost rather than soil conditioner. Expensive, inappropriate, and less effective than soil conditioner specifically formulated for in-ground improvement. See our soil conditioner vs compost guide.

Planting trees in pure compost. Produces poorly-established trees with stability problems as compost decomposes around the root ball. Solution: plant trees into prepared existing soil or topsoil-based blends, with compost incorporated as organic matter improvement only.

How Flourish specifies the right material

Material specification on Flourish Landscaping projects begins with understanding the application requirements: what the planting needs to achieve, over what timescale, with what maintenance commitment. For containers, John Innes No.3 for permanent specimens; multi-purpose for seasonal bedding. For raised beds, 50:50 topsoil and compost blend. For lawn establishment, BS3882 Premium topsoil at 100mm minimum depth. For ground-level border improvement, soil conditioner incorporated into existing soil rather than imported compost.

The wrong material specification is the single most common reason for landscaping projects requiring expensive remediation. Getting this decision right at the start – before plants are bought and before materials are delivered – prevents the problems that no amount of subsequent care can resolve.

Book a consultation with Flourish Landscaping for material specification advice.