Compost vs topsoil

The question of whether to fill a raised bed with compost, topsoil, or a mixture is one of the most common in Surrey landscaping. It is also one most frequently answered wrongly. Pure compost in raised beds settles dramatically. Pure topsoil lacks the organic matter for optimal plant performance. The 50:50 blend that professional landscapers specify combines the benefits of both materials and is the standard correct answer for most situations.

Understanding why neither material alone is the right answer – and why the blend works – matters because the wrong specification at the start of a raised bed project produces problems that no amount of subsequent care can resolve. Within 2–3 years, pure compost settles 100–150mm requiring complete refilling. Pure topsoil produces plants that grow slowly and require more feeding than the blend would demand. The 50:50 specification avoids both problems.

This guide explains the differences between compost and topsoil, why the blend works, how to specify proportions for different applications, and how to calculate quantities for raised beds of different dimensions. For broader context see our complete compost guide and topsoil complete guide.

The compositional difference

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

Topsoil is natural soil excavated from the upper soil layer, typically the top 150–300mm where organic matter concentration is highest. Composition is 85–95% mineral particles (sand, silt, clay) and 3–8% organic matter.

This is not a small difference. It determines how each material behaves over time, how it supports plant growth, and what applications it suits.

How they behave differently over time

Topsoil maintains structure indefinitely. The mineral skeleton – sand, silt, clay particles – does not decompose at any meaningful timescale. The organic matter component cycles through decomposition and replenishment but the underlying structure persists. Topsoil placed today will provide essentially the same growing conditions in 20 years.

Compost decomposes progressively. The organic matter that constitutes 90%+ of compost continues breaking down after placement. Within 12 months, multi-purpose compost has lost a significant proportion of its structure. Within 24 months, structural collapse is typically advanced. Pure compost in raised beds settles 30–40% in the first growing season and continues settling in subsequent years. Within 3–4 years, pure compost-filled raised beds require complete refilling.

This decomposition trajectory means pure compost is fundamentally unsuited to permanent applications. It is excellent material for temporary uses (containers, seasonal bedding) where regular replacement is acceptable. It is wrong material for raised beds expecting multi-year performance.

How drainage and water retention differ

Compost provides excellent drainage and aeration when fresh, deteriorating as structure collapses. After 12–18 months pure compost in containers or raised beds typically shows poor drainage with water sitting on the surface and anaerobic conditions developing in the lower zones.

Topsoil drainage characteristics vary by soil type. Sandy topsoils drain quickly. Clay topsoils drain slowly. Loam topsoils drain at the optimal middle rate. These characteristics remain stable over years – topsoil that drains well today continues draining well.

The combined effect of these characteristics is that compost in raised beds initially performs well but deteriorates progressively. Topsoil in raised beds performs adequately initially and continues performing adequately for years. The blend combines compost’s initial high performance with topsoil’s sustained characteristics.

How fertility differs

Compost provides high initial nutrient charge depleting over 6–8 weeks. Excellent for vigorous initial growth, requires supplementary feeding for sustained performance.

Topsoil provides moderate ongoing nutrient supply through cation exchange capacity (CEC). The mineral particles, particularly clay, hold and release nutrients over time. Topsoil-grown plants require less supplementary feeding than compost-grown equivalents but produce less vigorous initial growth.

For raised beds, the blend provides high initial fertility from compost combined with sustained nutrient supply from topsoil. As compost decomposes and releases its nutrients, topsoil’s CEC captures and retains them for ongoing plant use.

The 50:50 raised bed specification

The professional standard specification for raised beds is 50:50 topsoil and compost by volume. This proportion delivers the optimal balance between immediate performance and long-term stability.

At 50:50, the topsoil component (50%) provides permanent mineral structure preventing significant settling and maintaining drainage characteristics over years. The compost component (50%) provides immediate fertility, improves the topsoil’s structure, supports biological activity, and supplies organic matter that gradually decomposes adding ongoing fertility.

The blend settles 5–10% in the first growing season as the compost component decomposes – significantly less than pure compost’s 30–40% – and minimal settling thereafter. Annual organic matter top-up (compost or soil conditioner spread as 50mm mulch) maintains organic matter content as the existing compost continues decomposing.

Initial costs are similar to pure compost. The blend avoids the expensive complete replacement that pure compost requires every 3–4 years. Over a 10-year period, the 50:50 blend is significantly more economical than repeated pure-compost refilling.

When proportions might vary

The 50:50 standard works for most situations but specific applications justify variations.

60:40 topsoil to compost suits raised beds for trees or shrubs that need substantial root structure stability. The higher topsoil proportion provides better anchoring while the 40% compost still delivers adequate fertility.

40:60 topsoil to compost suits raised beds for vegetable growing where higher fertility for crop production justifies accepting more settling. The 40% topsoil still prevents the dramatic settling of pure compost while the 60% compost provides the high fertility vegetables benefit from.

30:70 topsoil to compost suits seasonal cut flower beds where the temporary nature of the planting and the demand for high fertility justify the more compost-heavy specification. Accept that the bed will require significant top-up or refilling within 5 years.

The standard 50:50 remains the appropriate default for permanent ornamental raised beds combining shrubs, perennials, and seasonal interest.

Calculating quantities for raised beds

The formula is straightforward: Length (m) × Width (m) × Depth (m) × 0.5 = Cubic metres of each material required for 50:50 specification.

Worked examples for typical Surrey raised bed dimensions:

Small raised bed 2m × 1m × 0.4m deep: Volume 0.8 cubic metres total. 0.4 cubic metres topsoil and 0.4 cubic metres compost.

Standard raised bed 3m × 1.2m × 0.4m deep: Volume 1.44 cubic metres total. 0.72 cubic metres topsoil and 0.72 cubic metres compost.

Large raised bed 4m × 2m × 0.45m deep: Volume 3.6 cubic metres total. 1.8 cubic metres topsoil and 1.8 cubic metres compost.

Long border raised bed 6m × 1m × 0.4m deep: Volume 2.4 cubic metres total. 1.2 cubic metres topsoil and 1.2 cubic metres compost.

Bulk bags typically contain 0.75–1 cubic metre per bag (always confirm with supplier). For a standard 3m × 1.2m × 0.4m bed, that’s approximately one bulk bag of topsoil and one bulk bag of compost.

Order 10% additional material to allow for settling in the first growing season. Topsoil settles approximately 5–10%; the compost component settles more. The additional material allows top-up to maintain levels.

Where to buy compost and topsoil in Surrey

For bulk supply suited to raised bed filling, several Surrey suppliers deliver reliable products. Bourne Amenity provides BS3882 certified topsoil and quality compost with delivery across Surrey. Bury Hill Landscape Supplies offers professional-grade materials including pre-blended topsoil and compost mixes. Topsoil Direct is a Surrey-based supplier with online ordering and reliable delivery.

Several suppliers offer pre-blended topsoil and compost mixes, which save the labour of mixing on site. Confirm the blend proportion when ordering – “topsoil and compost mix” can mean anything from 90:10 to 30:70 depending on the supplier. For the standard 50:50 specification, specify explicitly.

For domestic-scale projects (single small raised bed), bagged compost from garden centres combined with a single bulk bag of topsoil delivered to site provides a practical solution. For larger projects, bulk delivery of both materials is significantly more economical than bagged purchases.

How Flourish specifies raised bed materials

Raised bed construction is a core part of Flourish Landscaping projects across Kingston, Surbiton, and Surrey. Our standard specification for permanent ornamental raised beds is 50:50 BS3882 Premium topsoil and quality multi-purpose compost. For vegetable growing beds we shift to 40:60 to provide higher fertility. For tree planting in raised beds we shift to 60:40 for better root anchoring.

Material sourcing is from established Surrey suppliers with consistent quality. We test compost quality before delivery to large projects and specify BS3882 certification for all topsoil orders. The material specification is integrated into the construction phase rather than treated as an afterthought.

For client projects we provide the material specification alongside the construction quote, ensuring transparency about what is being delivered and why. The 50:50 blend specification is non-negotiable for permanent raised beds – the alternative is rebuilding the bed within 3–4 years.

Book a consultation with Flourish Landscaping for raised bed design and construction.