This methodology note explains how we test and interpret the compost samples used in our comparative reviews. Tests are conducted by hand using consistent equipment and procedure across all products in each group.
These tests are designed to support practical gardener-facing reviews. They are not formal laboratory trials, and they are based on one purchased bag per product. However, by using the same method across all samples, they provide a useful comparison of structure, moisture behaviour, particle size and likely use case.
For the full science explanation of compost water behaviour, see the HealthySoil authority article:
Water holding, water retention and available water: why compost tests need all three
This HealthySoil article provides the main technical framework. This page explains how we apply that framework in our compost reviews.
Bulk density (BD)
Bulk density measures how much a compost weighs relative to its volume. It is a useful proxy for physical structure and likely behaviour in use.
Why it matters
Light, low-density composts are typically fibre-dominant, often containing wood fibre, coir or bark-based fractions. Heavier composts tend to contain more mineral material, humified organic matter, composted green waste, loam or dense fine fractions.
Density affects:
- handling weight;
- pot stability;
- drainage;
- aeration;
- likely shrinkage;
- moisture buffering;
- and how a product behaves in pots, borders, baskets or raised beds.
A very light compost is not automatically poor, and a heavy compost is not automatically good. Bulk density is best interpreted alongside particle size, water behaviour and ingredient list.
Procedure
A fixed volume of compost is taken from the bag, weighed as received, then dried completely and weighed again.
We report two figures:
- BD as received — weight per ml of fresh material, including moisture.
- BD dry — weight per ml after drying, giving a better indication of structural density.
Dry BD removes much of the moisture variation between bags and gives a more comparable baseline across products.
Results are expressed in g/ml.
Indicative dry BD ranges
| Range | Typical character |
|---|---|
| Below 0.15 g/ml | Very light — usually high fibre content |
| 0.15–0.25 g/ml | Light to mid-range — typical peat-free organic products |
| 0.25–0.35 g/ml | Mid-weight — mixed, composted or well-humified material |
| Above 0.35 g/ml | Heavy — likely mineral content, loam, dense compost fraction or high moisture-buffering fines |
Water behaviour: water held, water retained and water available
Water holding capacity and water retention are related, but they are not the same. In addition, neither tells the whole story unless we also consider plant-available water.
This site therefore uses the HealthySoil three-part water model:
- Water held — how much water the compost holds after saturation and drainage.
- Water retained — how much of that water remains after exposure, surface drying and evaporation.
- Water available — how much of the retained water roots can actually use before the plant becomes stressed or wilts.
Why it matters
A compost can score well on water holding capacity and still dry too quickly in an exposed basket. Equally, a compost can hold less total water but keep plants going for longer if it loses water slowly and keeps enough of the retained moisture available to roots.
This is why our reviews do not treat WHC as a single winning score.
Instead, water behaviour is interpreted alongside:
- bulk density;
- particle size distribution;
- fibre and fines balance;
- visible structure;
- rewetting behaviour;
- likely air-filled porosity;
- drainage behaviour;
- and intended use case.
A good multipurpose compost should not simply hold water. It should hold enough water, retain enough of it between watering events, and still leave enough air around roots.
Water held: procedure
A weighed dry sample is saturated with water, allowed to drain freely for a standardised period, then weighed again.
This gives a practical measure of water held after drainage. It is broadly comparable to a simple water holding capacity test, but we avoid treating it as a complete prediction of growing performance.
The result is expressed as a multiplier of dry weight.
For example:
3.5× means the saturated, drained sample holds 3.5 times its own dry weight in water.
Water retained: procedure
We also record practical dry-down behaviour.
After saturation and drainage, the sample is observed under consistent ambient conditions to assess how quickly the surface becomes dry-feeling and how the structure changes during drying.
This gives a practical indication of water retained during exposure, which the WHC multiplier alone does not capture.
This is especially relevant for:
- hanging baskets;
- containers;
- shallow pots;
- exposed patios;
- raised-bed surfaces;
- and peat-free mixes with open fibre structures.
Water available: interpretation
We do not directly measure plant-available water in a formal laboratory sense. That would require more advanced testing across water tension ranges.
However, we interpret likely plant-available water from a combination of practical indicators:
- how much water the sample holds after drainage;
- how quickly the surface dries;
- whether the structure is open and fibrous or fine and moisture-buffering;
- whether the compost appears likely to drain too freely;
- whether it may compact and reduce air around roots;
- whether it rewets after drying;
- and whether the ingredient list suggests a seed, pot, basket, container, raised bed or soil-improver use case.
This means our reviews use available water as a practical interpretation, not as a formal laboratory value.
Indicative water held ranges
The older table called this WHC. The revised wording is clearer: these ranges describe water held after saturation and drainage, expressed as a multiplier of dry weight.
| Range | Typical character |
|---|---|
| Below 2.0× | Low water held — often mineral-heavy, coarse-screened or free-draining material |
| 2.0–3.5× | Mid-range water held — typical for many mixed peat-free products |
| 3.5–5.0× | High water held — often fibre-rich, fine-fraction-rich or wetting-agent-assisted |
| Above 5.0× | Very high water held — may indicate wetting agent, very absorbent fibre, high fines, or unusual formulation |
Important caution
A high water-held figure is not automatically better.
It may mean the product has a useful reservoir. However, it may also mean the product relies heavily on absorbent fibre, wetting agents or fine material that behaves differently after repeated wetting and drying.
Therefore, water-held results should always be interpreted with the dry-down observation, sieve profile, bulk density and product use case.
Particle size distribution (sieving)
Sieving separates a compost sample into size fractions to characterise its texture and identify the dominant material type.
Why it matters
Particle size often explains compost behaviour more directly than any single headline claim.
Fine fractions can:
- hold moisture;
- improve root contact;
- support nutrient retention;
- and suit seed trays or propagation mixes.
Coarser fractions can:
- improve aeration;
- increase drainage;
- reduce compaction;
- and indicate wood fibre, bark or incompletely screened material.
However, balance matters. Too much fine material can reduce air-filled porosity or increase compaction risk. Too much coarse fibre can produce a mix that dries rapidly, sheds water unevenly, or lacks root contact.
Procedure
A sample is passed as received through two sieves in sequence:
- 10 mm sieve
- 2 mm sieve
This produces three fractions:
- Above 10 mm — coarse material, such as woody fragments, large fibre, bark pieces or unscreened debris.
- 2–10 mm — mid fraction, often the dominant fraction in peat-free multipurpose products.
- Below 2 mm — fine fraction, including humified compost, mineral particles, fine coir, fine wood fibre and other small particles.
Each fraction is weighed and expressed as a percentage of the total sample weight.
Where a product is notably wet at the time of sieving, fine particles can clump and be retained on the 2 mm sieve rather than passing through. We note this on the data table because it means the sub-2 mm figure may be understated for that product.
Indicative sieve profiles
| Profile character | Typical pattern |
|---|---|
| Fine-dominant | High below 2 mm, low above 10 mm — often seed, propagation or fine potting grade |
| Balanced | Moderate across all three fractions — often best for general-purpose use |
| Coarse-dominant | High 2–10 mm, low below 2 mm — common in wood fibre or coarse peat-free products |
| Poorly screened | Elevated above 10 mm fraction — may include woody fragments, stones, plastics or oversized material |
Ingredient and label review
Where available, we also record the declared ingredient list from the bag or manufacturer website.
Why it matters
Ingredient lists help explain the test results. For example:
- coir and wood fibre often explain low bulk density and high visible fibre;
- composted green waste may explain higher density, darker colour and more fine material;
- digestate or PAS100-type material may explain higher nutrient load or stronger smell;
- loam or mineral fractions may explain weight and improved moisture buffering;
- wetting agents may explain unusually high water uptake;
- added fertiliser affects likely early plant performance but not necessarily long-term structure.
The label also tells us what the product is intended for. Many products sold as “multipurpose compost” are technically better understood as growing media for seeds, pots, containers or young plants, rather than true soil-improving composts.
That distinction matters throughout our reviews.
Personal handling observations
Each review includes direct handling observations from opening and testing the bag.
These are not laboratory measurements, but they are important because compost is a practical product. Gardeners notice smell, feel, visible fibre, clumping, stones, plastic fragments, white fungal growth, wetness, dryness and ease of handling.
We record observations such as:
- initial smell;
- visible fibre or woody material;
- fine versus coarse feel;
- visible contaminants;
- stones or grit;
- plastic fragments;
- white fungal growth;
- clumping;
- wetness or dryness on opening;
- ease of wetting;
- and whether the product looks suited to its claimed use.
These field notes help give the reviews a practical voice and can explain why two products with similar measured results may feel very different in use.
Notes on comparability
All measurements are taken from a single bag per product. Natural variation exists between bags and production batches, particularly for green waste, digestate-based and certified compost products.
Results represent the sample tested, not a statistical average across multiple bags.
Tests were conducted under consistent ambient conditions. As-received moisture content varies between products and affects BD-as-received figures. Dry BD and water-held figures are normalised for moisture and are therefore more reliable comparison points.
However, even normalised values are not complete performance predictions. Compost behaviour depends on the full matrix, including structure, particle size, fines, fibre, ingredients, wetting agents, maturity and use case.
Price-per-litre figures in reviews are calculated from the retail price and stated volume at the time of testing. They may not reflect current pricing.
How to read our review scores
Our reviews are designed to help gardeners choose products for real use, not simply to rank laboratory numbers.
A high score usually reflects a good balance between:
- appropriate structure;
- acceptable screening;
- useful moisture behaviour;
- practical handling;
- ingredient transparency;
- price per litre;
- and suitability for the claimed use.
A product can perform well in one use case and poorly in another. For example, a fine seed compost is not expected to behave like a basket mix. A raised-bed mix is not expected to behave like a seed tray medium. A soil improver is not the same as a multipurpose growing medium.
Therefore, each review should be read in context.
Short reusable water note for individual reviews
Use this note, or a shortened version of it, in the individual compost reviews:
Water behaviour note: In these reviews, we separate water held, water retained and water available to roots. A compost can hold water but still dry quickly, or retain water that roots cannot easily use. See our compost testing methodology for how we interpret water behaviour.
Summary
Our compost testing method is deliberately practical.
We measure bulk density, water held after drainage and particle size distribution. We also record dry-down behaviour, ingredient information and direct handling observations.
The most important water-method change is this:
We do not treat WHC as a stand-alone performance score. Compost water behaviour is interpreted through water held, water retained and water available to roots.
That approach is more realistic for baskets, containers, raised beds and peat-free growing media.
