Even within the same garden centre, two bags labelled “compost” can behave very differently. Here are the six most common reasons.
The wrong product for the job
A key distinction is between PAS100 compost and Multipurpose Compost (MPC).
- PAS100 compost is a recycled soil improver, not a precision growing medium.
- Multipurpose Compost, in contrast, is formulated for pots and seed trays.
Confusing these two markets — or mixing bulk PAS100 feedstock into retail blends — often leads to unpredictable results.
Input material quality
Most UK compost is based on green waste from councils or landscapers. The feedstock mix (grass, hedge trimmings, leaves) changes seasonally, altering texture, nutrient balance, and smell.
Some suppliers supplement with food waste, bark, or loam — each shifts performance. Poorly sorted inputs can also carry plastic, glass, or metal fragments.
Incomplete processing
Even certified material can vary in stability and humification. Compost leaving a site too early may still contain woody fragments or active decay, leading to nitrogen drawdown or later heating in bags.
See also “Wood-Rich Composts: The Hidden Variable” in our What’s in the Bag section.
Contamination and screening issues
When trommels or magnets fail in production, whole runs can escape QA checks — sometimes up to 10,000 bags before detection.
These faults introduce oversize wood, stones, or plastic, which buyers understandably interpret as “poor quality compost.”
Storage and re-bagging
Moisture content, fungal growth, and odour can change if bags sit outdoors for months. Bulk material is sometimes re-bagged under contract, meaning the brand on the front may not match the site that made it.
Inconsistent formulations
Retail MPCs are lightly standardised at best. Each brand tweaks ingredients (peat-free fibres, digestate, coir, bark fines, composted green waste) to hit price or performance targets.
This “recipe drift” explains why the same product name can look or feel different from season to season.
Summary
Variation in compost isn’t random — it reflects different goals, feedstocks, and quality-control systems.
- Use Multipurpose Compost for containers and pots.
- Use PAS100 compost only as a soil improver.
- Buy from reputable suppliers who publish feedstock details and batch QA data.
Disclaimer.
Brand names such as Westland, SylvaGrow, RocketGro, Dalefoot, Carbon Gold, and others mentioned on this site are registered trademarks of their respective owners. MultipurposeCompost.co.uk includes these examples for informational and comparative purposes only and does not claim endorsement, affiliation, or suitability for any specific use. Gardeners and buyers should always check current product specifications and manufacturer guidance before purchase or application.
