It’s one of the most common gardener frustrations: two bags labelled “Multipurpose Compost” can behave completely differently. One drains perfectly; another stays soggy and lifeless. The reason lies in what “multipurpose” really means — and what it doesn’t.

1) “Multipurpose” is not a strict standard

Unlike PAS100 (for recycled green compost) or Soil Association certification (for organic composts), there’s no binding standard defining what goes into a bag labelled Multipurpose Compost  (MPC). Each manufacturer chooses their own recipe to meet a price point or market niche — so the label tells you purpose, not specification.

• Budget brands often use high proportions of green waste compost or coarse wood fibre to hit low cost targets.
• Premium brands may blend imported coir, bark, or digestate to achieve more consistent texture and nutrient balance.
• Specialist or “organic” brands avoid synthetic wetting agents and fertilisers but rely on natural feedstocks such as bracken, wool, or composted bark.

For more background on recognised standards, see WTB-C2: Compost Quality & Standards.

2) Feedstock and process differences

Every base ingredient brings its own fingerprint:

• Coir – light, consistent, but imported.
• Wood fibre – UK-sourced, renewable, but can tie up nitrogen.
• PAS100 compost – biologically active, but variable and often coarser.
• Digestate or bark fines – add structure or slow-release nutrients.

The ratios and maturity of these materials vary widely. Even small differences in moisture, particle size, or curing time affect aeration, drainage, and biological activity.

3) Fertiliser charge, pH, and wetting agents

Each supplier adds their own “secret sauce”: slow-release fertiliser, lime, or surfactants that control water behaviour. These additives strongly influence short-term performance but fade over time.

• Fertiliser charge: usually supports 4–6 weeks of growth.
• Wetting agents: help initial water absorption but may leach quickly.
• pH correction: essential if coir or bark are major ingredients.

That’s why two MPCs can both look dark and crumbly, yet one supports strong seedlings and the other doesn’t.

4) Batch variability and quality control

Even within a single brand, batches may differ seasonally. Supply shortages, changing waste streams, or moisture at packaging all create noticeable differences between spring and autumn production runs.

The most reliable producers maintain tight quality control and traceable feedstock sourcing — usually visible on their back-of-bag information or websites.

5) How to use variation to your advantage

• Choose lighter, coir-based blends for seed sowing and pots.
• Pick composts with PAS100 or digestate for soil improvement or raised beds.
• Mix and test small quantities before bulk purchase — consistency matters.

For comparative scores and ingredient transparency, visit our Reviews Section where we rate brands side-by-side using the same evaluation criteria.

Summary

MPC is a broad category, not a fixed formula. What’s inside each bag reflects brand priorities — price, performance, or sustainability. Understanding those differences helps you buy and use more confidently.