TL;DR:

If your compost has gone hard and water just runs off, don’t panic. Break it up, add water slowly in stages, and give it time to re-absorb. For severely dried compost, the only reliable fix is either a commercial wetting agent or a natural humus suspension.

Why Compost Dries Out

Bagged Multipurpose Compost (MPC) can become hydrophobic if stored open, exposed to heat, or left unwatered. Peat-free blends containing coir, wood fibre, or PAS100 green waste compost are especially prone.

When the fine organic particles dry beyond a threshold, they lose their surface polarity — water beads up and drains away instead of soaking in.

Step-by-Step: Re-Wetting Dried Compost

Option 1 — Soak Method

• Break up clumps gently.
• Stand the pot or compost batch in a tray of water 2–4 cm deep.
• Leave several hours until evenly moist, then drain.

Avoid submerging delicate roots for too long — lack of oxygen can stress plants.

Option 2 — Layer-and-Mix

• Tip compost into a wheelbarrow or tub.
• Sprinkle water across each layer as you mix.
• Wait 10–15 minutes, then remix.
• Stop when compost holds together when squeezed but does not drip.

Option 3 — Use a Wetting Agent or Humus Suspension

Commercial wetting agents reduce surface tension so water can re-enter the dry organic matrix — but they’re rarely sold in garden-centre sizes.

Tony Callaghan’s natural alternative: use a liquid humus suspension — a dark, colloidal extract of mature compost or biochar-humus blend — which re-introduces organic acids and polar compounds that “re-open” hydrophobic material.

How to Make and Use a Natural Humus Suspension – Video & FAQ →

Tips for Prevention

• Keep bags sealed and shaded.
• Avoid letting pots dry completely between waterings.
• Mulch surfaces to reduce evaporation.
• Select MPCs with coir or wood-fibre bases — they re-wet more easily than PAS100 composts.

Why It Matters for Biological Activity

Re-wetting does not “revive microbes” in standard bagged MPC — most are microbially inactive by design. However, restoring moisture does re-activate nutrient release and root access.

If you wish to re-introduce biology, you can inoculate with home compost, worm castings, or a small dose of humus suspension.