Potting Mix Calculator — Make Your Own Potting Compost

Homemade potting mix is often cheaper in bulk and lets you tailor the mix to specific plants. Cacti need more grit; seedlings need finer, lighter mixes.

Perlite is expanded volcanic glass that improves drainage and aeration. It prevents compaction and keeps roots healthy. Use 10-30% in most mixes.

Garden soil alone is too dense for containers. It compacts, drains poorly, and may contain weed seeds or diseases. Always mix with other materials.

John Innes is a soil-based compost recipe (loam, peat-free compost, and grit). The classic base is 7 parts loam to 3 parts peat-substitute to 2 parts coarse sand by loose volume, giving good structure and nutrient retention for longer-term plantings.

Epiphytic orchids like Phalaenopsis grow on tree branches in the wild, with roots exposed to air. Standard compost holds too much water around the roots and quickly rots them. A free-draining bark-led mix (roughly 60% medium orchid bark with perlite and grit or charcoal) keeps roots aerated, which is why this calculator outputs bark — not compost — for the orchid recipe.

Yes, refresh it by mixing in 30-50% new compost, add perlite for drainage, and a slow-release fertiliser. Discard any mix from pots where plants had disease.