Indoor Cannabis Soil Mix Recipe: Peat, Perlite, Compost

A base indoor cannabis soil mix uses 1 part peat moss or coco coir, 1 part perlite, and 1 part mature compost by volume. Add 1 to 2 tablespoons of dolomite lime per gallon of mix. Target pH in the root zone is 6.0 to 7.0.

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The mix holds about 25 to 30 percent air space after watering. That number drives root health. Roots need oxygen. A compacted mix starves roots and slows growth.

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The base recipe

Measure by volume. A scoop is any container that holds the same amount each time.

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• 1 part peat moss or coco coir. Peat holds water and has a pH of 3.0 to 4.5. Coir holds water and drains faster than peat.
• 1 part perlite. Perlite is volcanic glass heated to 1,600 F. It expands and holds air.
• 1 part worm castings or screened compost. This is the nutrient base.
• 1 to 2 tablespoons dolomite lime per gallon. Lime raises pH and adds calcium and magnesium.

Mixing steps

1. Put on a dust mask. Perlite dust irritates lungs.
2. Dump peat or coir into a tub that holds twice the final volume.
3. Add perlite and compost.
4. Sprinkle dolomite lime on top.
5. Turn the pile with a trowel or gloved hands for 3 minutes.
6. Add water in 1 cup amounts until a squeezed handful holds shape and releases one drop.
7. Cover the tub and let it sit 24 hours. Lime needs time to react with peat.
8. Fill pots to 1 inch below the rim.

Indoor adjustments

Indoors there is no rain to leach salts. Salt from fertilizer builds in the pot.

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• Water with plain pH 6.0 to 6.5 water every 3 to 4 weeks. Pour 1 pot volume of water through the pot. This is a flush.
• Keep runoff at 10 to 20 percent of the water you pour.
• Use 3 to 5 gallon pots for plants vegged 4 weeks. Use 7 gallon pots for longer veg.
• Keep air at 68 to 78 F and humidity at 40 to 60 percent in veg.

Amendments and rates

Per gallon of base mix:

• Blood meal: 1 tablespoon. Nitrogen.
• Bone meal: 2 tablespoons. Phosphorus and calcium.
• Kelp meal: 1 tablespoon. Potassium and trace elements.
• Epsom salt: 1 teaspoon. Magnesium and sulfur. Use it if leaves show yellow between veins.

Coir holds calcium and magnesium. Add 1 teaspoon gypsum and 1 teaspoon Epsom salt per gallon when coir is the base.

Common errors

• Compost that is not mature. It heats in the pot and burns roots.
• pH below 6.0. Calcium and magnesium drop out of reach.
• Reusing mix for a third cycle without new lime or compost.
• Skipping the 24 hour rest. Lime does not blend into peat in 5 minutes.