Cannabis Soil Mix: Ingredients, Ratios, and Pot Size

The short answer

A cannabis soil mix is a potting medium that holds roots, water, air, and nutrients. A working blend has three parts: a base that holds water (peat moss or coco coir), an amendment that adds air (perlite or pumice), and a nutrient source (compost or worm castings). Target pH is 6.0 to 7.0. Roots need oxygen, so the mix stays loose after a full watering.

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Base ingredients and what they do

• Peat moss holds water and stays light. Its pH runs 3.5 to 4.5, so mixes add lime to lift pH.
• Coco coir holds water and air. pH runs 5.5 to 6.5. It comes from coconut husk fiber and rewets faster than dry peat.
• Perlite is expanded volcanic glass. It drains water and adds air space. Use 20% to 30% of volume.
• Pumice or rice hulls do the same job. Pumice weighs more and holds its shape longer.
• Compost or worm castings supply nitrogen, phosphorus, potassium, and microbes. Keep them at 10% to 20% of volume.
• Dolomite lime raises pH and adds calcium and magnesium. A common dose is 1 to 2 tablespoons per gallon of peat base.

Why aeration is the part people miss

Cannabis roots respire. Saturated soil pushes air out and plant growth slows. Perlite at 25% of volume leaves air space after watering. Let the top 2 to 3 inches (5 to 8 cm) of soil dry before the next water. If a pot holds water for a week, the mix is too dense.

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Pot size and root volume

Indoor plants finish in 3 to 5 gallon (11 to 19 L) pots. Outdoor plants take 10 to 30 gallon (38 to 114 L) pots. Root volume sets plant size. A 5 gallon pot supports a plant in the 2 to 4 foot range under lights. Size still depends on light, strain, and pruning. Repot before you flip to flower, because root growth slows during flower.

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Nutrients in the mix

Compost and worm castings feed a plant for 3 to 4 weeks. Cannabis uses more nitrogen during vegetative growth and more phosphorus and potassium during flower. Soil buffers mistakes: salts bind to organic matter instead of burning roots the way they do in water culture. Check runoff pH and ppm every 2 to 3 weeks.

How to water a soil mix

Water until runoff leaves the drain holes. Then stop. A 5 gallon pot of peat and perlite takes 1 to 1.5 gallons per watering in a warm room. Lift the pot to check weight. A light pot needs water. A heavy pot does not.

A mix recipe

1. Measure 3 parts peat moss or coco coir by volume.
2. Add 1 part perlite.
3. Add 1 part compost or worm castings.
4. Add dolomite lime at 1 tablespoon per gallon of base.
5. Blend on a tarp until the color is even. Wet the pile and let it sit 24 hours.

That recipe lands near 60% base, 20% air, and 20% organic matter. It drains in seconds and holds water for 2 to 3 days in a 5 gallon pot.

Bagged mixes

A bagged potting soil works if the label lists perlite and compost. Read the label for slow-release fertilizer. Many mixes add fertilizer beads that feed for 3 to 6 months, which makes nutrient control hard. A mix labeled for seedlings and cuttings holds less fertilizer. Soilless blends cost less per volume than cannabis-specific bags and use the same base ingredients.

Problems and fixes

• Yellow lower leaves: nitrogen runs low, or pH sits outside 6.0 to 7.0.
• Drooping leaves and wet soil: too little perlite, a pot that is too large, or water too often.
• Salt crust on the surface: fertilizer builds up. Flush with water at 2 to 3 times pot volume.
• Fungus gnats: wet topsoil. Let the surface dry between waterings.

Storage

Keep mixed soil in a bin with a lid and a few holes. Moist soil in a sealed bag grows mold. Dry soil in an open bag draws pests.