Quick answer
A homemade cannabis potting mix uses three parts by volume: one part sphagnum peat moss or coco coir, one part aeration, and one part compost or worm castings. Add minerals and lime for pH and plant food. A 30-gallon batch takes 10 gallons of each part. Water the mix to field capacity, then let it sit in a bin for 2 to 4 weeks before planting. Cannabis in soil grows at pH 6.0 to 7.0.
Soil Amendments for Cannabis Grow: Top Picks and Comparison
Base: peat moss or coco coir
Peat moss is the standard base. It holds water, holds nutrients, and has a pH of 3.5 to 4.5. That low pH needs lime. Coco coir is a substitute. Coir drains faster than peat and has a pH of 5.5 to 6.5. Rinse coir that came from a saltwater source. Both materials compress, so break up clumps before you mix.
homemade cannabis potting soil
Aeration
Aeration supplies oxygen to roots and stops the mix from turning into mud. Perlite is expanded volcanic glass. It weighs about 6 to 8 pounds per cubic foot. Pumice is heavier and lasts longer. Rice hulls break down in about a year. Use one of these at one third of the total volume. Do not pack the mix into the pot. Fill the pot, then water to settle the mix.
homemade cannabis potting soil
Compost and worm castings
Compost supplies organic matter, microbes, and slow-release nutrients. Worm castings cost more and carry more microbial life. Keep compost at 20 percent to 30 percent of the total volume. More than that makes the mix dense and can burn young plants. Compost must be finished. Unfinished compost heats up in the pot and pulls oxygen from roots.
Minerals and nutrients
Lime buffers pH and adds calcium and magnesium. Dolomite lime is the common choice. Gypsum adds calcium and sulfur without raising pH. Oyster shell flour breaks down over months. Kelp meal, neem meal, and crab meal feed soil microbes. Rock phosphate and bone meal supply phosphorus. Feather meal and blood meal supply nitrogen.
Recipe per cubic foot of base mix
One cubic foot is 7.5 gallons. Mix the base first, then add:
- 1 cup dolomite lime
- 1/2 cup gypsum
- 1/2 cup oyster shell flour
- 1 cup kelp meal
- 1/2 cup neem meal or crab meal
- 1 cup rock phosphate or bone meal
- 1/4 cup feather meal (optional)
Scale the numbers to your batch size. A 30-gallon batch holds 4 cubic feet, so use 4 cups of lime.
pH and water
Target pH is 6.0 to 7.0. Below 6.0, calcium and magnesium drop out of reach. Above 7.0, iron and manganese drop out of reach. Water the mix until a handful drips one or two drops when you squeeze it. That is field capacity.
Charge and rest
Put the mix in a bin or tote. Cover it. Turn it once a week for 2 to 4 weeks. Microbes break down the meals during this rest period. A mix that goes straight into a pot can run hot and damage seedlings.
What to leave out
- Garden soil. It compacts in pots and carries pests and weed seed.
- Unfinished manure. It burns roots and holds salts.
- Sand. It adds weight and does not improve drainage in a pot.
- Slow-release fertilizer beads. They release salts on a schedule you cannot control.
Perlite dust irritates lungs. Wear a mask when you pour it.
Watering and feeding
Water when the top inch of the mix is dry. A 5-gallon pot in flower can take 1 to 2 gallons per watering. The mix feeds plants for 3 to 5 weeks. After that, top dress with compost or worm castings at 1 cup per 5 gallons of soil, or water with compost tea every 2 weeks.
Legal note
Cannabis cultivation rules differ by state. Check your state law before you plant.