The short answer
Build the base from three equal parts by volume: peat moss or coco coir, an aeration material like perlite or pumice, and compost or worm castings. That mix drains fast, holds moisture, and gives roots something to eat. For a water-only grow, add dry amendments at about 1 to 2 cups per cubic foot, moisten the pile, and let it sit for two to four weeks before a plant touches it.
what soil to use for cannabis plants
The three-part base
Get this ratio right and most other problems stay small.
cannabis soil mixture recipe
- Peat or coco, one third. Peat is acidic and holds water well. Coco is closer to neutral and drains a touch faster. Either works. Coco needs a calcium and magnesium source because it binds both.
- Aeration, one third. Perlite is cheap and everywhere. Pumice lasts longer and does not float to the top. Rice hulls break down over a season and add silica as they go.
- Compost or worm castings, one third. This is the engine of the mix. Buy bagged castings from a shop you trust, or screen your own. Chunky, unfinished compost will steal nitrogen while it breaks down.
If you are mixing for seedlings, cut the compost back to a quarter and skip the amendments. Seedlings burn easy. Start them in a light starter mix, then move them into the hot stuff once they have three or four real nodes.
cannabis soil mixture recipe
Amendments by the cubic foot
One cubic foot is about 7.5 gallons, which fills a standard 7-gallon pot with a little left over. My standard set looks like this.
- Kelp meal, half a cup. Feeds the microbes more than the plant, and adds potassium over time.
- Neem meal or karanja cake, half a cup. Slow nitrogen plus mild pest pressure on fungus gnats and root aphids.
- Crustacean meal or oyster shell flour, half a cup. Calcium and chitin.
- Rock phosphate, half a cup. Phosphorus that releases across the whole run.
- Gypsum, half a cup. Calcium and sulfur with no pH shift.
- Dolomite lime, one cup in a peat mix. Skip it with coco, since coco does not run acidic the way peat does.
- Greensand, one cup, if you want a slow potassium and trace mineral source.
Blood meal and bone meal work, but they smell, they draw animals, and they hit hard and fast. Save those for a top dress in flower if you need a push. Bat guano sits in the middle, faster than rock phosphate and gentler than bone meal.
Let it cook, and cook it long enough
Mix everything on a tarp, add water until a handful holds together without dripping, and put it in a bin or tote. Turn it every few days for the first two weeks. The pile will warm up as microbes wake and start breaking down the meals. That heat is the point. Fresh, uncooked amendments are too concentrated and can burn roots.
Two weeks is the floor in warm weather. Four weeks is better. If the pile smells sour or like ammonia, it went anaerobic, so turn it and back off the water.
pH and watering
Target soil pH between 6.0 and 7.0. Most nutrients stay available in that window, and the lime or gypsum handles the drift. Water going in should land around 6.2 to 6.8. Do not chase runoff numbers in an amended organic mix. It lies.
Water on a wet-dry cycle. Soak the pot until a little runs out, then wait until the top inch or two dries and the pot feels light. Overwatering drowns the microbial life you just spent a month feeding, and it invites fungus gnats.
Container size and reuse
For water-only runs, give each plant 10 to 15 gallons. Smaller pots work if you plan to feed liquid or top dress in flower. After harvest, pull the root ball, add a fresh cup of compost and half the amendment list per cubic foot, and the mix can run again. Reused soil usually performs better the second time around once the biology is established.
Mistakes that cost a run
- Digging soil from the yard. It compacts, drains poorly, and often carries pests and weed seed.
- Doubling the lime. Too much magnesium locks out potassium and calcium, and the plant stalls in mid flower.
- Planting straight into a fresh mix. Give it the cook time, or start the plant in a pocket of plain base mix in the middle of the pot.
- Skipping aeration because the compost looks fluffy. It never stays fluffy in a pot.