What soil should I use for growing cannabis?

Q: What soil should I use for growing cannabis?

The best soil for growing cannabis indoors is a quality bagged potting mix, peat- or coco-based with compost and a light starter fertiliser, opened up with 20–35% perlite or pumice. Never use garden soil in pots. The starter charge feeds for 2–4 weeks; after that, feed at EC 0.8–1.6 mS/cm with water at pH 6.2–6.8.

Short answer

Use a quality bagged potting mix, peat- or coco-based with compost and a light starter fertiliser, and add 20–35% perlite or pumice. Don't use garden soil in pots. The starter charge feeds a plant for about 2–4 weeks; then feed at EC 0.8–1.6 mS/cm with water at pH 6.2–6.8. Supported for the container physics; brand choice matters far less than drainage.

Why

Indoor "soil" is really potting mix. It has three jobs: hold water, hold air and hold nutrients on its exchange sites. Compost and peat buffer pH, which is why soil forgives small mistakes.

Air matters as much as water. A dense mix stays saturated, roots run short of oxygen and the plant wilts in wet soil ^[1]. Perlite or pumice adds large pores that drain and refill with air.

Garden soil compacts in a container and can carry damping-off and root-rot organisms ^[2].

Organic or mineral feeding both work. Cannabis grew well in organic, coir-based media once nitrogen supply was high enough ^[3]; the plant mostly doesn't care where its ions come from.

The numbers

Item	Target
Base	Peat- or coco-based potting mix with compost
Perlite or pumice	20–35% of volume, mixed through
Starter charge lasts	About 2–4 weeks
Irrigation pH	6.2–6.8 (soil range 6.0–7.0)
Feed EC, once charge runs out	0.8–1.6 mS/cm (400–800 ppm500 / 560–1,120 ppm700)
Pot size, photoperiod	11–25 L (3–7 US gal)
Pot size, autoflower	11–15 L (3–4 US gal), planted directly

What to do

Buy a mix labelled for containers; avoid "moisture control" mixes with wetting gels.
Add 20–35% perlite or pumice and mix it through evenly.
Water with plain, pH-adjusted water while the starter charge lasts.
Start feeding at half the label rate when lower leaves begin to pale.
Check runoff EC and pH monthly; if runoff EC climbs well above input, do a root-zone flush with 2–3× the pot volume.

Common mistakes

Garden soil or topsoil in pots.
A hot, heavily fertilised mix for seedlings, which burns them.
Hydro-strength feed on top of a fertilised mix.
A gravel layer at the bottom instead of perlite through the mix.

Full article: Growing in soil
Answers: when soil runs out of nutrients · mixing soil and coco · super soil · reusing soil
Terms: potting mix · perlite

Potting mix is a manufactured container medium, usually peat or coco with perlite, lime and starter fertiliser, designed to drain and hold air better than garden soil.Perlite is a light, white, puffed volcanic glass added to growing mixes to increase drainage and air space; it is chemically inert and holds no nutrients.EC (electrical conductivity) measures how well a solution conducts electricity, a proxy for total dissolved salts; growers read it in mS/cm to gauge nutrient strength.mS/cm (millisiemens per centimetre) is the standard unit of EC for nutrient solutions; 1 mS/cm equals 1 dS/m and 1,000 µS/cm.pH measures how acidic or alkaline a solution is on a 0–14 scale; in growing it controls nutrient uptake, with soil best near 6.0–7.0 and hydro 5.5–6.5.Runoff is the water or nutrient solution that drains out of the bottom of a container after irrigation; growers test it to see what is happening in the root zone.Peat is partially decomposed sphagnum moss from bogs, the base of most potting mixes; it holds water and nutrients well but is acidic and needs lime.The collapse of seedlings or cuttings when soil-borne pathogens such as Pythium, Rhizoctonia solani and Fusarium rot the stem at the soil line.ppm500 is the TDS scale that converts EC to ppm with a factor of 0.5 (based on sodium chloride), so 1.0 mS/cm reads 500 ppm.ppm700 is the TDS scale that converts EC to ppm with a factor of 0.7 (the 442 natural-water standard), so 1.0 mS/cm reads 700 ppm.Pot size is the volume of medium a plant roots into; bigger pots give bigger plants within reason, but feeding frequency can let small pots punch well above their size.The daily length of light versus dark; photoperiod cannabis flowers when nights grow long enough, rather than at a fixed age.A day-neutral cannabis plant that starts flowering by age, usually a few weeks after germination, regardless of light schedule; the trait is mainly recessive.Running a large volume of low-EC water or mild solution through the medium to wash out accumulated salts; it treats the medium, not the plant, and suits any stage.A seedling is a young cannabis plant from emergence of the cotyledons until it has several sets of true leaves, typically the first 2-3 weeks.

Follow-up questions

Do I need to add perlite to potting soil?

Yes, add perlite or pumice to potting soil for cannabis, at about 20–35% of the volume. Most bagged mixes are made for small houseplants and hold too much water for a big, thirsty plant in a deep pot. Extra perlite raises air space so roots get oxygen between waterings. Mix it evenly through the whole pot, never as a bottom layer.

Can I use hydro nutrients in soil?

You can use hydro mineral nutrients in soil, but only at soil strength. Wait until the mix's starter charge runs out, usually 2–4 weeks, then start at half the label rate and feed at EC 0.8–1.6 mS/cm every second or third watering. Check runoff EC monthly, because a rising reading means salts are building up in the pot.

Can I use garden soil for cannabis in pots?

No, don't use garden soil in pots. Garden soil compacts in a container, drains poorly and starves roots of air, and it can carry pathogens that cause damping-off and root rot. Outdoors in the ground, native soil can work well once amended. Indoors, use a bagged potting mix with added perlite or pumice.

Sources

Raviv M, Lieth JH, Bar-Tal A (eds), 2019. Soilless Culture: Theory and Practice, 2nd ed. Elsevier (container media physics and air-filled porosity). doi.org/10.1016/C2015-0-01807-5

University of Minnesota Extension. How to prevent seedling damping off (sanitation; avoid garden soil in containers). extension.umn.edu/solve-problem/how-prevent-seedling-damping

Caplan D, Dixon M, Zheng Y, 2017. Optimal rate of organic fertilizer during the vegetative-stage for cannabis grown in two coir-based substrates. HortScience 52(9):1307–1312. doi.org/10.21273/HORTSCI12401-17