Can I use regular garden fertiliser on cannabis?

Q: Can I use regular garden fertiliser on cannabis?

Yes, regular garden fertiliser works on cannabis in soil, because roots take up the same ions whatever the label says. Pick a complete feed with calcium, magnesium and micronutrients, and dose by element: aim for roughly 150-200 mg/L nitrogen in veg and 30-60 mg/L phosphorus. All-purpose feeds suit veg, tomato feeds suit flower. Avoid lawn feeds with herbicides, and use fully soluble salts in hydro.

Short answer

Yes, in soil. Cannabis roots take up nitrate, phosphate, potassium and the rest exactly as any crop does, so a complete garden fertiliser works if you dose it by element. All-purpose feeds suit veg, potassium-rich tomato feeds suit flower. Avoid weed-and-feed lawn products, and use fully soluble, complete salts in hydro. Supported.

Why

Plants absorb ions, not brands ^[1]. A "cannabis" bottle is a blend of the same salts sold for tomatoes and houseplants.

What matters is the analysis. NPK percentages convert to dose: 1 g/L gives the element percentage × 10 in mg/L, after converting P2O5 × 0.44 and K2O × 0.83 ^[2].

Nitrogen is the main lever in veg. In a controlled cannabis trial, vegetative growth peaked at about 160 mg/L nitrogen ^[3]. Phosphorus has a ceiling: elevated root-zone phosphorus did not improve yield or quality ^[4], so a high-P "bloom" garden feed adds nothing.

Garden feeds often skip calcium, magnesium or micros. Soft water or RO then leaves a gap.

The numbers

Product type	Typical use	Watch for
Complete soluble all-purpose (e.g. 20-20-20 + micros)	Veg in soil	High P relative to need
Tomato feed (K-rich)	Flower in soil	Low N for veg; check Mg
Controlled-release pellets	Soil, outdoor pots	Can't taper or adjust
Lawn feed	Avoid	Herbicides, very high N
Agricultural-grade granular salts	Soil only	Insoluble fillers in hydro

Targets: N ~150–200 mg/L in veg, P 30–60 mg/L, soil feed EC 0.8–1.6 mS/cm.

What to do

Read the label for Ca, Mg and micronutrients, not just NPK.
Convert to mg/L and compare with stage targets.
Start at half the label rate in soil and check runoff EC.
Switch from an all-purpose to a K-rich feed in flower.
In hydro and coco, use complete fully soluble feeds only.

Common mistakes

Using weed-and-feed lawn fertiliser.
Assuming a cannabis label means better chemistry.
Doubling up a garden feed on fertilised potting mix.
Granular field fertiliser in a hydro reservoir.

Full article: Building a cannabis feed by stage
Answers: what NPK means · which nutrients to buy · kitchen scraps
Articles: organic vs mineral
Terms: mineral fertiliser · NPK ratio

Follow-up questions

Is tomato feed good for cannabis in flower?

Tomato feed is a reasonable flowering feed for cannabis in soil. It is usually potassium-rich with moderate nitrogen, which suits flower. Check the label for magnesium and micronutrients, dose to about EC 0.8-1.6 mS/cm in soil, and add a calcium source if your water is soft. It is too low in nitrogen for veg.

Can I use slow-release fertiliser pellets on cannabis?

Controlled-release fertiliser pellets work for cannabis in soil and outdoor containers, mixed into the potting mix at planting at the label rate for container plants. They release nutrients with temperature and moisture, so you can't taper or adjust them quickly. Avoid them in coco, rockwool and hydro, where you need precise, adjustable feeding at every irrigation.

Why shouldn't I use lawn fertiliser on cannabis?

Avoid lawn fertiliser on cannabis because many products are weed-and-feed mixes containing broadleaf herbicides, which cannabis is very sensitive to. Plain lawn feeds are also very high in nitrogen, often with little phosphorus, magnesium or micronutrients, and some use urea or ammonium heavily. A complete general-purpose or tomato feed is safer.

Sources

Marschner P (ed.), 2012. Marschner's Mineral Nutrition of Higher Plants, 3rd ed. Academic Press. doi.org/10.1016/C2009-0-63043-9

Waltz C, 2003. Know the numbers on a fertilizer bag. University of Georgia CAES Field Report. fieldreport.caes.uga.edu/?p=19743

Saloner A, Bernstein N, 2020. Response of medical cannabis (Cannabis sativa L.) to nitrogen supply under long photoperiod. Frontiers in Plant Science 11:572293. doi.org/10.3389/fpls.2020.572293

Westmoreland FM, Bugbee B, 2022. Sustainable cannabis nutrition: elevated root-zone phosphorus significantly increases leachate P and does not improve yield or quality. Frontiers in Plant Science 13:1015652. doi.org/10.3389/fpls.2022.1015652