Do banana peels, coffee grounds or eggshells feed cannabis?

Q: Do banana peels, coffee grounds or eggshells feed cannabis?

No, not in any useful way: feeding cannabis with kitchen scraps is a Myth. Banana peels hold some potassium but release it only as they rot; peel water is a trace dose. Raw coffee grounds reduced plant growth in a controlled study. Eggshells are calcium carbonate that dissolves very slowly and acts more like lime than feed. Compost the scraps first, and feed the plant properly.

Short answer

Not usefully. The idea that kitchen scraps make a cannabis fertiliser is a Myth. Peel water is a trace dose, raw coffee grounds can stunt plants, and eggshells dissolve too slowly to feed anything this season. Compost the scraps, then use the compost; feed the plant with a complete fertiliser.

Why

Roots take up dissolved ions, not food waste ^[2]. Organic material has to be broken down by microbes first, and that takes weeks to months. Even purpose-made organic liquid feeds need higher nominal nitrogen to match mineral ones, around 389 mg/L N in one cannabis veg trial ^[3]. A banana peel soaked overnight is nowhere near that.

Coffee grounds are worse than useless raw. Applied directly to soil, spent grounds greatly reduced growth of the plants tested, regardless of soil type or added fertiliser, with evidence of nitrate being immobilised ^[1]. Composting changes them.

Eggshells are calcium carbonate. Coarse shell dissolves very slowly in soil and acts more like a liming agent, raising pH a little, than a calcium feed ^[2].

Plants mostly don't care where ions come from. They do care whether the ions are there, now, in the right amounts.

The numbers

Scrap	Main element	Available this grow?	Better use
Banana peel / peel water	K	Trace	Compost
Raw coffee grounds	N (locked up at first)	No; growth fell in trials ^[1]	Compost (small share)
Eggshell, coarse	Ca (as carbonate)	Barely	Compost or skip
Eggshell, finely ground	Ca	Slowly, liming effect	Pre-mix months ahead
Complete fertiliser	All	Yes	Feed the plant

What to do

Put scraps in a compost heap or worm bin, not the pot.
Use finished compost or castings as part of a soil mix or top-dress.
Feed the plant from a complete fertiliser at stage strength.
Fix calcium problems through pH, watering and the feed.

Common mistakes

Pouring peel water into a hydro reservoir.
Top-dressing raw coffee grounds on a pot.
Crushing eggshells into a pot to cure calcium spotting.
Assuming "natural" means "available".

Full article: Organic vs mineral nutrition
Answers: garden fertiliser on cannabis · super soil
Articles: living soil
Terms: organic fertiliser · top-dressing

Follow-up questions

Does banana peel water add potassium?

Banana peel water adds only a trace of potassium, far below what a flowering cannabis plant takes up, and soaking peels mostly grows microbes and smells. Peels release their potassium as they decompose, which takes weeks. If you want to use them, compost them. For potassium in flower, a complete feed or potassium sulfate does the job reliably.

Are coffee grounds good for cannabis?

Raw coffee grounds are not good for cannabis as a fertiliser. In a controlled study, spent coffee grounds applied directly to soil greatly reduced plant growth across several species, regardless of soil type or fertiliser, with signs of nitrogen being locked up. Composted grounds behave differently and are a reasonable compost ingredient.

Do eggshells add calcium to soil?

Eggshells are mostly calcium carbonate, so they can add calcium in the long run, but whole or coarse shells dissolve extremely slowly and behave more like a liming material that nudges pH up than a feed. Finely ground shells work faster. They cannot fix a calcium deficiency in an active grow; fix root-zone pH, watering and the feed instead.

Sources

Hardgrove SJ, Livesley SJ, 2016. Applying spent coffee grounds directly to urban agriculture soils greatly reduces plant growth. Urban Forestry & Urban Greening 18:1-8. doi.org/10.1016/j.ufug.2016.02.015

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

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/HORTSCI11903-17