Cannabis Nutrients, Soil & Feeding: A Practical Guide

Feeding cannabis well is less about buying expensive bottles and more about understanding what the plant actually needs — and when to leave it alone. This practical guide covers your growing medium, the all-important pH, the main nutrients, and how to read the plant so you can tell the difference between a hungry plant and an over-fed one.

A quick legal note: Under the 2018 Constitutional Court “Prince” judgment, adults (18+) in South Africa may privately grow and use cannabis in a private place for personal consumption. Public cultivation is not allowed and selling or dealing remains illegal. See our overview of whether dagga is legal in South Africa for the details.

Choosing a growing medium: soil vs coco

Your medium is the foundation of good feeding, and for beginners the choice usually comes down to soil or coco coir.

• Soil is the most forgiving option. A good-quality potting soil already holds nutrients, so it feeds the plant for the first few weeks with little input from you. It buffers mistakes, making it ideal for a first grow.
• Coco coir is an inert medium made from coconut husk. It drains beautifully and encourages fast growth, but because it holds almost no nutrients of its own, you must feed the plant deliberately from early on. It rewards attention but punishes neglect.

If you are just starting, soil is the gentler teacher. Coco is a great next step once you are comfortable reading your plants.

Why pH matters more than almost anything

You can have perfect nutrients in your water and still starve your plant if the pH is wrong. pH measures how acidic or alkaline your water and medium are, and it controls whether roots can actually absorb the nutrients present — a phenomenon called “nutrient lockout”.

• In soil, aim for a pH of roughly 6.0–7.0.
• In coco, aim slightly lower, around 5.5–6.5.

A cheap pH meter or drops pay for themselves many times over. If a plant looks deficient despite regular feeding, check pH before you add anything — the answer is often here.

Understanding NPK and the main nutrients

Nutrient bottles show three numbers, the NPK ratio, standing for the three macronutrients a plant needs in the largest amounts:

• N (Nitrogen) drives leafy, green vegetative growth. Plants want more of it while they are growing bushy and before they flower.
• P (Phosphorus) supports roots and, crucially, flower development.
• K (Potassium) supports overall plant health, water movement and bud formation.

In simple terms, plants generally want more nitrogen during vegetative growth and more phosphorus and potassium during flowering. This is why many feed lines sell a separate “grow” and “bloom” formula.

Cal-mag and the deficiency you will probably meet

Beyond NPK, plants need secondary nutrients — most notably calcium and magnesium, often sold together as “cal-mag”. A cal-mag deficiency is one of the most common issues home growers face, especially in coco.

• Magnesium deficiency typically shows as yellowing between the veins of older, lower leaves while the veins stay green.
• Calcium deficiency often appears as brown or rust-coloured spots on newer leaves, sometimes with curling.

A cal-mag supplement usually corrects these, but as always, rule out a pH problem first. For a broader troubleshooting reference, see our guide to common cannabis growing problems.

Watering: the skill everyone underestimates

How you water is as important as what you feed. Cannabis roots need both moisture and oxygen, and constantly soggy soil suffocates them.

• Water thoroughly, then wait until the top few centimetres of the medium feel dry before watering again.
• Lift the pot — a light pot means dry, a heavy pot means there is still plenty of moisture. This “weight test” is a reliable habit.
• Water less for small seedlings and more as the plant and its root system grow.

Overwatering is far more common than underwatering, and its drooping leaves are easy to mistake for thirst — resist the urge to add more water.

Reading over-feeding and under-feeding

Plants tell you what they need if you learn their signals. The most important skill is telling too much from too little.

• Under-feeding (hunger): older, lower leaves turn pale and yellow first as the plant moves nutrients to new growth. Growth may slow. This calls for a gradual increase in feeding.
• Over-feeding (nutrient burn): leaf tips turn brown, crispy and slightly clawed, and deep-green leaves may look glossy and stiff. This calls for less feed and, often, a plain-water flush.

When in doubt, feed lightly. It is far easier to add more nutrients to a hungry plant than to reverse the damage of over-feeding. A useful mantra for beginners is to start at half the strength a bottle recommends and build up only if the plant asks for it.

Putting it together

Good feeding is a rhythm: the right medium, correct pH, the appropriate nutrients for the stage, sensible watering, and a close eye on the leaves. Get those basics right and your plant will largely look after itself. To see how feeding fits into the wider journey, follow our week-by-week growing guide and the full South African home cultivation overview, then explore more how-to content on the grow hub.