Cannabis Nutrients: Feeding Schedule, Deficiencies and Nutrient Burn

Which nutrients does cannabis need?

Every fertilizer bottle shows an NPK value, for example 4-2-6. The numbers give the share of nitrogen (N), phosphorus (P) and potassium (K) — the three primary nutrients. On top of that come secondary nutrients, which cannabis needs in smaller amounts, and trace elements.

The rule of thumb: in the vegetative stage the focus is on nitrogen (about 2:1:1), in flowering on phosphorus and potassium (about 1:2:2 to 1:2:3). That is why grow and bloom fertilizers are sold as separate products.

Which soil for cannabis?

The soil is your plant’s nutrient store. Two types suit cannabis:

• Lightly pre-fertilized soil (light mix): few nutrients, ideal for seedlings and for anyone who wants to control feeding themselves. You start feeding earlier, but the risk of overfeeding is low. Good for autoflowers.
• Heavily pre-fertilized soil (all mix): lasts for the first 3–5 weeks. Too “hot” for delicate seedlings — better used from repotting onwards.

Mix 20–30% perlite into the soil. The light white granules loosen the soil, let air reach the roots and drain excess water faster. That prevents waterlogging and root rot.

Ordinary potting compost from the garden centre works in an emergency, but often contains slow-release fertilizer in unknown amounts, or is very peaty and compacts quickly. If you use it, mix in perlite and don’t feed extra in the first weeks. In the garden: dig a generous planting hole (at least 50 × 50 cm / 20 × 20 in) and mix the garden soil with mature compost and perlite or sand.

On pot size: photoperiod strains indoors usually manage with 11–20 litres (3–5 gallons), autoflowers with 11–15 litres (3–4 gallons). In bigger pots the soil holds nutrients for longer, and you need to feed less often.

When to start feeding cannabis — and how often

• Seedling (weeks 1–2): no fertilizer. The seedling lives on the reserves in the seed and the seed leaves.
• In pre-fertilized soil: only feed when the soil is exhausted — usually after 2–4 weeks. The first sign: the lower leaves turn paler.
• In a light mix: from week 2–3, start with about a quarter of the manufacturer’s dose and increase slowly.
• How often: fertilizer with every second or third watering, plain water in between. That way no salts build up in the soil.
• When to stop: only plain water in the last 1–2 weeks before harvest.

Only water when the top 2–3 cm (about an inch) of soil is dry and the pot feels noticeably lighter. Cannabis copes better with a short dry spell than with permanently wet soil.

Cannabis feeding schedule by stage

The following schedule applies to soil and a photoperiod strain with about 8–9 weeks of flowering. The EC values (electrical conductivity of the nutrient solution) are guide values for anyone who measures — without a meter, simply follow the dose guidance.

Important: the manufacturer’s full dose is an upper limit, not a target. Many growers manage with half to three-quarters — the plant shows you whether it needs more. What happens to the plant in each week of flowering is described in our guide Cannabis flowering stage week by week.

Feeding autoflowers

Autoflowering strains are smaller and have a shorter life, so they need considerably less fertilizer. The most important rules:

• Sow directly into the final pot with light soil — repotting costs autoflowers time they don’t have.
• Use at most half the manufacturer’s dose.
• Switch to bloom fertilizer from about week 3–4, when the first pistils appear.
• Give only plain water in the last week before harvest.

More on how autoflowering strains work is explained in our guide Autoflowering seeds explained.

pH and nutrient lockout

A wrong pH is the most common cause of an apparent nutrient deficiency. If it is outside the right range, the roots can no longer take up certain nutrients even though they are present in the soil — this is called nutrient lockout. Feeding even more then makes things worse.

• Soil: pH 6.0–7.0, ideally 6.2–6.8.
• Coco and hydro: pH 5.5–6.5.
• Measuring: add the fertilizer to the water first, then measure the pH and adjust if necessary — fertilizer changes the pH.
• Tap water: in many areas the water is hard and chalky, and its pH is often 7.5 or higher. Water left to stand or a pH-down product helps.

The rule for every deficiency: check the pH first, then think about more fertilizer.

Cannabis nutrient deficiency chart

Where a symptom appears first tells you a lot. Nitrogen, phosphorus, potassium and magnesium are mobile in the plant: when they run short, the plant pulls them from the old leaves, so the symptoms show at the bottom. Calcium and iron are not mobile — their deficiency shows on young and middle leaves.

Easy to confuse: yellow leaves can also come from overwatering, cold or light stress. Brown spots that feel soft and rotten point to mold rather than a deficiency — how to recognise it is shown in our guide Bud rot and cannabis mold.

Nutrient burn: how to recognise and fix overfeeding

Too much fertilizer does damage faster than too little. Typical signs of nutrient burn and overfeeding:

• Nutrient burn: the leaf tips turn yellow, then brown and dry.
• The claw: leaves are very dark green, shiny and bend down at the tips — a classic sign of too much nitrogen.
• White crusts on the soil or at the edge of the pot: built-up fertilizer salts.
• The plant droops its leaves despite moist soil.

How to fix nutrient burn:

• Flush: water slowly with clean, pH-adjusted water — about two to three times the pot volume — until plenty runs out at the bottom. This washes out excess salts.
• Pause: then let the soil dry out well and don’t feed for 1–2 weeks.
• Restart: continue at half the dose. Burnt leaf tips don’t recover; what matters is that new leaves grow back healthy.

Organic or mineral nutrients?

For a first grow in soil, organic fertilizer is the more forgiving choice. The only important thing: don’t mix both systems at random, and stick to one line.

Home remedies: coffee grounds, eggshells, ash, nettle tea

Home remedies supplement a fertilizer but rarely replace it in flowering. The amounts of nutrients are small and hard to dose.

Feeding cannabis outdoors

• Prepare the soil: work in mature compost in spring. With it, good garden soil often carries a plant until summer.
• Rain: heavy rain washes nutrients out, especially from containers. After wet weeks, watch for pale lower leaves — a common sight in rainy climates such as the UK, Ireland or the Pacific Northwest.
• Flowering: outdoors in the northern hemisphere it starts from late July to early August — switch to bloom fertilizer then.
• Last feed: for a harvest in mid to late September, feeding ends in early September.

The most common feeding mistakes

• Feeding too early: seedlings don’t need fertilizer.
• Feeding too much: the manufacturer’s full dose is an upper limit.
• Ignoring pH: a deficiency despite feeding is usually nutrient lockout.
• Feeding with every watering: salts build up — give plain water in between.
• Stopping too late: only plain water in the last 1–2 weeks. When the plant is really ripe is explained in our guide When to harvest cannabis.

Strains that forgive growing mistakes

Especially on a first grow, not everything goes smoothly with feeding. Robust strains forgive a wrong pH or one dose too many much better. All seeds come direct from the breeder, with a germination guarantee and discreet shipping to the UK, Europe, the USA, Canada and worldwide.