Cannabis nutrients: what the plant really needs

TL;DR

• Veg: high N (3-1-2), flower: high P/K (1-2-3 to 1-3-4).
• pH window soil 6.0-7.0, coco/hydro 5.5-6.2 – outside = lockout.
• Mobile deficiencies (N, P, K, Mg) show on old leaves; immobile (Ca, Fe, S) on young.
• Overfeeding is more common than deficiency – less is more.

pH and nutrient availability

Each nutrient has an optimal pH uptake window. Outside it the nutrient is chemically present but not bioavailable – classic lockout. pH monitoring matters more than any nutrient brand.

Nutrient requirements throughout the cycle and how deficiencies occur

Cannabis shifts its requirements significantly: nitrogen dominates in the vegetative phase, phosphorus and especially potassium in the flowering phase. Calcium, magnesium, and sulphur are not additives, but primary nutrients — magnesium deficiency with interveinal chlorosis on middle leaves is one of the most common misdiagnoses and is often incorrectly treated as nitrogen deficiency. Micronutrients act through availability: iron and manganese simply precipitate if the pH is too high, regardless of the dosage.

Most nutrient problems are in fact pH or EC problems. If the pH is too low, calcium is locked out; if it is too high, iron and zinc are locked out; if the EC is too high, the nutrient solution osmotically draws water from the root, which looks like drought stress. Therefore, the sequence is: first measure runoff values, then interpret, and only fertilize last. Those who fertilize first worsen the problem in half of the cases.

• Vegetative EC 1.2–1.8, flowering EC 1.6–2.4 (substrate-dependent)
• Deficiency symptoms first on old leaves = mobile nutrients (N, P, K, Mg)
• Symptoms on young leaves = immobile nutrients (Ca, Fe, S)
• If overfertilization is suspected, flush with pH-corrected water, do not add more fertilizer

Residues and Mandatory Analysis in Switzerland

For marketable Swiss goods, it is not only the cannabinoid content that counts, but also the absence of residues. Pesticide and heavy metal analyses are part of the batch release process for reputable producers; customers can trace them via the batch number on the certificate of analysis. Excessive fertilisation is also reflected in nitrate levels and burning behaviour — a practical quality indicator that even laypersons will notice.

In Switzerland, the total THC value (THC + THCA × 0.877) alone determines whether a plant is considered industrial hemp or a narcotic. Below 1 %, cultivation is generally permitted; above that, only within the framework of an authorised pilot project under BetmG Art. 8b or with a medical prescription under Art. 8a. Anyone who cultivates themselves bears the burden of proof for the value — the analysis of the harvested batch is decisive, not the variety specification of the seed dealer.

Organic versus mineral fertilization compared

The choice between organic and mineral fertilizers determines the overall maintenance effort and the reaction time in the event of nutrient problems. Mineral fertilizers are present in the form of salts that can be absorbed by the roots immediately and independently of the soil temperature. This allows precise control of the EC value, but carries a high risk of over-fertilization and salinization of the substrate. Organic fertilizers, on the other hand, are based on complex carbon compounds that must first be converted into plant-available ions by soil microorganisms. This process is delayed, but builds up a stable humus layer and is much more forgiving of dosing errors.

In practice, organic fertilization requires a living substrate supported by beneficial bacteria and mycorrhizal fungi. Since the release of nutrients is dependent on temperature and moisture, the system reacts sluggishly to acute deficiencies. Those who grow on coco coir or in hydroponic systems therefore almost exclusively use mineral fertilizers to maintain full control over the nutrient concentration. In soil-based cultivation, on the other hand, the organic approach promotes the biosynthesis of terpenes and ensures a stable biological balance that naturally buffers the pH value in the root zone.

• Mineral fertilizers act immediately and allow precise control via the EC value.
• Organic fertilization protects soil life and minimizes the risk of overdosing.
• Hydroponic systems strictly require mineral, completely water-soluble nutrient salts.
• The buffering capacity of organic soil automatically compensates for slight fluctuations in the pH value.

Influence of Swiss tap water quality on the nutrient solution

The composition of tap water in Switzerland varies greatly depending on the region and significantly influences fertilisation. Many Swiss municipalities have very calcareous, hard water that is rich in calcium and magnesium. Anyone who applies additional Cal-Mag fertiliser here without testing risks quickly blocking other essential nutrients such as potassium. Before preparing any nutrient solution, it is therefore essential to check the local water values of the municipality or to carry out your own measurement. Soft water, as is often found in Ticino or alpine regions, on the other hand, requires targeted supplementation to prevent deficiencies.

A high carbonate content in Swiss tap water also acts as a strong pH buffer. This makes it difficult to lower the pH value to the optimal range for cannabis, as the value often rises again after adding acids. For legal cultivation within the framework of Swiss pilot trials or for medical cannabis, strict quality standards apply, which is why professional operations often use reverse osmosis systems. Filtration produces highly purified source water, which is then remineralised in a controlled manner with the required macro- and micronutrients to guarantee standardised and reproducible conditions.

• Hard Swiss tap water already contains high amounts of calcium and magnesium.
• The high carbonate hardness stabilises the pH value, but often requires a stronger pH correction.
• Soft water from alpine regions has hardly any buffering capacity and is prone to sudden pH drops.
• Reverse osmosis filters out disruptive mineral salts and creates a defined base for fertilisation.

Flushing the substrate and targeted salt elimination

Flushing describes the intensive watering of the substrate with pure, pH-corrected water without any added fertiliser. This process is primarily carried out at the end of the flowering phase to force the plant to use up its stored nutrient reserves in the leaves. Although the benefit of final flushing for improving taste is scientifically debated, in practice it has been proven to reduce the remaining mineral salt content in the tissue. An excessively high nitrogen content in the flowers leads to a harsh smoking experience and impairs the even burning behaviour of the final product.

In addition to flushing before harvest, therapeutic flushing in the event of acute over-fertilisation is a vital measure for the plant. When the EC value in the substrate exceeds critical limits due to salt accumulation, osmotic pressure blocks water uptake by the roots. In this case, the substrate must be flushed with water equivalent to several times the pot volume to physically wash out the excess nutrient salts. After this, the plant is restarted with a low-dose, balanced nutrient solution to immediately support the regeneration process of the fine root hairs.

• The final flush reduces the nitrogen content in the flowers for a cleaner burn.
• Therapeutic flushing saves the plant in the event of acute over-fertilisation and extremely high EC values.
• Only water with a pH value adjusted to the substrate should be used.
• Flushing too frequently leaches out beneficial soil microorganisms and damages the soil structure.

Risks and safety aspects when handling fertilizers

Highly concentrated fertilizers and pH regulators, especially phosphoric acid or nitric acid for lowering pH, pose significant health and ecological risks. These chemicals are highly corrosive and must always be handled with extreme caution, protective goggles, and gloves. Improper storage, particularly within reach of children or pets, can lead to severe chemical burns or poisoning. In the event of accidents or accidental ingestion of fertilizer components, the Swiss emergency number from Tox Info Suisse provides rapid, specialist assistance 24/7 on the telephone number 145.

The improper use of cannabis fertilizers also poses a burden on the environment. The runoff water generated during watering contains highly concentrated nitrates and phosphates, which must never enter the groundwater or natural bodies of water unfiltered. In professional Swiss cultivation, for example in the strictly controlled pilot projects according to Art. 8b BetmG, strict environmental regulations apply to the disposal and recycling of wastewater. Private growers should ensure they minimize the amount of runoff water and dispose of it in an environmentally friendly manner via the sewage system to protect local ecosystems from eutrophication.

• Acids for pH regulation are highly corrosive and require personal protective equipment.
• In case of poisoning or chemical burns from fertilizers, Tox Info Suisse provides assistance via the number 145.
• Runoff water from mineral fertilization must not be allowed to seep into the garden due to nitrate contamination.
• Strict disposal guidelines in Swiss pilot projects prevent the pollution of surface waters.

FAQ