Nutrient Oversupply in Cannabis – What the Controlled Fertilisation Studies Show

In cannabis cultivation the assumption is widespread that higher nutrient supply produces higher yields and stronger potency. Over the past five years several research groups have tested this assumption element by element, running the same cultivar in the same system at different supply rates and measuring yield at the end of flowering. The result can be summed up in one sentence: the optima for the three primary nutrients lie considerably lower than the common recommendations suggest – and for phosphorus the data additionally show that more supply produces neither more yield nor more potency.

How the optimum is determined in the first place

Such a study grows the same cultivar in the same system at several supply rates of the nutrient under test, while the other nutrients are held constant. If yield rises with supply at first and then levels off into a plateau, the value at the beginning of the plateau is the agronomic optimum: additional supply no longer increases yield. Everything above that is luxury – the plant takes up the nutrient and stores it in the tissue without any additional flower or potency resulting from it. For nitrogen, phosphorus, and potassium such studies now exist in multiple replicates, and the findings largely agree between research groups.

Nitrogen

The Bernstein group tested five nitrogen concentrations between 30 and 320 mg per litre on a medical cultivar in perlite culture in a 2020 study. The optimum for yield and photosynthesis during the vegetative stage was 160 mg per litre. At higher values the leaf concentrations of calcium, magnesium, and manganese dropped measurably, even though the nutrient solution contained these elements in sufficient amounts – the additional ammonium nitrogen displaced the divalent cations at the root uptake site (an example of ion antagonism, see the article on lockout). A follow-up study by the same group in 2021 extended the test to the flowering phase and found a similar range there: 160 to 320 mg per litre were optimal, below 80 mg per litre yield collapsed.

An independent study on the cultivar “Gelato” (Bevan and colleagues, 2021) confirmed this magnitude for the flowering phase using a different methodology. In a systematic experimental design with five N, five P, and five K rates in deep water culture, the best value for N in flowering was 194 mg per litre; below 160 and above 230 mg per litre yield declined markedly. The range of best supply therefore lies between roughly 160 and 230 mg per litre. Many commercial cannabis recipes run at 250 mg per litre or above, which sits at the upper edge of or already within the range of yield depression.

Phosphorus

For phosphorus the data show a remarkable drop compared to established practice. The common recommendation was long 60 to 90 mg per litre, sometimes far higher; the controlled studies find the optimum well below that. Shiponi and Bernstein (2021) tested P concentrations of 5, 15, 30, 60, and 90 mg per litre on two cultivars. One cultivar reached peak yield already at 30 mg per litre; in the other yield continued to rise up to 90 mg per litre – but only by around 20 percent for a tripling of the P amount. A toxicity pattern did not appear up to 90 mg per litre in either cultivar. The authors recommend 30 mg per litre as an application value and 15 mg per litre as a minimum.

The Utah group around Bugbee arrived at an even lower value on a CBD-dominant cultivar. In the 2025 study by Hershkowitz and colleagues, five P rates between 15 and 90 mg per litre were combined with two electrical conductivity levels. Result: 15 mg per litre was sufficient for full flower yield; neither an increase to 30, 45, 60, nor 90 mg per litre produced any additional yield or higher cannabinoid concentration. An additional argument against high P rates is environmental: at 90 mg per litre supply, phosphorus accumulated in the substrate solution to several hundred mg per litre and drained into the environment with the irrigation surplus.

The apparent contradiction between older recommendations and newer measurements resolves itself when the tested range is considered: the older studies had not tested values below 60 mg per litre and therefore missed that the plateau begins much earlier. Anyone who does not test the lower range does not see the optimum.

Potassium

Potassium is the quietest of the three primary nutrients. The 2021 Bevan study tested K rates between 60 and 340 mg per litre and found no significant yield difference across the entire range. The tested range lies above the deficiency threshold and below a toxicity limit that has not been determined for cannabis – in between the plant does not respond in yield. The choice of the exact K amount is therefore, by the current data, not a yield question but a question of ion balance in the overall system. The common commercial values of 300 to 400 mg per litre sit at the upper end of the tested range and cannot be justified from the yield data.

The trade-off between yield and potency

An important secondary finding of the nitrogen studies: higher N supply lowers the cannabinoid concentration in the flower while yield increases. In the 2021 Saloner and Bernstein study the THCA concentration in plants at 30 mg per litre N was roughly 30 percent higher, and the CBDA concentration roughly 20 percent higher, than in plants at 160 mg per litre N. The yield was the reverse: at 30 mg per litre it was half as large as at 160 mg per litre.

This is not a recommendation to cut N – the total amount of active compound per unit area is still higher at 160 mg per litre despite the lower concentration, because yield has doubled. What the finding shows is a choice: anyone aiming for maximum potency per gram of flower must run N supply lower than for maximum yield per unit area. There is no single “right” rate, because the two targets diverge.

For phosphorus the data look different. Neither in the Bevan study nor in the Hershkowitz study did the cannabinoid concentration respond to P supply. The trade-off is specific to nitrogen and not transferable to every nutrient.

Luxury uptake as a concept

When a plant takes up a nutrient above its requirement and stores it without that storage turning into growth, plant physiology calls this luxury uptake. In cannabis the behaviour is documented for several nutrients: leaf phosphorus in the Hershkowitz study rose from 15 to 90 mg per litre supply by several hundred percent without any accompanying yield increase. The taken-up nutrient sits in the tissue because the root carries it in passively with the water stream or stores it actively, but it is not converted into additional growth. Luxury uptake is therefore not evidence of need; it is more the sign of an overfeeding that can be measured in the leaf tissue without any advantage showing in the yield.

Electrical conductivity as a ceiling

The summary value of a nutrient solution is the electrical conductivity (EC), measured in millisiemens per centimetre. It rises with ion concentration and is used as a rough gauge of overall nutrient concentration. For cannabis the practical EC range is between 1.5 and 3.0 mS per centimetre. Controlled studies show that the species tolerates the upper end of the EC range well, as long as the salts come from fertiliser: Hershkowitz found no yield or cannabinoid difference between EC 2 and EC 4 mS per centimetre. Only at considerably higher values does yield drop. With sodium chloride the sensitivity is greater – phytotoxicity occurred in a referenced study from EC 9 mS per centimetre onward when NaCl was raising the EC. An EC value without specifying the salt type is incomplete for assessment; the details are described in the article on lockout.

Why less is often more

The sum of the studies paints a consistent picture: the controlled optima for cannabis lie below what practice and vendor recommendations suggest. There are several reasons for the discrepancy. Older studies did not test the lower range. Commercial nutrient programmes are often designed with safety margins that end up being deposited in the tissue. And grower culture carries a “more is more” expectation that overlooks the demonstrable trade-off between potency and yield. Anyone who runs cannabis with a lean but complete nutrient solution achieves, according to the current data, the same yield at more stable potency values and with considerably less fertiliser washing out.