NPK Ratio in Cannabis: Values and Myths

Every fertiliser bottle carries three numbers such as 5-12-26, which give contents in percent: nitrogen as N, phosphorus and potassium as oxides. Many growers read an NPK ratio from them, and bloom fertilisers carry more phosphorus and potassium. The cannabis trials give no reason for this, since little phosphorus is enough in flower and more potassium cost potency. A ratio also says nothing about the amount, which is given in ppm and in the feed solution means milligrams per litre.

Which NPK ratio fits veg and flower?

In perlite, a heat-expanded volcanic rock that binds hardly any nutrients, supported single values give a calculated ratio of about 5:1:2 to 5:1:6 in veg and 5:1:2 in flower. The range in veg comes from potassium, whose optimum depended on the cultivar. In flower, nitrogen and phosphorus stay the same and only potassium drops. The single values per phase and the nutrient calculator are in the article on calculating a nutrient solution. No trial tested a whole ratio; the values come from separate trials with one or two cultivars each.

In peat, flower needs less phosphorus than in perlite, and with the perlite values for nitrogen and potassium this gives a calculated ratio of about 6:1:2.4 there. No trial reviewed here tested potassium in peat. The figures always mean the elements N, P and K, not the three numbers on the bottle.

In veg, plan about 5 parts nitrogen and 2 to 6 parts potassium per part of phosphorus.
In flower, plan about 5 parts nitrogen and 2 parts potassium.
Look for suitable fertilisers in the linked nutrient calculator under “Find matching fertilisers”.

Is a ratio enough, or do I need the amount?

A ratio is not enough, because the plant responds to the amount. The same 5:1:6 ratio can mean 80, 160 or 320 ppm nitrogen, depending only on the dose. Leaf colour does not reliably show whether it is too much, and the EC in the runoff is more reliable. Runoff, which growers also call drain, is the water draining out of the bottom of the pot, and EC, the electrical conductivity, measures all dissolved salts together.

A fertiliser with a good ratio can therefore still overfeed, while one with a poor ratio can be usable at the right dose. That is why you work out the amount of each nutrient first and let the ratio follow from it.

For each fertiliser, decide how many grams or millilitres per litre go into the feed solution.
Enter these amounts in the linked nutrient calculator, and it works out the ppm for N, P and K.
Take the target values per phase from its table.
If runoff EC is clearly above the feed solution, give less.

Does flower need a PK booster?

The trials give no reason for a PK booster, an additive with a lot of phosphorus and potassium for flower. No trial reviewed here tested such an additive, and what it supplies simply counts towards the phosphorus and potassium in the solution. Above the requirement, extra potassium left the flower yield unchanged.

More potassium did, however, cost potency. With rising potassium, THCA and CBDA fell, the acid forms of THC and CBD in the plant, and so did the terpenes, the aroma compounds of the flower.

Look up the phosphorus and potassium of the base solution in the nutrient calculator.
Give a PK booster only if phosphorus or potassium fall below the target values without it.
How much phosphorus each growing medium needs is in the article on phosphorus.
How much potassium veg and flower need is in the article on potassium.
If you lower pH with phosphoric acid, count its phosphorus in the nutrient calculator. The pH value describes how acidic a solution is.

How do I convert a fertiliser label into nutrients?

You multiply the oxide values on the label by a fixed factor. Five nutrients usually appear there as oxides, meaning compounds with oxygen: phosphorus, potassium, calcium, magnesium and sulphur. In the EU the fertilising products regulation requires this form, and the element itself may be stated in addition or instead. Nitrogen appears as N. A 5-12-26 label thus contains 5 % N and, by calculation, 5.2 % P and 21.6 % K. By calculation the element ratio is then about 1:1:4, while the label numbers give 1:2.4:5.2.

Liquid fertilisers may state their content either in percent by mass, relative to weight, or in percent by volume, relative to volume. The density is on the data sheet and tells you how many grams one millilitre weighs. With percent by mass and 1 ml per litre, the percentage times 10 times the density gives mg/L. A fertiliser with 5 % P₂O₅ contains about 2.2 % phosphorus by calculation. At a density of 1.2 g/ml and a dose of 2 ml per litre, it puts about 52 mg/L phosphorus into the can by calculation. With percent by volume the density drops out of the sum.

Check on the label whether the oxide or the element is given.
Take P₂O₅ times 0.4364 to get phosphorus.
Take K₂O times 0.8302 to get potassium.
Take CaO times 0.7147 to get calcium.
Take MgO times 0.6030 to get magnesium.
Take SO₃ times 0.4005 to get sulphur. Do not enter SO₃ directly as sulphur, or you get 2.5 times too much by calculation.
Check on the label whether percent by mass or by volume is meant.
Only then compare the converted values with 5:1:2.

What are 3:1:2, the Lucas formula and Jack’s 321 worth?

None of the three rules fully matches the values from cannabis trials. The common rule of thumb 3:1:2 only fits for nitrogen against phosphorus: in a hydroponic model the optima for nitrogen and phosphorus were about 3:1 by calculation. For potassium, flower yield in that model stayed the same over a wide range. The Lucas formula mixes 2 parts FloraBloom with 1 part FloraMicro, which according to the manufacturer are 0-5-4 and 5-0-1. The label numbers are then in a ratio of 5:10:9 by calculation, and if both liquids have the same density, this is about 1.1:1:1.7 as elements.

With Lucas, each part of phosphorus thus comes with only just over one part nitrogen, whereas the flower values give about five by calculation. Jack’s 321 mixes 3 g of Part A, 2 g of Part B and 1 g of Epsom salt, which is magnesium sulphate. Part A is 5-12-26 and Part B is 15-0-0 according to the manufacturer, and as elements the mix gives about 3:1:4 by calculation. If that puts 160 ppm nitrogen into the solution, it also gives about 56 ppm phosphorus and 230 ppm potassium by calculation. For flower, that is by calculation almost four times the potassium from the trials.

With Lucas, add the missing nitrogen with a nitrogen fertiliser until the target value is reached.
If you use calcium nitrate for this, include its calcium and its ammonium share in the calculation.
With Jack’s 321, choose a mix with less potassium for flower.
Enter the flower values for N, P and K under “Find matching fertilisers” in the linked nutrient calculator.