Potassium (K) in Cannabis – Deficiency and Requirements

The margins of the lower leaves turn yellow, starting at the serrations. That is how potassium deficiency begins. Potassium is one of the six macronutrients. It controls the stomata, the tiny pores in the leaves, and with them the water balance. It also helps move sugar through the plant. In flowering, 60 mg/L gave the same yield as 175 mg/L. The unit mg/L means milligrams per litre of feed solution. Too much potassium does harm mainly indirectly, because it crowds out calcium and magnesium.

How do I recognise potassium deficiency?

Potassium deficiency shows as yellow margins on the lower leaves that start at the serrations. The centre along the midrib stays green at first. The midrib is the main vein running down the middle of each leaflet. Later the margin turns brown and looks scorched. Older leaves often bend downwards, and the plant stays smaller.

In flowering two patterns appear on the same plant. At the top, dark brown spots form along the side veins of the sugar leaves. These are the small leaves inside the buds. At the bottom, the tips and teeth of the fan leaves turn yellow, then light brown. Fan leaves are the large fingered leaves on the stem and branches.

Every stage with its look-alikes is described in the article on nutrient deficiency and overdose.
Rule out magnesium deficiency. It keeps the veins green and yellows the area between them, while potassium yellows more evenly from the margin.
Rule out salt burn. It runs as an even brown margin all around the leaf.
Watch the progress over a week: potassium deficiency advances more slowly than salt or drought damage.

How much potassium does cannabis need?

In flowering, 60 mg/L was enough for full yield. In vegetative growth 15 mg/L was too little. One cultivar grew best at 175 mg/L, another equally well between 60 and 175 mg/L. At 240 mg/L growth was inhibited.

In flowering, yield did not differ between 60 and 175 mg/L. In a hydroponic trial, where the roots hang in the nutrient solution, it even stayed the same between 60 and 340 mg/L. Cannabinoid acids such as THCA and terpenes, the aroma compounds of the buds, fell as potassium rose. Retailers often write ppm instead of mg/L. In irrigation water both units mean the same.

Vegetative growth: give 60 to 175 mg/L, not more.
Flowering: start with 60 mg/L.
If you feed for potency and aroma, stay at 60 mg/L in flowering.
Plant nutrition specialists at NC State University give mother plants 80 to 100 mg/L.
Work out the potassium in your own fertiliser from the label.

What does too much potassium do?

Usually you do not see the potassium itself but a calcium or magnesium deficiency. In a vegetative trial both fell in leaf, stem and root as potassium rose. Magnesium began to fall there from 60 mg/L. Zinc, iron and manganese fell in the root as well.

In flowering, 175 to 240 mg/L caused moderate yellowing, mainly of the older leaves. One of two cultivars also suffered physiologically at 240 mg/L. The researchers explain this with the lower calcium and magnesium in the leaves.

Check potassium-rich bloom boosters and potassium bicarbonate. Potassium bicarbonate is a product for raising pH, the measure of how acidic a solution is.
After raising pH with potassium bicarbonate, water with plain water the next day.
Have a lab measure the feed solution and the runoff. Runoff drains out of the bottom of the pot.
In perlite with plenty of runoff, potassium there exceeded the feed from 175 mg/L upwards. At such high rates the comparison is worth making.
Lower the potassium. New growth shows whether it works.
As a rule of thumb, growers give potassium, calcium and magnesium at a ratio of 4 : 2 : 1. This ratio has not been tested on cannabis.

How much potassium is in my fertiliser?

The label usually gives potassium as K₂O, and that value times 0.83 gives the potassium. In the EU the fertiliser regulation requires this oxide form. A bloom booster with 8 % K₂O therefore contains about 6.6 % potassium by calculation.

For mg/L you also need the density from the data sheet. Percent by mass refers the content to weight, percent by volume to volume. With percent by mass, 1 ml per litre gives: percent times 10 times density. At a density of 1.3 g/ml, just 1 ml per litre supplies about 86 mg/L potassium by calculation. That is more than the 60 mg/L that gave full yield in flowering.

The full calculation including tap water is in the article on calculating a nutrient solution.
Multiply the K₂O value on the label by 0.83.
Check whether the label gives percent by mass or by volume. With percent by volume the density drops out.
Add up the potassium from base fertiliser and additives.

What do I do about potassium deficiency?

Measure EC first, and if it is normal, raise the potassium. EC, the electrical conductivity, measures all dissolved salts together. A high EC scorches leaf margins much like potassium deficiency does.

Potassium can be missing even when the fertiliser contains enough. In soil it becomes scarce below pH 6 and between pH 7.6 and 8.4. A lot of magnesium or sodium suppresses potassium uptake.

Measure EC and pH in the runoff. For vegetative growth the NC State group gives 2.0 to 3.0 mS/cm in runoff collected with the PourThru method.
Check tap water and fertiliser for high magnesium and sodium. For greenhouse crops, water with less than 34.5 mg/L sodium counts as unproblematic.
If both look normal, raise the potassium. The NC State group considers 40 to 80 mg/L sufficient and advises 60 to 120 mg/L to be safe.
Watch the new growth. Brown margins do not recover, while new leaves stay green if it works.
Once new leaves stay green, hold the value and do not raise it further.