Phosphorus (P) in Cannabis – Deficiency and Requirements

A plant stays compact, and its lower leaves turn dark green and blotchy. That is often how phosphorus deficiency begins. Phosphorus is one of the six macronutrients. The plant builds it into its genetic material and into its cell membranes, the envelopes around its cells. It also uses phosphorus to move energy. In flowering, cannabis stored about 80 % of its phosphorus in the buds. Still, it needs little: in peat and in a closed hydroponic system, phosphorus above 15 to 25 mg/L brought no extra yield. The unit mg/L means milligrams per litre of feed solution.

How do I recognise phosphorus deficiency?

Phosphorus deficiency shows as dark green lower leaves with spots and as compact growth. How the deficiency develops depends on temperature. In cool or wet conditions the lower leaves and leaf stalks turn purple. In warm conditions there is hardly any purple. The lower leaves then turn pale yellow and carry olive-green spots that look water-soaked.

Purple alone does not prove phosphorus deficiency. Diseased roots and a very acidic root zone also turn lower leaves purple, and in some plants so does nitrogen deficiency. In flowering the deficiency began with tiny pale dots on the lower fan leaves. These are the large fingered leaves on the stem and branches. The dots merged into brown patches.

Every stage with its look-alikes is described in the article on nutrient deficiency and overdose.
Check the bottom first: phosphorus deficiency starts on old leaves because the plant moves phosphorus from them into young leaves.
Read purple as a clue only together with dark green foliage, compact growth, and spots.
Look at the roots. Young, healthy roots are clear to translucent.

How much phosphorus does cannabis need?

In vegetative growth 30 mg/L is enough, while in flowering the amount depends on the growing medium. The figure always refers to the feed solution, meaning what comes out of the can or dripper. Retailers often write ppm instead of mg/L. In irrigation water both units mean the same.

In vegetative growth two cultivars showed deficiency at 5 mg/L. One was fully supplied from 15 mg/L, the other only from 30 mg/L. In flowering, phosphorus above 25 mg/L brought no further benefit in peat. In perlite the best range was 30 to 90 mg/L. Perlite is a heat-expanded volcanic rock that holds almost no nutrients itself. Above 5 mg/L the cannabinoid acids THCA and CBDA fell by up to 25 % in perlite, while bud mass grew.

Vegetative growth: give 30 mg/L.
Flowering in peat: 15 to 25 mg/L is enough.
Flowering in perlite with daily irrigation and plenty of runoff: 30 to 90 mg/L.
Hydroponics, where the roots hang in the nutrient solution: the findings contradict each other. In a closed system 15 mg/L was enough, with weekly solution changes the calculated optimum was about 59 mg/L.
Garden soil, compost soil, and coco were not tested in these trials. There a lab analysis of substrate and leaf shows whether enough arrives.
If you feed for potency rather than bud weight, stay at the low end of the range.

Does too much phosphorus cause damage?

Cannabis showed no damage pattern of its own in three trials, but the surplus is wasted. Two cultivars stayed free of damage up to 90 mg/L. In one hydroponic trial phosphorus at the roots rose above 300 mg/L without any visible difference.

Whatever the plant does not take up ends up in the runoff. That is the water draining out of the bottom of the pot. In peat, tripling the phosphorus feed raised the phosphorus in the runoff twelvefold. In other crops a lot of phosphorus crowds out iron, manganese and zinc. For cannabis this is not settled.

Limit phosphorus to what the growing medium needs.
If new growth yellows between green veins, consider iron deficiency caused by too much phosphorus.
Work out the phosphorus in your fertiliser from the label.
Then lower the phosphorus. New growth shows whether it works.

How much phosphorus is in my fertiliser?

The label usually gives phosphorus as P₂O₅, and that value times 0.44 gives the phosphorus. In the EU the fertiliser regulation requires this oxide form. A fertiliser with 5 % P₂O₅ therefore contains about 2.2 % phosphorus 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 1.2 g/ml and 2 ml per litre this puts about 52 mg/L phosphorus into the can by calculation. That is above the whole range that was enough in peat.

The full calculation including tap water is in the article on calculating a nutrient solution.
Multiply the P₂O₅ value on the label by 0.44.
Check whether the label gives percent by mass or by volume. With percent by volume the density drops out.
Count the acid used to lower pH. The pH value describes how acidic a solution is.
Phosphoric acid adds phosphorus. In greenhouses the water is acidified until 0.5 mmol/L bicarbonate remains.
For water with 2 mmol/L bicarbonate, phosphoric acid adds about 46 mg/L phosphorus this way by calculation.
Take the bicarbonate value from the water supplier’s analysis.
With a lot of bicarbonate choose nitric acid instead of phosphoric acid. It lowers pH without phosphorus but adds nitrogen.

What do I do about phosphorus deficiency?

Check the root zone first, then raise the phosphorus. Cool, wet soil and diseased roots slow uptake even when the fertiliser contains enough phosphorus. In soil, phosphorus becomes scarce below pH 6.

Phosphorus deficiency itself acidifies the root zone. In the trial the runoff pH fell to 4.5, compared with 5.8 in well-supplied plants. The authors of this trial see this as one route to calcium and magnesium deficiency.

Check temperature and wetness in the root zone, and fix waterlogging.
Measure the pH in the substrate. In soilless substrates plant nutrition specialists recommend 5.8 to 6.2 for cannabis. In the runoff the value can read somewhat higher.
Work out the phosphorus in the fertiliser and raise it to what is needed, in peat to 15 to 25 mg/L.
Watch the new growth. Spotted leaves do not recover, while new leaves stay unspotted if it works.
Once new leaves stay clean, hold the value and do not raise it further.