Sulphur (S) in Cannabis – Deficiency and Requirements

The leaves in the middle of the plant turn yellow while the lower ones stay dark green. The pattern suggests nitrogen deficiency. At normal EC, sulphur deficiency is the more likely cause. Sulphur, spelled sulfur in American English, is one of the six macronutrients. It is part of the protein building blocks cysteine and methionine. From cysteine the plant also builds glutathione, a protective molecule against cell damage. Without sulphur a THC cultivar gave 34 % lower bud yield in flowering.

How do I recognise sulphur deficiency?

In vegetative growth the newly matured leaves in the middle of the plant yellow first. The yellow runs from the leaf base to the tip and later reaches the youngest growth. In vegetative growth the lower leaves stay dark green. Late on, pale spots that look sunburnt appear, and the plant stays stunted.

In flowering the youngest sugar leaves yellowed first, the small leaves inside the buds. A week later the upper fan leaves stood steeply upwards, and the lower ones yellowed from the midrib outwards. Fan leaves are the large fingered leaves on the stem and branches. Where sulphur deficiency first shows in cannabis is described differently by the sources depending on the phase.

  • Every stage with its look-alikes is described in the article on nutrient deficiency and overdose.
  • In vegetative growth, rule out nitrogen deficiency. It starts at the bottom and climbs, while sulphur deficiency leaves the lower leaves green.
  • Measure EC, the electrical conductivity of all dissolved salts together. For vegetative growth the NC State group considers 2.0 to 3.0 mS/cm normal in runoff collected with the PourThru method.
  • If EC is in that range, yellowing in the middle points to sulphur rather than nitrogen.
  • Check the fertiliser. If it contains no sulphate, the usual sulphur source is missing.

How much sulphur does cannabis need?

No trial with graded sulphur rates on cannabis was found. The base solutions of trials on other nutrients contained between 54 and 180 mg/L sulphur. The unit mg/L means milligrams per litre of feed solution. Retailers often write ppm instead of mg/L, and in irrigation water both mean the same.

The NC State group recommends 50 to 200 ppm sulphur from fertiliser and substrate, with at least 25 ppm. It names no primary source for this. Across 6119 hemp samples, the middle half of leaf values lay at 0.25 to 0.36 % sulphur in the dry matter.

  • Add up the sulphur from fertiliser, additives and water. According to the NC State group, tap water brings 5 to 30 ppm.
  • Make sure there is at least 25 mg/L sulphur.
  • Compare a lab leaf sample with 0.25 to 0.36 %, together with an analysis of substrate or solution.

Where does the sulphur in my solution come from?

From sulphate, because the plant takes up sulphur as sulphate. Sulphate is a compound of sulphur and oxygen. It comes in salts such as Epsom salt with about 13 % sulphur by calculation or potassium sulphate with about 18 %. On EU fertiliser labels sulphur is usually listed as SO₃, and that value times 0.40 gives the sulphur. Entering SO₃ as sulphur overstates it 2.5 times.

Sulphuric acid used to lower pH also adds sulphur. The pH value describes how acidic a solution is. Bicarbonate is the part of the water that absorbs acid and holds the pH up.

  • Check calcium-magnesium additives. If they supply magnesium as nitrate instead of sulphate, they bring no sulphur according to the NC State group.
  • With the complete fertiliser 20-10-20 and in rockwool or hydroponics, make sure there is a sulphur source. The NC State group sees sulphur deficiency here too.
  • Multiply the SO₃ value on the label by 0.40.
  • Count sulphuric acid from pH correction. If it acidifies water with 2 mmol/L bicarbonate down to 0.5 mmol/L, it adds about 24 mg/L sulphur by calculation.
  • The full calculation including tap water is in the article on calculating a nutrient solution.

Does too much sulphur cause damage?

No leaf pattern for too much sulphur was found described for any crop. The NC State group warns that above about 300 ppm the uptake of boron and molybdenum can fall. It names no primary source for this.

In a magnesium trial, sulphate rose along with the highest magnesium rate. The damage at that level therefore cannot be attributed to magnesium alone.

  • In standing feed, keep sulphur below about 300 mg/L.
  • If boron deficiency persists despite boron feeding, also work out the sulphur.
  • Do not give Epsom salt repeatedly without measuring, because it supplies magnesium and sulphur at once.

What do I do about sulphur deficiency?

Give Epsom salt once and switch the fertiliser to one with a sulphate source. The NC State group uses the same shock dose as against magnesium deficiency, 2.4 g Epsom salt per litre. By calculation that is about 312 mg/L sulphur and 238 mg/L magnesium. It is a one-off drench and not a standing value.

Yellow leaves usually do not recover. New growth shows whether the remedy works.

  1. Check the fertiliser for a sulphur source.
  2. Measure EC. If it is normal, that points to sulphur rather than nitrogen.
  3. Water once with Epsom salt, letting 5 to 10 % run off. Amounts and limits are in the article on magnesium.
  4. Afterwards use a fertiliser containing sulphate as standing feed.
  5. Watch the new growth. If it stays green, do not repeat shock doses.

Sources