Sulfur is a secondary macronutrient absorbed as the sulfate ion (SO4 2-). It is built into the amino acids cysteine and methionine, into coenzymes and into many defence compounds. It is only partly mobile, so deficiency tends to show on younger leaves first.
In practice
A sulfur-short plant looks like it lacks nitrogen, but the even pale yellowing starts on new or middle growth rather than the bottom leaves. True deficiency is uncommon in hydro, because magnesium sulfate and potassium sulfate bring plenty of S along. Feeds typically land somewhere around 50-100 mg/L (ppm) S, and plants tolerate a wide range.
Don't confuse with
- Sulfur burner or elemental sulfur sprays: used against mildew, not as a feed.
- Nitrogen deficiency: starts on old leaves; sulfur deficiency starts higher up.
- Volatile sulfur compounds: the skunky aroma molecules in cannabis contain sulfur, but feeding more S has not been shown to raise them.
Myth check
"More sulfur makes buds smell stronger" has no controlled support (Evidence: Myth until tested). The plant needs adequate S; extra sulfate mostly raises EC. See Mythbusters.
Sulfate (SO4 2-) is the form in which plants take up sulfur, delivered mainly by magnesium sulfate and potassium sulfate in mineral feeds.A nutrient deficiency is a shortage of an essential element in the plant tissue, causing characteristic symptoms; the cause is often pH or uptake rather than an empty feed.Magnesium sulfate heptahydrate, sold as Epsom salt, is the common hydro magnesium source, supplying about 9.9% magnesium and 13% sulfur.Potassium sulfate (K2SO4) supplies roughly 41-45% potassium and 17-18% sulfur with no nitrogen, useful for adding K while tapering N in late flower.A heated vessel that vaporises elemental sulfur to suppress powdery mildew and mites in veg rooms; it carries fire, gas and timing risks and is never used in flower.Nitrogen (N) is the macronutrient plants use most, built into proteins, chlorophyll and nucleic acids; it drives leaf and stem growth and is fully mobile.Volatile sulfur compounds (VSCs) are trace sulfur-containing aroma molecules, especially prenylated thiols, that give cannabis its skunky, gassy smell at concentrations far below terpenes.EC (electrical conductivity) measures how well a solution conducts electricity, a proxy for total dissolved salts; growers read it in mS/cm to gauge nutrient strength.Secondary nutrients are calcium, magnesium and sulfur: macronutrients needed in substantial amounts but not included in the N-P-K numbers on a label.
