Gypsum for Cannabis: When It Helps and When It Doesn’t
Gypsum, or calcium sulfate, gives cannabis calcium and sulfur, but Penn State Extension says it is not a liming material and cannot neutralize acidic soil. UF/IFAS lists gypsum as one sulfate source for hemp when a soil or tissue test shows a need. In one Oregon on-farm trial on serpentine soil, hemp did equally well with or without gypsum.
What Is Gypsum?
Gypsum is calcium sulfate. UF/IFAS lists it among sulfate sources for hemp, along with ammonium sulfate, magnesium sulfate and potassium sulfate, and notes sulfate is used because elemental sulfur converts too slowly for the current crop (UF/IFAS EDIS). For sulfur’s role in the plant, see sulphur deficiency.
Does Gypsum Change Soil pH?
Penn State Extension says calcium sulfate (gypsum) and magnesium sulfate are not liming materials, even though they contain calcium and magnesium, because they cannot neutralize soil acidity (Penn State). If your soil is acidic, lime is the tool, as our pH guide explains.
Source: Penn State Extension, Soil Acidity and Aglime.
Data for this chart:
| Material | Neutralizes acidity? | Source |
|---|---|---|
| Limestone (calcitic, dolomitic) | Yes | Penn State Extension |
| Gypsum | No | Penn State Extension |
| Magnesium sulfate | No | Penn State Extension |
Source: UF/IFAS EDIS SL521, nutrient management recommendations for hemp. The rate is pounds of sulfur, not pounds of gypsum.
Data for this chart:
| Material | Role in SL521 | Source |
|---|---|---|
| Gypsum (calcium sulfate) | Listed sulfate source | UF/IFAS SL521 |
| Ammonium sulfate | Listed sulfate source | UF/IFAS SL521 |
| Magnesium sulfate | Listed sulfate source | UF/IFAS SL521 |
| Potassium sulfate | Listed sulfate source | UF/IFAS SL521 |
| Sulfur rate when required | 15 to 20 lb S per acre | UF/IFAS SL521 |
When Does Cannabis Benefit From Gypsum?
UF/IFAS says sulfur, calcium and magnesium needs for hemp may be identified through soil or leaf tissue tests, and gives 15 to 20 lb of sulfur per acre if sulfur is required (UF/IFAS EDIS). In southwest Oregon, a hemp farm that had used large amounts of gypsum on serpentine soils ran a side-by-side trial, and the crop did equally well with and without it, and the grower reported saving $5,000 to $6,000 (Oregon State).
How Do Growers Apply Gypsum?
UF/IFAS says sulfur, calcium and magnesium needs for hemp may be identified through soil or leaf tissue tests (UF/IFAS EDIS). Check the product label and local rules for your crop. If leaves show calcium problems, our calcium deficiency post covers diagnosis.
Preparing ground for spring planting? Have a look at our outdoor seeds.
Sources
Some sources below studied hemp or other crops rather than drug-type cannabis, and their findings are reported as results for those crops.
- Penn State Extension: Soil Acidity and Aglime
- UF/IFAS EDIS SL521: Nutrient Management Recommendation Series, Hemp
- Oregon State Extension: Serpentine Soils Hemp Gypsum Trial
- Oregon State EM 9239: Soil, Seedbed Preparation and Seeding for Hemp
Read this as general growing information. Kind Seed Co drew it from the references above.
FAQ
Gypsum vs lime: what is the difference?
Lime raises soil pH because its carbonate, oxide or hydroxide neutralizes acidity. Penn State Extension says gypsum, which is calcium sulfate, is not a liming material because it cannot neutralize soil acidity, even though it contains calcium. Lime corrects acidity, while gypsum is a source of calcium and sulfur.
Do you need a soil test before using gypsum?
Yes. UF/IFAS says sulfur, calcium and magnesium needs for hemp may be identified through soil or leaf tissue tests. In an Oregon on-farm trial, a hemp grower who had applied large amounts of gypsum for years found the crop did equally well without it. The grower reported saving $5,000 to $6,000, Oregon State Extension says.
Did gypsum help hemp in the Oregon trial?
No difference was found. In a side-by-side trial on serpentine soils in Josephine County, Oregon State Extension reported that hemp performed equally well with and without gypsum, tissue tests showed no problems in untreated areas, and the grower reported saving $5,000 to $6,000 in material and labor without losing yield or quality.
Does gypsum supply sulfur?
Yes. Gypsum is calcium sulfate, and UF/IFAS lists it among sulfate sources for hemp, along with ammonium sulfate, magnesium sulfate and potassium sulfate. UF/IFAS recommends sulfate forms because elemental sulfur converts too slowly to meet the current crop's needs, and gives 15 to 20 lb sulfur per acre when needed.
