| Pot | Dimensions | Estimated mix volume | Gypsum per pot |
|---|---|---|---|
| Small nursery pot | 18 cm × 16 cm | 3.66 L | 8.1 g |
| Medium patio pot | 30 cm × 25 cm | 15.90 L | 35.3 g |
| Large tree tub | 45 cm × 35 cm | 50.10 L | 111.3 g |
| Window box | 60 cm × 18 cm × 18 cm | 17.50 L | 38.9 g |
| Raised planter | 80 cm × 35 cm × 30 cm | 75.60 L | 168.0 g |
| Herb trough | 50 cm × 20 cm × 15 cm | 13.50 L | 30.0 g |
- Pot volume (round): V = π × (d ÷ 2)² × h
- Pot volume (rectangular): V = L × W × h
- Fill adjustment: Vused = V × (fill% ÷ 100)
- Volume method: Gypsum(g) = Vused(L) × rate(g/L) ÷ purity
- Surface method: Gypsum(g) = Area(m²) × rate(g/m²) ÷ purity
- Weight method: Soil(kg) = Vused(L) × density(kg/L), then Gypsum(g) = Soil(kg) × rate(g/kg) ÷ purity
- Totals: Total gypsum = gypsum per pot × number of pots
- Pick the pot shape and your measurement unit.
- Enter pot dimensions and fill level percentage.
- Select a method: volume, surface, or weight.
- Enter an application rate that matches your plan.
- Adjust purity, then set number of pots.
- Optionally add bag size and price for budgeting.
- Press Calculate Gypsum to view results above.
- Gypsum improves soil structure and helps replace sodium with calcium.
- For container mixes, use conservative rates and mix evenly.
- If you already use high-calcium fertilizers, avoid overapplication.
- When uncertain, start low and observe plant response.
Gypsum Per Pot Guide
Why Gypsum Matters
Gypsum supplies calcium and sulfur while improving structure in sodic or compacted mixes. In containers it can reduce crusting, increase water movement, and support root aeration without raising pH like lime. Use it when soil tests show sodium issues or low calcium availability. Pair gypsum with deep watering to leach salts, and ensure drainage holes stay open to prevent buildup. For seedlings, start at the low end and observe growth closely.
Rates You Can Defend
For mix-in applications, many growers start near 1–5 g per liter of potting mix. For surface topdressing, a practical planning band is 80–200 g per square meter, split across seasons. When dosing by soil weight, 2–6 g per kilogram often lands in a conservative zone. Log EC readings monthly; reapply only after clear improvement, because excess sulfate adds cost without benefits.
Pot Geometry Drives Quantity
The calculator converts your pot to volume or surface area using standard geometry. A 30 cm round pot filled to 25 cm holds about 17.7 L at 100% fill. At 90% fill and a 2.0 g/L rate, gypsum is roughly 39 g per pot before purity adjustment. Large boxes multiply volume fast, so verify inside dimensions rather than rim size for accuracy.
Purity, Bags, and Budget
Not all products are equal. If purity is 90%, divide by 0.90 to avoid under-applying. Multiply the per-pot dose by the number of pots to get totals, then round up by bag size. With 6 pots at 39 g each, total is about 0.234 kg, so one 10 kg bag easily covers it. Budget planning is simple: total grams become kilograms, then bags round up. If price is entered, multiply bags by price to estimate spend.
FAQs
What rate should I start with in containers?
Start conservatively. Many mixes tolerate 1–2 g per liter as a trial. If a soil test shows sodium problems, increase gradually and leach well. Track plant response and electrical conductivity rather than guessing.
Does gypsum change soil pH?
Gypsum is generally pH neutral, so it adds calcium without the strong pH lift you get from lime. It can improve structure and infiltration, but it will not replace pH correction when soil is too acidic.
When should I use the surface method instead of volume?
Use the surface method when you topdress existing pots. Measure pot surface area, apply the chosen g/m² rate, then water in. It is also handy when you do not know the exact soil depth.
Why does purity affect the result?
Lower purity means less active gypsum per kilogram, so you need more product to deliver the same calcium and sulfate. The calculator divides by the purity fraction, increasing grams per pot when purity is reduced.
How accurate are pot volume estimates?
They are as accurate as your measurements. Use inside dimensions, not rim-to-rim. Irregular shapes, sloped sides, and thick drainage layers reduce true volume, so the fill percentage helps keep estimates realistic.
What soil density should I enter for the weight method?
Most light potting mixes fall around 0.4–0.8 kg/L, while heavier mineral blends can be higher. If unsure, start with 0.7 kg/L and adjust after weighing a known container volume of your mix.
Is it safe to overapply gypsum?
Excess gypsum usually wastes money and can raise sulfate levels unnecessarily. In containers, heavy doses may increase salt load if drainage is poor. Apply in steps, water through, and retest soil or runoff before repeating.