Calcium/Magnesium Amendment Calculator Guide
Use this calcium and magnesium amendment calculator to estimate pounds of dolomitic lime or gypsum for your garden area. Dolomitic lime raises pH and supplies Ca and Mg; gypsum adds calcium without changing pH for clay improvement.
How to use the calcium/magnesium amendment calculator
Enter garden area in square feet and select dolomitic lime or gypsum. For lime, enter current and target pH. For gypsum, the calculator applies a general improvement rate.
Results show total amendment weight in pounds — confirm with a soil test before applying.
Amendment rate formulas
Total lbs = (Area ÷ 1,000) × Rate per 1,000 sq ft. Dolomitic lime: ~50 lbs/1,000 sq ft to raise pH ~0.5 units; ~100 lbs for ~1.0 unit. Gypsum (general): 40 lbs/1,000 sq ft. Gypsum (severe): 80 lbs/1,000 sq ft.
Lime is only recommended when current pH is below target. Gypsum does not replace lime for acid soils needing pH correction.
- Total lbs = (area / 1000) × rate
- Dolomitic lime: Ca + Mg + raises pH
- Gypsum: Ca + S; neutral pH effect
- Always soil test before amending
Worked example
A 1,000 sq ft garden with pH 6.0 targeting 6.5 using dolomitic lime: pH gap = 0.5 → rate = 50 lbs per 1,000 sq ft.
Total lime = (1000 ÷ 1000) × 50 = 50 lbs. For gypsum at general rate on the same area: 40 lbs.
When to use lime vs gypsum
Dolomitic lime corrects acidic pH and supplies calcium and magnesium — ideal when soil tests show low pH and need for both nutrients. Gypsum improves clay structure and supplies calcium without raising pH — use when pH is acceptable but calcium or sodium issues exist.
Application tips
Apply lime or gypsum evenly and incorporate into the top 6–8 inches. Lime reacts slowly — retest pH after 3–6 months. Do not apply lime and sulfur in the same season.
- See soil pH adjustment calculator for sulfur needs.
- Gypsum application calculator uses similar rate math.
- Split heavy lime rates across seasons on sensitive crops.
- Pelletized lime is easier to broadcast than pulverized.