Soil pH Adjustment Calculator
Calculate garden lime or sulfur needed to adjust soil pH.
Enter current and target pH with soil type to get the application rate in pounds per 100 sq ft.
Soil pH measures the acidity or alkalinity of your soil on a scale from 0 to 14. Most plants grow best in soil with a pH between 6.0 and 7.0. Adjusting pH ensures plants can absorb nutrients effectively.
pH Scale Reference:
| pH | Classification | Common Plants |
|---|---|---|
| 4.5–5.5 | Strongly acidic | Blueberries, azaleas, rhododendrons |
| 5.5–6.0 | Moderately acidic | Potatoes, strawberries |
| 6.0–6.5 | Slightly acidic | Most vegetables, flowers, herbs |
| 6.5–7.0 | Neutral | Lawns, beans, lettuce |
| 7.0–7.5 | Slightly alkaline | Asparagus, beets, cabbage |
| 7.5–8.0 | Moderately alkaline | Some desert plants |
To raise pH (more alkaline), add lime:
- Ground limestone (calcium carbonate): the baseline every rate table is written against
- Dolomitic limestone: same neutralising power, and it adds magnesium
- Hydrated lime (calcium hydroxide): about a third stronger, so you need roughly a quarter less
- Wood ash: around half the strength of limestone, so about twice the quantity
- Clay soils need more than sandy soils for the same pH move, because there is more to buffer
To lower pH (more acidic), add sulfur:
- Elemental sulfur: the standard, and the slowest at 3 to 6 months
- Iron sulfate: faster, but it takes roughly six times the weight for the same pH drop
- Aluminium sulfate: fastest of the three, also about six times the weight, and easy to overdo
Amendment rates per 100 sq ft (9.3 m²), for a 1 point change
The kilogram figures are the ones the calculator uses. The pounds beside them are the same number converted, not a separate rule of thumb.
| Soil type | Ground limestone | Elemental sulfur |
|---|---|---|
| Sandy | 1.5 kg (3.3 lb) | 0.5 kg (1.1 lb) |
| Loam | 3.5 kg (7.7 lb) | 1.0 kg (2.2 lb) |
| Clay | 5.0 kg (11.0 lb) | 1.5 kg (3.3 lb) |
When to use this calculator:
- Preparing garden beds for specific plants
- Correcting pH after a soil test
- Establishing blueberry patches (need acidic soil)
- Improving lawn health
Tips:
- Always test your soil pH before amending: guessing can make things worse.
- Apply amendments in fall for best results; they take weeks to months to fully work.
- Never change pH by more than 1 point per season.
- Retest soil 2–3 months after application to check results.
- Organic matter (compost) gradually moves soil toward neutral pH over time.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
SuperGlobalCalculator is independently built and maintained. See how we build and verify our calculators.