OneTaskBy Tim King · Updated
Soil pH Calculator
How Much Lime or Sulfur to Change Soil pH
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How the Soil pH Calculator works
This soil pH calculator tells you how much ground limestone to add to raise acid soil, or how much elemental sulfur to add to lower alkaline soil. Enter the area, the pH from your soil test, the pH you want and whether the soil is sandy, loam or clay, and it gives the total amount and the rate per square metre or per 100 square feet.
Soil pH controls how well plants take up nutrients. Most vegetables and lawns grow best between pH 6 and 7; blueberries, rhododendrons and azaleas want 4.5 to 5.5. Clay and soils rich in organic matter resist change (they have a high buffering capacity), so they need more lime or sulfur than sandy soil to move the same distance.
How the soil pH calculator works
The formula is rate = soil factor × pH change, where the factor is the rule-of-thumb amount to move the top 15 cm (6 in) of soil by one unit. For lime it is about 3 lb per 100 ft² on sand, 6 on loam and 8 on clay; for sulfur about 1, 1.5 and 2. For example, raising 20 m² of loam from 5.5 to 6.5 takes about 293 g of lime per m², or 5.9 kg in all. A second example: lowering 100 ft² of sandy soil from 7.0 to 6.0 needs about 1 lb of sulfur.
These are starting figures, not a prescription. A lab soil test that reports buffer pH gives a more exact lime requirement, and you should follow it where you have one. Apply no more than about 250 g of sulfur per m² at once; split bigger doses over several months.
Lime works in two to three months and sulfur takes longer, because soil bacteria have to convert it. Retest a season later to see how many more applications you need before planting acid-sensitive or lime-hating plants.
Lime to raise pH by one unit, and sulfur to lower it by one unit, by soil type
| Area | Sandy lime | Loam lime | Clay lime | Sandy sulfur | Loam sulfur | Clay sulfur |
|---|---|---|---|---|---|---|
| 5 m² | 0.73 kg | 1.46 kg | 1.95 kg | 0.24 kg | 0.37 kg | 0.49 kg |
| 10 m² | 1.46 kg | 2.93 kg | 3.91 kg | 0.49 kg | 0.73 kg | 0.98 kg |
| 15 m² | 2.20 kg | 4.39 kg | 5.86 kg | 0.73 kg | 1.10 kg | 1.46 kg |
| 20 m² | 2.93 kg | 5.86 kg | 7.81 kg | 0.98 kg | 1.46 kg | 1.95 kg |
| 25 m² | 3.66 kg | 7.32 kg | 9.76 kg | 1.22 kg | 1.83 kg | 2.44 kg |
| 30 m² | 4.39 kg | 8.79 kg | 11.72 kg | 1.46 kg | 2.20 kg | 2.93 kg |
| 35 m² | 5.13 kg | 10.25 kg | 13.67 kg | 1.71 kg | 2.56 kg | 3.42 kg |
| 40 m² | 5.86 kg | 11.72 kg | 15.62 kg | 1.95 kg | 2.93 kg | 3.91 kg |
| 45 m² | 6.59 kg | 13.18 kg | 17.58 kg | 2.20 kg | 3.30 kg | 4.39 kg |
| 50 m² | 7.32 kg | 14.65 kg | 19.53 kg | 2.44 kg | 3.66 kg | 4.88 kg |
| 60 m² | 8.79 kg | 17.58 kg | 23.43 kg | 2.93 kg | 4.39 kg | 5.86 kg |
| 70 m² | 10.25 kg | 20.50 kg | 27.34 kg | 3.42 kg | 5.13 kg | 6.83 kg |
| 80 m² | 11.72 kg | 23.43 kg | 31.24 kg | 3.91 kg | 5.86 kg | 7.81 kg |
| 90 m² | 13.18 kg | 26.36 kg | 35.15 kg | 4.39 kg | 6.59 kg | 8.79 kg |
| 100 m² | 14.65 kg | 29.29 kg | 39.06 kg | 4.88 kg | 7.32 kg | 9.76 kg |
| 110 m² | 16.11 kg | 32.22 kg | 42.96 kg | 5.37 kg | 8.06 kg | 10.74 kg |
| 120 m² | 17.58 kg | 35.15 kg | 46.87 kg | 5.86 kg | 8.79 kg | 11.72 kg |
| 130 m² | 19.04 kg | 38.08 kg | 50.77 kg | 6.35 kg | 9.52 kg | 12.69 kg |
| 140 m² | 20.50 kg | 41.01 kg | 54.68 kg | 6.83 kg | 10.25 kg | 13.67 kg |
| 150 m² | 21.97 kg | 43.94 kg | 58.58 kg | 7.32 kg | 10.98 kg | 14.65 kg |
FAQ
How much lime do I need to raise soil pH by 1?
As a rule of thumb, about 150 g per m² on sandy soil, 290 g on loam and 390 g on clay, or 3, 6 and 8 lb per 100 ft². Use ground limestone or dolomitic lime and dig it into the top 15 cm, then retest after a couple of months.
How do I lower the pH of my soil?
Work in elemental sulfur, which soil bacteria slowly turn into sulfuric acid. Allow about 50 g per m² on sand to 100 g on clay for each unit of pH. Aluminium sulfate and iron sulfate act faster but are easy to overdo, so sulfur is safer for most gardens.
When is the best time to add lime to soil?
Autumn or winter is best, so the lime has several months to react before spring planting. Do not lime at the same time as applying manure, because the reaction releases nitrogen as ammonia gas; leave a few weeks between the two.
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