How the calculator works it out
The amount of lime needed to shift pH depends on how strongly the soil resists change. That resistance is called buffering. Sand has little of it, clay and organic matter a lot. The calculator uses the texture formula published by South Australia's Department of Primary Industries (PIRSA), which comes from CSIRO work:
texture factor: sand 200 · sandy loam 300 · loam and clay loam 400 · heavy clay 500 · volcanic or peaty 1,000
bagged product = lime × 100 ÷ neutralising value · −25 % on soils low in organic matter
The heavy clay figure is the top of Tasmania's published range for clay (333–500 g/m² per unit). The volcanic and peaty figure reflects New Zealand's Fertiliser Association guidance: about 1 t/ha of lime raises pH by only 0.1 on typical NZ soils. That is 3–5 times more lime per unit than the Australian texture tables, so NZ gardeners on rich volcanic loams should choose that option.
Each dose is capped at one pH unit and 750 g/m². Anything more is split into applications a year apart. Both limits come from state agriculture departments. Overshooting is harder to fix than undershooting, and lime keeps working for more than a year.
Home kit or lab test?
Home colour kits and meters measure pH in water. Most Australian laboratory tests, and nearly all the lime tables used by agronomists, measure it in a calcium chloride solution. That usually reads 0.5–0.8 lower for the same soil, although the gap ranges from nothing to 2 units depending on the soil. The calculator keeps your target on the same scale as your reading, so choose the method that matches your test. The difference in pH is what sets the lime rate, so the result is similar either way. But a lab result of 5.5 is not the same soil as a home kit reading of 5.5.
Which pH for which plants?
| Plants | Target (home kit, water) | Notes |
|---|---|---|
| Most vegetables and flowers | 6.0–7.0 | Most nutrients are available across this range. |
| Lawns | 5.5–6.5 | Couch and kikuyu tolerate acid soil well. |
| Brassicas with clubroot | 7.0–7.5 | Liming reduces clubroot. Above 7.5, iron, manganese, boron, zinc and copper become harder to take up. |
| Camellias | 5.0–6.5 | |
| Blueberries, azaleas, rhododendrons | 4.5–5.2 | Easier to grow in pots or mounds of acid mix than to acidify a whole bed. |
| Australian natives | Test first | In a trial of 810 native species, about 82 % showed no phosphorus toxicity. The sensitive group is the protea family: banksias, grevilleas, hakeas and waratahs. They need low-phosphorus feeding more than a pH change, so don't lime them unless a test shows very acid soil. |
Worked example: a 10 m² vegie bed at pH 5.5
- Home kit reads 5.5, target 6.5 for vegetables: a rise of 1.0 unit.
- Loam: 400 g/m² of pure lime per unit. Bagged lime at a neutralising value of 95 is 400 × 100 ÷ 95 = 421 g/m².
- For 10 m²: 4.2 kg, one 5 kg bag. One application, dug into the top 10 cm.
- Retest in 12 months. If you had needed to go from 4.8 to 6.5, that is 1.7 units: the calculator would give two doses of about 358 g/m², a year apart.
Lowering pH
Garden sulfur is turned into sulfuric acid by soil bacteria. That makes it slow, taking 3–6 months, and slower in cold, wet soil. Rates per pH unit come from Purdue University's extension guide: about 37 g/m² on sand, 73–98 g/m² on loam and 146 g/m² or more on clay. The calculator splits anything above 100 g/m² into separate doses, the highest single rate on Australian and New Zealand garden sulfur labels. On soils that fizz when you drip vinegar on them, which is common in parts of Adelaide and on Perth's coastal limestone sands, the free lime neutralises the sulfur. There, acidifying a bed is not realistic. Grow acid-lovers in pots.
The soil pH guide covers testing, symptoms and the difference between lime and dolomite in more depth.
Soil pH questions
How much lime do I add to raise pH by 1?
About 200 g/m² on sand, 300 g/m² on sandy loam and 400 g/m² on loam or clay loam (pure lime, top 10 cm). Bagged garden lime is a little weaker, so add about 5 %. Strongly buffered volcanic or peaty soils need far more, around 1 kg/m² per unit.
Should I use lime or dolomite?
Both raise pH by about the same amount. Dolomite also adds about 12 % magnesium. Use it where a soil or leaf test shows magnesium is short, otherwise choose on price. Using only dolomite year after year can oversupply magnesium.
How long does lime take to work?
It starts once the soil is moist and does most of its work over 12–18 months. Lime moves only about 2.5 cm a year on its own, so dig it into the top 10 cm rather than leaving it on the surface.
Sources and method
- PIRSA / Acid Soils SA, Lime for acid soils (2018) – texture formula, 25 % reduction on low-carbon soils, one unit per application.
- NSW DPI, Agfact AC.19 Soil acidity and liming (3rd edition) – neutralising values, timing, CaCl₂ versus water readings, upper pH limit.
- Tasmania NRE, Soil pH and liming – rates by soil type, 750 g/m² maximum per application, target ranges, dolomite.
- Fertiliser Association of New Zealand, Lime use booklet (2019) – buffering of NZ soils.
- Purdue University, HO-65, and Oregon State University, EC 1560 – sulfur rates by texture, calcareous soils.
- AUSVEG, Clubroot management in brassica vegetables (2024); Australian Native Plants Society, Phosphorus needs of Australian plants.
Written by DueRiva · Last reviewed 23 September 2026 · Report an error