How to test and change soil pH

Why pH matters

pH controls which nutrients plants can take up. In acid soil, aluminium and manganese dissolve and become toxic. Aluminium toxicity shows as short, stubby roots, and it rarely occurs above pH 5.2 in a lab (CaCl₂) test. Molybdenum becomes scarce, which twists the leaves of cauliflowers and other brassicas. In alkaline soil, iron, zinc, manganese, copper and boron get locked up. Across Australia, around 50 million hectares of agricultural land, about half of it, has surface soil at pH 5.5 or below. Yet more than 60 % of South Australia's farming soils are alkaline. Your garden can sit anywhere on that range, and years of compost, manure and fertiliser can shift it further.

How to test at home

1. Sample the root zone. Manutec suggests soil from 10–15 cm deep for shallow-rooted plants and 25–30 cm for trees. Scrape the mulch away first.
2. Mix several samples. NSW DPI's field method bulks 30 cores for each area. For a garden bed, a handful of samples mixed together does the same job. One spot can mislead.
3. Use a colour kit or a meter. Australian colour kits use an indicator dye and a white barium sulfate powder, a method developed from CSIRO research. Mix the soil with the dye, dust it white, and match the colour. Colour charts are accurate to about ±0.5 pH. A calibrated meter or a laboratory test is more precise.
4. Don't test right after feeding. Results can mislead for about three months after adding lime, fertiliser or compost. Soil can also read up to 0.5 higher in summer than in winter.

Why kits and labs disagree

Home kits and most meters measure pH in water. Most Australian soil laboratories, and nearly all Australian lime tables, measure it in a weak calcium chloride solution, written pH(CaCl₂). That usually reads 0.5–0.8 lower for the same soil, although NSW DPI records gaps from nothing to 2 units. New Zealand labs, by contrast, generally report pH in water. So a lab report of 5.3 in CaCl₂ may be about 6 on your kit. Before you compare numbers or follow a rate table, check which method was used. The soil pH calculator asks for it.

Signs of acid and alkaline soil

Symptoms overlap with other problems such as waterlogging and pests, so test before you treat.

Raising pH: lime or dolomite

Garden or agricultural lime is calcium carbonate. Dolomite is calcium magnesium carbonate, with about 12 % magnesium. Both neutralise acidity by about the same amount. Their neutralising value is around 95 for good commercial products, against 100 for pure lime. NSW DPI's advice is to choose mostly on price, and to use dolomite where a soil or leaf test shows magnesium is short. Tasmania's agriculture department warns that constant dolomite use can unbalance calcium and magnesium.

How much depends on texture, because clay and organic matter buffer the soil. South Australia's texture formula gives about 200 g/m² per pH unit on sand, 300 on sandy loam and 400 on loam or clay loam, for lime worked into the top 10 cm. Use a quarter less on pale soils low in organic matter. Tasmania's figures run to 500 g/m² on clay. New Zealand soils, especially volcanic and peaty ones, are far more strongly buffered. The NZ Fertiliser Association puts the effect of 1 t/ha of lime at about 0.1 pH. That is roughly 1 kg/m² per unit, so a dressing that would shift an Australian sandy loam may barely move a New Zealand volcanic soil.

Limit each application to one pH unit and no more than about 750 g/m², work it into the top 10 cm, and retest after a year before adding more. Surface-applied lime moves down only about 2.5 cm a year. NSW DPI also advises not to lift topsoil much above pH 6.0 in CaCl₂ (about 6.5–6.8 on a kit), because trace elements start to lock up.

Lowering pH: sulfur and what doesn't work

Elemental sulfur is the standard product. Soil bacteria turn it into sulfuric acid, which takes 3–6 months and slows in cold, wet soil. Purdue University's extension guide gives about 37 g/m² per pH unit on sand, 73–98 g/m² on loam and 146 g/m² or more on clay. Garden sulfur labels in Australia and New Zealand suggest a little less per dose. Split large amounts and retest between doses.

• Iron sulfate acts faster but takes about eight times the sulfur rate, and can tie up phosphorus.
• Aluminium sulfate takes about seven times the rate and risks aluminium build-up. Oregon State University's extension service does not recommend it.
• Sulfate of ammonia fertiliser acidifies slowly, only about 0.1–0.2 pH a year at normal feeding rates.

Calcareous soils can't realistically be acidified. If a spoonful of soil fizzes when you drip vinegar on it, it contains free lime, as parts of Adelaide and Perth's coastal limestone sands do. Every gram of sulfur is then neutralised by the carbonate. Purdue calls correcting such soils futile, and the Royal Horticultural Society says they cannot realistically be acidified. Grow blueberries, azaleas and camellias in pots or raised beds of acid potting mix instead.

Timing and mistakes

• Lime early. Lime starts working once the soil is moist, but most of the change comes over 12–18 months. New Zealand guidance says the maximum effect comes after 18–24 months. Lime a new bed months before planting if you can.
• Keep lime and nitrogen fertiliser apart. Ammonia is lost fastest from alkaline surfaces, so freshly limed soil wastes urea and manure nitrogen. Some Australian lawn guides suggest leaving about two weeks between liming and fertilising.
• Don't overshoot. Over-liming is harder to undo than under-liming, and it causes trace element deficiencies.
• Don't lime natives by habit. The group that needs special care, the protea family (banksias, grevilleas, hakeas, waratahs), is sensitive to phosphorus. Low-phosphorus feeding matters more for them than a pH change. Test before you lime.

Questions