Lime, gypsum, dolomite or oyster shell: which calcium for which job

Choose by what the soil test says needs to move. If pH is low, use a carbonate: Superfine AgLime in most cases, dolomite only when magnesium is also short, oyster shell when you want a slow release. If pH is already where you want it and calcium is low, use gypsum, which adds calcium without lifting pH.

pH is not a calcium test

pH measures acidity. It does not tell you how much calcium the soil holds. On a Hill Laboratories report, calcium, magnesium, potassium and sodium all count as bases, and total base saturation (their sum as a share of the cation exchange capacity, or CEC) rises and falls with pH (Hill Laboratories n.d. b). So a soil can reach a respectable pH on magnesium, potassium or sodium and still be short of calcium.

Calcium has its own line. Hill prefers the absolute amount, in me/100g, and gives 6 to 12 me/100g as the desirable range for agricultural and horticultural soils (Hill Laboratories n.d. a). A sandy soil with a CEC of 8 and 65% calcium saturation holds 8 × 0.65 = 5.2 me/100g, under the range. A loam with a CEC of 25 and only 40% calcium holds 25 × 0.40 = 10 me/100g, inside it.

That said, New Zealand soils generally have plenty of calcium for pasture, and the reason to apply lime is its carbonate, not its calcium (Morton 2019). Liming an acid soil for pH supplies calcium as well. Low calcium at a good pH is the job for gypsum.

Carbonate or sulphate: why some raise pH and some do not

Lime is calcium carbonate. The carbonate reacts with soil acid to form water and carbon dioxide, and pH rises. The calcium is what is left behind (Morton 2019). Gypsum is calcium sulphate, a neutral salt. It supplies calcium and sulphur but does not correct acidity, so it is not a liming material (Chen and Dick 2011; Heckman 1998).

Product What it is Effect on pH Choose it when
Superfine AgLime Calcium carbonate. Pure calcium carbonate is 40% calcium (Heckman 1998) Raises it pH is low and magnesium is adequate
Dolomite Lime Calcium magnesium carbonate Raises it, more slowly pH and magnesium are both low
Crushed Bluff Oyster Shells Shell, mostly calcium carbonate, with a liming potential very similar to it (Lee et al. 2008) Raises it, slowly You want a slow, long pH hold in a bed or mix
Gypsum and Granulated Gypsum Calcium sulphate. Pure gypsum is 23.3% calcium and 18.6% sulphur (Chen and Dick 2011). Granulated is the same mineral No lift pH is fine and calcium is low, or sodium is high, or sulphur is short
BioPhos A phosphorus input that also carries calcium The soil needs phosphorus. Every dose adds phosphorus, so do not use it to chase calcium
Calsil Boost Wollastonite, a calcium silicate Raises it. Calcium silicate has 60 to 90% of the neutralising value of calcium carbonate (Heckman 1998) You want silicon. Count its liming effect

New Zealand lime is sold on neutralising value (NV): its carbonate content, with pure calcium carbonate as 100. Otago limestones typically run 85 to 95 and dolomite is listed at 98 (Morton 2019). Pure dolomite is 109, because magnesium carbonate neutralises more acid per kilogram (Heckman 1998).

Speed: particle size sets the clock

Lime dissolves where it touches soil moisture and acid, so surface area sets the pace. In the Fertiliser Association's figures, limestone particles averaging 0.12 mm were 95% dissolved in six months, 0.23 mm particles 60%, 0.56 mm particles 22% and 1.2 mm particles 11% (Morton 2019). Fine lime acts sooner and runs out sooner, for a similar total pH lift. Ordinary ag lime typically needs 18 to 24 months for full effect. See our particle size clip.

Dolomite is harder rock that needs fine grinding to match limestone (Morton 2019), and its magnesium carbonate reacts more slowly than calcium carbonate (Heckman 1998). Crushed oyster shell is the same carbonate as a coarse grit, so expect it at the slow end. That is inference from the particle size figures, not a trial result.

Gypsum is different. About 2.5 g dissolves per litre of water, roughly 200 times the solubility of lime, so its calcium moves down with rain and irrigation. In a wet climate it can leach out within a couple of years (Chen and Dick 2011).

Dolomite: not on everything, and not never

Two opposite rules do the rounds: the old New Zealand garden habit of dolomite on everything, and the imported living soil rule of never using it. Both are grower lore, and both skip the soil test.

Dolomite is the right choice when pH and magnesium are both low. Compared on an equal carbonate basis it limes like limestone and supplies magnesium as well (Morton 2019), and where a soil test calls for magnesium, a magnesian limestone is the most economical way to supply it (Heckman 1998). For container substrates, Purdue Extension recommends dolomitic or calcitic limestone to set pH, and notes that calcitic reacts faster (Owen and Lopez 2015).

The trouble is the dose. Worked example 3 shows a full dolomite liming adding more magnesium than Hill's whole desirable range. Calcium is the stronger flocculant of the two, and magnesium can disperse clay when it dominates the exchange sites, although those studies used magnesium saturations above 75%, well beyond field soils (Chaganti and Culman 2017). Excess magnesium takes work to undo: calcium from gypsum has to displace it from the exchange sites (Chen and Dick 2011). If pH is fine and magnesium is low, use magnesium sulphate, not a lime.

The calcium to magnesium ratio: a wide target

The "ideal soil" of 65% calcium, 10% magnesium and 5% potassium came from Firman Bear's work in New Jersey in the 1940s. Kopittke and Menzies (2007) reviewed the trials, including Bear's and Albrecht's own data, and concluded that within the ranges commonly found in soils, chemical, physical and biological fertility is generally not influenced by the ratios of calcium, magnesium and potassium. A later review agreed: no study found a best ratio, and in Pennsylvania field trials calcium to magnesium ratios from 1.8 to 36.9 had no effect on maize grain yield (Chaganti and Culman 2017).

Hill's "ideal" base saturation ranges are 50 to 75% calcium and 5 to 15% magnesium (Hill Laboratories n.d. b). That spans ratios from 50 ÷ 15 = 3.3 to 75 ÷ 5 = 15. Dan's position fits the evidence: there is no single magic number, and the workable range is wide. Check first that calcium and magnesium are each sufficient. When a test shows magnesium crowding calcium, fix the ratio with gypsum, ag lime or oyster grit.

Choosing from a Hill Laboratories report

Read three lines: pH, calcium and magnesium. Hill's desirable ranges are 6 to 12 me/100g for calcium and 1 to 3 me/100g for magnesium (Hill Laboratories n.d. a).

pH Calcium Magnesium Choice
Low Low or fine Fine or high Superfine AgLime. Oyster shell for part of it if you want a slower tail
Low Low or fine Low Dolomite for the magnesium, ag lime for the rest of the pH lift
In range Low Fine or high Gypsum
In range Fine Low Magnesium sulphate. No lime
High Low Any Gypsum for calcium, no carbonates. Elemental sulphur if pH must come down
In range Fine Fine Nothing. You may not need any of these

"Low" depends on what you grow in. Hill rates pH 5.6 to 6.5 as medium for a loam (Hill Laboratories n.d. a). Dan's range for living soil is 6.2 to 7.2, with a target of 6.6 for his peat based mix, which is the range for licensed medical cannabis cultivators running it. The soil planner reads a Hill report the same way: it brings in dolomite only when pH is low and magnesium is short, and caps it at the lime requirement.

Worked examples

1. A liming rate

The Fertiliser Association's rule of thumb is that 1 tonne per hectare of good lime lifts the pH of a mineral soil by about 0.1. Its table gives 1,111 kg/ha per 0.1 unit for lime with an NV of 90, which is 1,000 ÷ 0.90 (Morton 2019).

  • Bed at pH 5.7, target 6.2: five steps of 0.1.
  • 5 × 1,111 = 5,555 kg/ha. One kg/ha is 0.1 g/m², so 556 g/m², or 5.6 kg for a 10 m² bed.
  • With an NV of 95: 5 × 1,000 ÷ 0.95 = 5,263 kg/ha, or 526 g/m².

The rule is written for mineral soils under pasture. It is a starting point for a garden bed, not for peat based mixes. The lime requirement on a Hill report assumes lime that is 90% pure (Hill Laboratories n.d. a), so adjust it the same way.

2. A gypsum rate

A bed tests at pH 6.4 with calcium at 4.0 me/100g and a volume weight of 0.90 g/mL. The aim is 6.0 me/100g, a lift of 2.0. Hill's conversion is me/100g × 200 × volume weight = mg of calcium per litre of soil (Hill Laboratories n.d. a).

  • 2.0 × 200 × 0.90 = 360 mg of calcium per litre of soil.
  • One square metre dug to 150 mm is 150 litres: 360 × 150 = 54,000 mg, or 54 g of calcium.
  • Pure gypsum is 23.3% calcium: 54 ÷ 0.233 = 232 g/m², or 2.3 t/ha.

This assumes all the calcium stays on the exchange sites. Some will leach, so retest.

3. Why dolomite overloads magnesium

  • The same 0.5 pH lift with dolomite at an NV of 98: 5 × 1,020 = 5,100 kg/ha, or 510 g/m² (Morton 2019).
  • Pure dolomite is 13.2% magnesium (24.3 ÷ 184.4), the ceiling for a commercial product: 510 × 0.132 = 67 g of magnesium per square metre at most.
  • Hill's magnesium conversion uses 122 in place of 200: 1 me/100g is 122 × 0.90 × 150 = 16,470 mg, or 16.5 g per square metre.
  • 67 ÷ 16.5 = up to 4.1 me/100g of added magnesium once it has all dissolved, against a desirable range of 1 to 3.

What to do

  1. Get a soil test before you buy any calcium product.
  2. Read pH, calcium and magnesium as three separate lines.
  3. If pH is low, lime. Ask for the neutralising value and adjust the rate.
  4. Use dolomite only when magnesium is also low, and only for part of the lime.
  5. If pH is fine and calcium is low, use gypsum.
  6. Build the order in the fertiliser calculator. Its pH amendment step offers lime at 500, 1,000 or 2,000 g/m², gypsum at 250, 400 or 600 g/m², and elemental sulphur. Examples 1 and 2 land beside its light rates.
  7. Retest before repeating.

References

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