Gypsum vs lime: which one does your soil need?

What each one does

The Victorian Brown Book says the only way to reverse soil acidification is to raise pH with alkaline minerals such as agricultural lime. A 2022 GRDC Update paper's soil amelioration framework lists lime, not gypsum, for acidic topsoil or subsoil. For lime rates, see how much lime per hectare.

Which problem do you have?

Sour (acidic) soil signs in Victorian pastures include uneven growth, poor legume nodulation, stunted roots, acid-tolerant weeds such as sorrel, and trouble establishing lucerne and phalaris. Confirm it with a lab pH test; see reading a soil test.

Sodic (dispersive) soil signs and tests, from the Brown Book unless noted:

• Milky puddles or dams, surface crusting, hard-setting topsoil, and 'soupy' soil when wet.
• Dispersion test: place three air-dry, pea-sized crumbs in rain or distilled water. A milky cloud around them after 20 hours means the soil disperses. If not, retest with moist, worked soil.
• Jar test: two jars of soil and rain water, with gypsum added to one. If the soil settles only in the gypsum jar after 24 hours, it is likely to respond to gypsum.
• ESP (exchangeable sodium percentage) on a soil test: 6–14% is sodic, 15% or more is strongly sodic. ESP alone can mislead, so do a dispersion test too.
• Queensland guidance says there is little point applying gypsum unless your soil disperses.
• Sodic subsoils are often alkaline (pH above 8.5).

Gypsum rates and catches

• Southern Victoria: typically about 2.5 t/ha, applied every 3–5 years.
• Queensland: usually about 2.5 to more than 10 t/ha, often set by cost as much as anything. The Queensland Government says there is no defined rate.
• Surface gypsum improves dispersive topsoil quite quickly but may take several years to reach the subsoil. It helps less in texture contrast soils, or where the dispersive clay starts deeper than 40 cm (Queensland).
• Soils with less than 15% clay show little response. Gypsum may not fix crusting caused by slaking alone (often low organic matter).
• By-product gypsum may contain cadmium or fluoride; mined gypsum can contain lime, salt, sand or clay. Check the label. Added calcium may cause magnesium deficiency.
• Gypsum is a short-term fix. Build organic matter for the long term.

When lime helps sodic soil, and when you need both

Lime also supplies calcium, but it is much less soluble. The Brown Book says lime on sodic soil acts like gypsum but is much slower and less effective. The 2022 GRDC paper re-examined 1970s trials on sodic grey clays in northern NSW: lime was slow to suppress dispersion, but significant yield benefits eventually showed up.

• Lime is only an option for sodicity where topsoil pHCa is below about 6.6, which is acidic enough for it to dissolve (GRDC).
• Surface dispersion on neutral or acidic soil: the GRDC framework lists a gypsum–lime blend.
• Acidic and dispersive soil: lime plus gypsum.
• Alkaline sodic soil: the framework lists gypsum (with elemental sulphur, whose economics it calls uncertain), not lime.

Blended granules exist. Calciprill S 14 is made from gypsum and calcium carbonate (14% S, per the maker's brochure). Its crop rate guide (70–500 kg/ha, depending on crop) is aimed at sulphur supply, far below the tonnes per hectare used to treat sodic soil. Compare it with straight Calciprill lime.

Calcium, sulphur and magnesium as nutrients

• At about 23% Ca and 18% S, a tonne of gypsum carries roughly 230 kg calcium and 180 kg sulphur.
• If you only need sulphur, compare sulphur products per kg of S. See sulphur deficiency in canola.
• Lime supplies calcium; browse calcium products, such as Calciprill.
• Dolomite adds magnesium. The Brown Book says it is used as a magnesium source on magnesium-deficient soils; Magprill is a granulated dolomite.

Always check with your agronomist before acting.