Why liquid gypsum is a suspension, not a solution

Soil chemistry

Why liquid gypsum is a suspension, not a solution

By Joe Barrington, Founder of Dr Forest · August 2026 · Edited 24 Sep 2026

What is liquid gypsum, and how is it different from bagged gypsum?

Gypsum barely dissolves — about 2.5 g per litre of water, then it stops. Almost everything about the liquid form follows from that number.

Liquid gypsum is natural calcium sulphate dihydrate (CaSO₄·2H₂O), milled to a few microns and held as a suspension in a liquid carrier. Ours is mined gypsum in fulvic acid. It is not a true solution. "Liquid" on the label does not mean "fully soluble." Ours settles — shake it.

This page is composition and form. Whether it helps heavy clay is a different question — that sits in the clay-soil guide.

What is gypsum, chemically?

Gypsum is a naturally occurring sulphate mineral. The formula is CaSO₄·2H₂O: one calcium ion, one sulphate ion, and two molecules of water built into the crystal itself. That water is not dampness on the surface. It is structural, which is why heating gypsum drives the water off and converts it to plaster.

Work the percentages out from the formula and pure gypsum comes to 23.3% calcium and 18.6% sulphur by mass, with water of crystallisation taking 20.9%. Those are arithmetic rather than marketing figures, and the same numbers appear in agronomic guidance for agricultural gypsum. Anything sold as gypsum claiming much more calcium has either been baked dry or is not gypsum.

One wrinkle on labels. Gypsum is a pure calcium sulphate, so those two figures are the whole of it, but UK and European fertiliser rules declare calcium as CaO and sulphur as SO₃. The convention is left over from when analysis meant burning a sample and weighing the oxides. The same pure gypsum therefore reads 32.6% CaO and 46.5% SO₃ on a label. Nothing has been added and no oxide is present. It is the same calcium and the same sulphur, counted a different way.

Dr Forest Liquid Gypsum carries 19.55% calcium and 15.31% sulphur (27.35% CaO, 38.23% SO₃). The first pair are the elemental figures by weight in volume; the oxide equivalents are given alongside because some soil reports quote them that way. Its neutralising value is zero, so it is not a liming material and will not correct an acid soil. It adds calcium without raising pH.

It is also very soft. Gypsum defines hardness 2 on the Mohs scale, soft enough to mark with a fingernail, which is why it mills cheaply enough for a micronised grade to make commercial sense.

Is liquid gypsum a solution or a suspension?

A suspension. This is the point at which most descriptions of the product go quiet.

Put gypsum in water and it stops dissolving at about 2.5 grams per litre, near room temperature. Laboratory measurements land between 2.53 and 3.13 g/L as calcium sulphate dihydrate depending on who did the work and how, and FAO puts it at about a quarter of one percent, which is the same figure said another way. Take whichever you like. The limit is low.

What that ceiling means with a watering can in your hand: nine litres of water carrying all the gypsum it can hold has about 22 grams in it. To deliver 100 grams in true solution you would need around 40 litres of water. No one is feeding a border that way.

So the only practical route to a useful dose through a can or a sprayer is to carry the gypsum as solid particles, small enough to stay suspended and to pass a nozzle. That is what liquid gypsum is, and it is why the honest word for it is suspension.

What does micronising do?

We wet-mill to ~5 microns. Dissolution rate tracks surface area; geometry alone says a 5 µm particle offers roughly hundreds of times the surface of a 2 mm top-cut grain — treat that as order-of-magnitude, not a measured rate. Fine particles pass a 200 µm sprayer mesh and move into pore space instead of crusting.

Finer does not raise final solubility. Above ~1 µm, particle size changes how fast gypsum dissolves, not how much will dissolve [Lebedev & Kosorukov]. You get there faster, not further. Fine acts quickly; coarse lasts — that trade-off belongs with form comparisons, not this page.

How does liquid gypsum work once it reaches the soil?

Diluted and watered on, particles travel with the water into the top of the soil and start dissolving — calcium and sulphate into the soil solution. Roots take both up. Calcium also occupies exchange sites on clay and organic matter. As plants and soil pull Ca out of solution, more solid dissolves to replace it. Low solubility is a feature: undissolved particles are a reserve.

Detail on structure response to clay sits elsewhere; this page stops at form and delivery.

How do you make liquid gypsum at home?

You can make a gypsum liquid at home. You cannot reproduce a micronised suspension, and the two obvious approaches each fail for a different reason.

Dissolving it. Stir garden gypsum into a bucket and you get a saturated solution at roughly 2.5 g/L, with everything above that sitting on the bottom refusing to co-operate. It is liquid, and it is gypsum. As a way of applying calcium to a bed it is close to useless, though it demonstrates the solubility ceiling very effectively.

Grinding it. Gypsum is soft, so a blender or a coffee grinder will break it down. Domestic equipment lands in the hundreds of microns at best. Particles that size drop out of suspension within seconds, clog a sprayer, and sit on the surface rather than working down into the gaps between soil particles. Single-figure microns take wet bead milling, which is a machine rather than a kitchen attachment, and the milling is most of what you are paying for.

The approach that does work. Spread powdered or granular garden gypsum and water it in properly. Slower, cheaper per kilogram, perfectly sensible on an open bed you can dig. The reasons to want the liquid form come down to reaching a root zone without cultivation and dosing accurately. On established lawns, where you cannot dig at all, that is most of the argument.

Dr Forest Liquid Gypsum

• Natural mined gypsum, wet-milled to an average particle of about 5 microns
• 19.55% calcium and 15.31% sulphur, the full analysis printed rather than summarised
• A suspension rather than a solution, so it carries a dose worth applying
• Two ingredients, mined calcium sulphate suspended in fulvic acid, with no synthetic dispersants

See Liquid Gypsum500 ml and 1 litre, from £12.99

Not sure whether you need calcium, sulphur, or neither? Start with a soil test, then read the limits of what it does before you buy anything.

Leave a bottle standing for a fortnight and you will see a band of white mineral settled at the bottom. That separation is the most honest thing about the product. The calcium and sulphur are still solid, still gypsum, waiting for soil water to do the rest.