Natural vs synthetic gypsum: does the source matter?

Provenance

Natural vs synthetic gypsum: does the source matter?

By Joe Barrington, Founder of Dr Forest · August 2026

Edited 24 Sep 2026

Two very different industries make calcium sulphate. One digs it out of rock. The other scrubs it out of a chimney.

This post covers where the mineral came from before it reached the bottle. The chemistry sits in the post on calcium sulphate dihydrate, what it does in clay is a separate and more qualified question, and which form to choose covers bottle against bag.

Where does agricultural gypsum actually come from?

Natural gypsum is an evaporite. It forms where shallow seas dry out and calcium sulphate drops out of solution, which is why British deposits sit in Permian and Triassic rock, laid down two to three hundred million years ago, under Nottinghamshire and Cumbria. You quarry it, crush it, mill it. The gypsum mineral that goes in is the gypsum mineral that comes out.

FGD gypsum starts as pollution control. Coal-fired power stations pass their exhaust gas through a limestone slurry, which grabs the sulphur dioxide out of it, and air is then blown through to finish the reaction. What settles out is calcium sulphate. The chemistry is sound and the output is real gypsum. It is also, by volume, a large part of what agriculture uses. The US Environmental Protection Agency put American output at roughly 23 million tons in 2019, with around a million tons a year going onto farmland. That is the biggest agricultural use of anything left over from burning coal.

The scale explains the marketing. When the American Coal Ash Association petitioned in 2014 to have synthetic calcium sulphate allowed in US organic production, its argument was that mined gypsum is expensive and not available everywhere. Cheap and abundant is a fair description of a by-product.

Two smaller sources exist: milled plasterboard waste, and phosphogypsum, left over from making phosphoric acid, which carries naturally occurring radium and is restricted for that reason. Neither goes in a Dr Forest bottle.

Is gypsum organic, and what does organic gypsum mean?

Two different questions wear the same word, which is why this one keeps getting searched.

Chemically, gypsum is inorganic. Organic chemistry means carbon-based compounds, and calcium sulphate has no carbon in it at all. A mineral cannot be organic in that sense any more than a lump of chalk can.

In growing terms, gypsum is permitted in organic systems, with a condition attached that most shoppers never see. The retained Great Britain version of Regulation 889/2008 lists calcium sulphate (gypsum) in Annex I, and against it, in the conditions column, four words: only of natural origin. European rules say the same at greater length, permitting a product of natural origin containing calcium sulphate at various degrees of hydration.

So when someone types organic gypsum into a search box, the useful answer is that the mineral qualifies if it was mined and does not if it came off a scrubber. American standards land in the same place by a different road: the Coal Ash Association petition went to the National Organic Standards Board in April 2015 and was not recommended for rulemaking. Gardeners hunting for a vegan fertiliser arrive at this from another direction, since both are questions about what an input is made of and who can tell you.

Does the source change what happens in your soil?

Here I have to be careful, because the honest answer is less dramatic than the marketing on either side.

FGD gypsum is widely approved for agricultural use. The EPA reviewed it in March 2023 and concluded that beneficial use in agriculture can provide meaningful benefits whilst remaining protective of human health and the environment, with a risk of selenium washing into streams only in the extreme case where it goes onto every available field, at the highest rates, as often as allowed. A 2023 study in Scientific Reports found 80 to 90% of the heavy metals in FGD gypsum stayed locked into the mineral rather than becoming available, and that was at farm reclamation rates far above anything a garden would use.

Cut the other way, mined gypsum is not automatically purer. Ohio State University Extension tested the agricultural gypsum sold in that state and found one mined sample that was up to 19% material which would not dissolve at all, mostly dolomite, against under 3% in the FGD product. Mercury sat below the detection limit in every sample, and trace metals came in hundreds to thousands of times below the limits set for spreading treated sewage sludge on farmland.

What does differ, reliably, is what travels alongside the calcium sulphate. The EPA's own comparison has FGD gypsum carrying more mercury, more selenium, more arsenic and more boron than mined gypsum, and by wide margins rather than narrow ones. The table below has the figures. Those gaps are consistent. Whether they matter on a vegetable bed at garden rates is a different question, and the published evidence does not say they do.

Trace element medians from the US EPA 2023 beneficial use evaluation, Table 4-3, comparing 38 to 96 FGD samples with 11 to 19 mined samples. Insoluble residue from Ohio State University Extension factsheet ANR-20. Organic status from Annex I of Regulation 889/2008 as retained in Great Britain and Annex II of Regulation (EU) 2021/1165. There is no carbon footprint row because the published life-cycle work compares calcined plasters rather than soil amendments.

One thing here cuts against the intuition. Count the carbon and a by-product often comes out ahead of a mined mineral, because nothing had to be quarried to get it. A 2021 study in Sustainability found plasters made from FGD gypsum carried roughly half the carbon footprint of the natural-gypsum equivalent. That work covers building plaster, which is gypsum that has been baked, rather than a soil amendment, so it does not transfer directly, and the authors flag how hard the published environmental figures are to compare. Nobody should be selling mined gypsum as the low-carbon option.

So which should you buy?

If you grow to organic standards, the question is settled for you: natural origin, or nothing. If you do not, the case for mined gypsum is the conservative one. You get a material whose trace metals are lower and more predictable and whose origin can be named, over one that reads as fine in the published work but carries a longer list of things that vary. That is a preference about knowing what you are putting on your soil, rather than a claim that the alternative harms it.

The UK angle is quietly interesting. Ratcliffe-on-Soar, the last coal-fired power station in the country, stopped generating on 30 September 2024. Britain no longer makes FGD gypsum. Anything synthetic sold here now arrives from somewhere that still burns coal.

Dr Forest Liquid Gypsum

• Mined natural mineral gypsum, never scrubber output or plasterboard waste
• Traceable back to a named deposit rather than to a power station
• 19.55% calcium and 15.31% sulphur, the analysis printed in full
• Wet-milled to about 5 microns and held in suspension in fulvic acid

See Liquid Gypsum500 ml and 1 litre, from £12.99

Wondering whether you need calcium and sulphur at all before you worry about their provenance? Read what gypsum will and will not do first.

Chemically, gypsum is gypsum. Every route ends at the same formula. The rest is a question about which industry you would rather have spread across your vegetable beds, which is a fair thing to have an opinion about even when the toxicology says both are fine.