Do I need liquid gypsum?

Gypsum is a soft natural mineral, chemically calcium sulphate. Its first job is on clay. Some clay holds too much sodium, a type of salt, which makes the tiny particles drift apart. That soil turns to paste when wet and sets hard when dry. Gypsum swaps the sodium for calcium, the particles clump into crumbs, and water gets through. Its second job is simpler. It feeds plants calcium and sulphur without changing how acid the soil is.

Shainberg, I., Sumner, M.E., Miller, W.P., Farina, M.P.W., Pavan, M.A. & Fey, M.V. (1989). Use of gypsum on soils: a review. Advances in Soil Science 9, 1–111.

Gypsum is widely used on farms. Its two best-proven uses are on soils with a particular problem, and on those soils it reliably raises yields. The first is farmland spoiled by salt and sodium: across published trials in China, gypsum nearly doubled crop yields on average. The second is subsoil so acid that roots stop short of it: a review of 43 Brazilian studies found gypsum raised grain yields by 7 to 14 per cent on average, more in dry years, and cereals gained in most trials. On farmland without either problem, the gain largely disappears. Across ten trial sites in the United States, yields did not change overall. That is why this test asks about your soil before it recommends anything. Neither problem is common in British gardens, and the dispersion test is how you find the sodium kind.

Wang, Y., Wang, Z., Liang, F., Jing, X. & Feng, W. (2021). Application of flue gas desulfurization gypsum improves multiple functions of saline-sodic soils across China. Chemosphere 277, 130345. · Pias, O.H.C., Tiecher, T., Cherubin, M.R., Silva, A.G.B. & Bayer, C. (2020). Does gypsum increase crop grain yield on no-tilled acid soils? A meta-analysis. Agronomy Journal 112, 675–692. · Kost, D. et al. (2018). Meta-analysis of gypsum effects on crop yields and chemistry of soils, plant tissues, and vadose water at various research sites in the USA. Journal of Environmental Quality 47, 1284–1292.

Most heavy clay in Britain is heavy because it has been packed down, or because the ground underneath drains slowly. Sodium is rarely the cause. The dispersion test tells the two apart. Clay with too much sodium clouds still rainwater; other clay leaves it clear. If yours stays clear, gypsum will not change the clay. It can still help the surface, where heavy rain beats bare soil into a thin skin that water runs off. Our clay soil guide covers this in full.

Emerson, W.W. (1967). A classification of soil aggregates based on their coherence in water. Australian Journal of Soil Research 5, 47–57. · Agassi, M., Shainberg, I. & Morin, J. (1981). Effect of electrolyte concentration and soil sodicity on infiltration rate and crust formation. Soil Science Society of America Journal 45, 848–851.

If water is still in a 30 cm hole a day later, the problem is below what you can see. The ground may be waterlogged from beneath, or a hard layer may be stopping the water. No bottle fixes that. It needs a drainage pipe, a gravel-filled trench, or the hard layer broken up. Once the hole empties within a day, it is worth asking whether gypsum would help.

The sunken dark patch on the bottom of a tomato, pepper or courgette is called blossom end rot. The fruit has run short of calcium. Usually that is because calcium only travels through a plant in water, and the compost dried out. But the supply itself can run short too. That happens in pots and grow bags, in peat-free and coir composts, in light sandy soil, in acid soil, and under heavy potash feeding. Steady watering comes first. A calcium feed covers the supply. Spraying calcium on the leaves has little evidence behind it.

Ho, L.C. & White, P.J. (2005). A cellular hypothesis for the induction of blossom-end rot in tomato fruit. Annals of Botany 95, 571–581. · Taylor, M.D. & Locascio, S.J. (2004). Blossom-end rot: a calcium deficiency. Journal of Plant Nutrition 27, 123–139. · Saure, M.C. (2001). Blossom-end rot of tomato (Lycopersicon esculentum Mill.): a calcium- or a stress-related disorder? Scientia Horticulturae 90, 193–208.

Plants need sulphur. For most of the last century it arrived free in the rain, from coal smoke. Since the air was cleaned up, light and sandy soils can run short. The cabbage family needs the most: in Canadian trials on sandy soil, gypsum raised cauliflower yields by about a quarter. Calcium fed to potatoes as the tubers swell reduces hollow and brown centres. Sulphur also gives onions their flavour. These trials used more gypsum than a watering can delivers, so take them as reasons to feed, not as promises.

McGrath, S.P. & Zhao, F.J. (1995). A risk assessment of sulphur deficiency in cereals using soil and atmospheric deposition data. Soil Use and Management 11, 110–114. · Sanderson, K.R. (2003). Broccoli and cauliflower response to supplemental soil sulphur and calcium. Acta Horticulturae 627. · Palta, J.P. (2010). Improving potato tuber quality and production by targeted calcium nutrition. Potato Research 53, 267–275. · Randle, W.M., Kopsell, D.E. & Kopsell, D.A. (2002). Sequentially reducing sulfate fertility during onion growth and development affects bulb flavor at harvest. HortScience 37, 118–121.

Grass very rarely runs short of calcium, so do not expect a greener lawn from the calcium alone. Gypsum earns its place on a lawn in three cases. The first is clay that clouds the water in the dispersion test. The second is a surface that seals and holds puddles. The third is an edge damaged by road or path salt. In a pot trial, gypsum greatly reduced salt damage to plants. Roadside trials needed very heavy doses, so a badly salted strip calls for granulated gypsum.

Shaddox, T. & Munshaw, G. Calcium for Kentucky turfgrasses. University of Kentucky Extension, AGR-242. · Dirr, M.A. & Biedermann, J. (1980). Amelioration of salt damage to cotoneaster by gypsum. Journal of Arboriculture 6, 108–110.

Gypsum does not make soil more or less acid, so it will not replace lime. For acid soil short of calcium, use Garden Lime for a quicker change, or Sea Shell Meal for a slow, lasting one. It will not help water into ground so dry that rain runs off; that needs a wetting agent. And it will not loosen a lawn packed down by feet and mowers. Spiking it with a garden fork does that.