What is a soil conditioner? Soil improver explained

By Joe Barrington, founder of Dr Forest · November 2026

Two names for one idea: something you add to make the soil better, rather than to feed the plant. Most of it is compost. The part that usually gets missed is mineral.

Soil conditioner or soil improver: is there a difference?

Not in practice. Soil improver is the usual British term; soil conditioner is more common in the trade. The RHS defines it as any substance dug into the soil to improve its structure, usually organic matter, though lime or gypsum serve the purpose too [1]. Two bags with different names can hold the same thing, so read the ingredients, not the front of the bag.

The two halves: organic and mineral

The organic half: compost and manure

Bagged soil improver is usually composted green waste or bark. Green waste can carry contaminants: bits of plastic and glass, and herbicide residues that damage crops. I don't use bagged soil improvers myself. I just add compost. Good compost improves drainage on clay and water-holding on sand, and feeds the soil life that builds crumb structure. The RHS suggests digging in 5 to 10 kg per square metre, about half to one bucket [2]. If your soil has never had any, start here. Nothing I sell replaces it.

The mineral half: what compost cannot add

Compost carries only the minerals that were in the plants it came from. It cannot put calcium into soil that lacks it. That is the mineral half:

• Gypsum (calcium sulphate) supplies calcium and sulphur without changing pH. It can also help some clays drain [3].
• Lime and sea shell raise pH on acid soil. Use them only when a test says so.
• Rock dust (basalt) is a slow reserve of silica, calcium, magnesium, iron and trace elements. University of Sheffield researchers found basalt dust raised sorghum yields by about 21% in UK farm soil in a controlled-environment trial, and maize and soybean yields by 12 to 16% over four years in a US field trial [4] [7]. Both used heavy doses: 10 kg per square metre in the controlled trial, and 50 tonnes a hectare (5 kg per square metre) every autumn in the field. Basalt weathers slowly and builds over years.
• Clay minerals such as montmorillonite raise the holding capacity of light soil. In a five-year field trial on sandy loam, bentonite raised water-holding by up to 12% [5].

Which one suits your soil?

Match the conditioner to the problem. Test pH before adding anything that moves it [6]. I'd avoid cheap digital meters and stick to a simple chemical test kit.

The crumb test for clay: drop a dry, pea-sized crumb of soil into a glass of rainwater and leave it for two hours without stirring. A milky cloud spreading out means the clay disperses and gypsum will help. Clear water means it won't. More on this in does gypsum work on clay soil?

Is a soil conditioner a fertiliser?

No. A fertiliser supplies nitrogen, phosphorus and potassium in stated amounts: the numbers on the bag. A soil conditioner changes the soil, and supplies little or no NPK. Neither does the other's job, so use a conditioner for the soil and a feed for the crop. For the feed, see the organic fertiliser collection.

The best time to apply

Autumn suits most of them: beds are emptying and winter rain carries minerals down. Gypsum and rock dust go on in autumn, and so do lime and shell, on beds and lawns alike. Sulphur, for lowering pH, is slow in cold soil, so an autumn dose acts mainly from spring. Rake mineral conditioners into the top few centimetres, water in, and compare a treated patch with an untreated strip after a season.

Everything in one place: the soil improver collection.

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