Gardening glossary
Soil amendment
Soil amendments and fertilisers are often confused because both come in bags from the garden centre, but they do different jobs. A fertiliser delivers nutrients in a plant-available form, usually for a single season. A soil amendment changes the structure, chemistry, or biology of the soil itself, with effects that last years.
Broadly, amendments fall into four families:
**Organic amendments — feed the soil.** - Compost, well-rotted manure, leaf mould, biochar, worm castings, peat-free coir. - Improve water retention in sandy soils and drainage in clay soils. - Slowly release nutrients over months to years. - Feed soil microbes, fungi, and earthworms.
**Mineral amendments — adjust texture and pH.** - Lime raises pH (calcium carbonate or dolomitic limestone). - Elemental sulphur lowers pH. - Gypsum loosens compacted clay without changing pH. - Sand and grit improve drainage in heavy soils (but never use builder's sand alone — it can make clay worse).
**Inorganic structural amendments — improve aeration and drainage.** - Perlite, vermiculite, pumice, expanded clay (LECA), horticultural grit. - Used mainly in container mixes and seed-starting blends.
**Inoculants — add or feed soil biology.** - Mycorrhizal fungi inoculants pair with most plant roots and extend nutrient and water uptake. - *Rhizobium* inoculants help legumes form effective nitrogen-fixing nodules.
The single highest-leverage amendment in a home garden is well-made compost. A 2–5 cm annual layer worked into beds (or laid on top in a no-dig system) fixes the majority of texture, fertility, and biology problems before they start. Match amendments to symptoms: drainage issues call for grit and compost; pH issues call for lime or sulphur; lifeless soil calls for compost and mulch.