Calcium crystals in compost. Coloured scanning electron micrograph of calcium crystals in a compost heap. Compost is made by stacking vegetable trimmings, grass cuttings and non- seeding weeds together in a heap. The plant material degrades and the resulting brown, crumbly compost is used to enrich the soil. Calcium contained in the decomposing layers of material is subject to great changes in acidity (pH). When the pH is low (acid) calcium exists in a creamy-white solution. When the pH rises (alkaline) the calcium crystallizes, as seen here (blue). Magnification: x350 at 6x6cm size.

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