Ferrous and ammonium sulphate, polarised light micrograph. Ammonium sulphate is an inorganic salt which is widely used as a fertilizer for lowering the pH of alkaline soils. It contains 21 per cent nitrogen and 24 per cent sulphur. When present in the soil, the ammonium ions are released from the molecules which form an acid, thereby lowering the pH level of the soil whilst contributing nitrogen, essential for plant growth. Magnification: x70 when printed 10cm wide.