Sulphur deficiency is very common in New Zealand. Since superphosphate contains about 10 per cent of phosphorus and 11 per cent of sulphur, this fertiliser serves as the main source of sulphur. In addition, small amounts of sulphur, commonly between ½–5 lb/acre and rarely exceeding 7 lb per acre, are added to the soil in rainfall. Many soils in the drier districts of Otago, Canterbury, and Marlborough are only slightly phosphorus deficient but strongly deficient in available sulphur. There is, moreover, the possibility that with regular phosphatic top-dressing the level of available phosphorus increases more rapidly than that of available sulphur because applied phosphorus is mainly retained in the topsoil while applied sulphur may leach quite rapidly out of the topsoil. Following superphosphate topdressing, therefore, the supply of available sulphur may not increase as much as the supply of available phosphorus. To cater for the soils on which sulphur deficiency is more severe than phosphorus deficiency but in which both deficiencies are present, a superphosphate-sulphur mixture is used in increasing quantities. The sulphur used is screened elemental sulphur. It contains a considerable proportion of fines which ignite easily on friction or if near an open flame or spark. The danger of combustion is greatly reduced by dilution with superphosphate or other materials. Hence the proportion of sulphur in a mixture designed for distribution by air is strictly controlled. The safe upper limit is set at 500 lb of screened sulphur per ton of fertiliser material or diluent.