Sulphur Availability in Soils of the South African Sugar Industry

An investigation was conducted in the laboratory and glasshouse to assess the sulphur (S) supplyingability of the important soil forms of the sugar industry. The amount of S in the soil as measured by a 0,5N ammonium acetate extraction procedure, was compared with the availability ofS during continuouscroppingof sorghumin pots. The number ofcropsrequired to exhaust the reserves of S was fairly consistent with the S status of each prior to planting. Soilswith a Scontent of20 ppm or less required between one and three crops to produce symptoms of a S deficiency compared with four to six crops for soils with S values between 20 and 30 ppm and seven or more crops for soils with values of S greater than 30 ppm. The response of the crop and data on the uptake ofS indicate that the current threshold value of 15 ppm is too low for advisory purposes, and that a value of 20 ppm would be more appropriate. Sugarcane growing in the grey, light textured soils of the Cartref Fernwood,Glenrosa, Longlands and Kroonstad forms, is therefore likely to be most prone to a S deficiency. A random survey of soil and leaf samples indicated that 13% of the sampleswere deficient in S. In future, greateremphasis willhave to be placed on the plant's requirement ofS, particularlywith the increased use of high grade fertilizers that have a low S content.