- Sulfur fertiliser inputs have declined with the use of DAP and MAP rather than single superphosphate.
- Sulfur can be lost via leaching.
- Most of the sulfur in the soil is contained in organic matter and is mineralised by micro-organisms.
for Farming in Tasmania - Bill Cotching
Sulphur (S) aids in nitrogen fixation by legumes and is involved in the formation of amino acids, proteins and vitamins. S is an especially important nutrient for brassica and onion crops. Nitrogen and sulfur are important in the formation of chlorophyll (the green pigment in plant leaves) and deficiency symptoms of both are similar. Sulfur is responsible for many flavour and odour compounds in plants such as the typical smell and pungency of onions and cabbage. Some of the S compounds (glucosinolates) responsible for pungency are produced in large amounts by biofumigant brassica crops that can be grown and managed in order to inhibit the growth of some soil-borne fungi and bacteria (Hort Innovation 2022).
Most of the S in the soil is contained in the organic matter and is mineralised by micro-organisms to plant-available sulphate ions (SO42-). These biological processes are negatively influenced by a low pH and waterlogging. Occurrence of S deficiency appears to be a complex interaction between the mineralisation of S from soil organic matter, seasonal conditions, crop species and plant availability of subsoil S making it difficult to predict plant available S from a soil test. Historically, soil S has been adequate for plant growth because S is supplied in superphosphate and in rainfall in coastal areas, but S inputs have declined with the use of DAP and MAP which are low in S. This means more attention must be paid to S nutrition of crops and pastures. Plant testing is one way of checking whether plant uptake is adequate.
Measuring soil Sulfur
The main test used for S is the Potassium Chloride-40 (KCl- 40) test.
Desirable soil Sulfur levels
Optimum levels of soil S for pasture growth are in the range 8 – 16 mg/kg. Less than 8 mg/kg is low, and more than 32 mg/kg is high (Appendix 6). A level of around 20 mg/kg is considered adequate for vegetable crops. Too much sulphur in grain dominated diet of calves has been reported as being associated with calf blindness in Victoria (Dairy Extension Centre and Gipps Dairy 2008).
Sulfur can be applied in two ways, as elemental sulfur (S) or as a sulfate (SO42-) in combination with another nutrient, e.g. ammonium sulfate (24% S), potassium sulfate (18%S) or calcium sulfate (gypsum 19% S). Gypsum is a component of single superphosphate fertiliser making up 11%. Sulfate of Potash (potassium sulfate, SOP) is a more expensive source of sulfur than superphosphate. SOP is generally not used to supply sulfur unless there is a reason to avoid using the more economically priced Muriate of Potash as a potassium fertiliser, e.g., where the chloride in Muriate of Potash may be detrimental. SOP is more commonly used in higher value horticultural crops.
Sulfur deficiency in late winter in Tasmanian pastures due to waterlogging and slow release from organic matter when soils are cold, has been addressed locally with the application of ammonium sulfate in combination with urea for the application of nitrogen to pastures in early to mid-winter (June/July). The timing of this application needs to be considered in terms of length of pasture rotation and when the pasture needs to be grown. When using ammonium sulfate and or urea, the acidifying value of these fertilisers must be considered when next applying lime or dolomite.