Sulfur

Why is sulfur important for soil and plants?

Sulfur is a constituent of amino acids, enzymes, co-enzymes and other metabolic compounds that are critical for plant function. It is essential for nitrogen fixation.

Sulfur is relatively immobile in the plant, meaning a sustained supply from the soil is required. Sulfur is required in relatively larger amounts when growing canola, with typically at least twice the amount of sulfur being required in wheat or lupin.

Sulfur is an essential part of proteins in plants, meaning it also influences grain quality. In recent decades, fertilisers containing sulfur have been used less in farming systems. At the same time, Australia’s area planted for canola which has a high sulfur requirement (three to five times higher than other crops and pastures), has increased. This has meant sulfur deficiency has become more evident in some agricultural soils. Legumes in particular, are sensitive to sulfur deficiency.

Sulfur cycle

Soil mineralisation, recycling of crop residues and leaching are the critical soil processes determining the levels of plant available sulfur during the growing season.

Most sulfur exists in soil within soil organic matter. Sulfur supply relies on the concentration of organic matter and the rate of breakdown to maintain nutrient supply. Organic matter can mineralise organic sulfur as sulfate (SO₄^2-) where the carbon to sulfur ratio is sufficiently narrow to result in net supply. Sulfate that is not absorbed by plants is subject to leaching so that in coarse textured soils under high rainfall, supplies of available sulfur are normally limited.

What are the causes of sulfur deficiency?

In more recent decades, fertilisers containing sulfur have been used less in farming systems. At the same time, Australia’s area planted to canola, which has a high sulfur requirement, has increased. This has meant sulfur deficiency has become more evident in many agricultural soils. Expression of sulfur deficiency is more likely in soils with high nitrogen availability.

What are the symptoms of sulfur deficiency?

Assessing sulfur in the paddock

Plant tissue testing

Tissue tests are an effective method to check for suspected sulfur deficiency.

Soil testing

Available sulfur is measured by the CPC or Blair sulphur (also called the KCl 40) test methods in milligrams per kilogram (mg/kg) or parts per million (ppm). As the KCL 40 test includes sulfur that becomes available from the mineralisation of organic matter, it is generally a few units higher than the CPC test. Sulfur is considered adequate between 9 to 12 mg S/kg using the CPC sulfur test.

Sulfur available in the surface layer (0 to 10 cm) is a poor indicator of fertiliser requirements for either crops or pastures. Therefore, deep soil tests should be used for determining available soil sulfur levels.

Sulfur management

Replacing the sulfur removed in grain or biomass (2-5 kg S/t) may be a good strategy for most crops and pastures, but for canola an additional allowance should be included to account for its greater needs during growth (10 kg S/t). Application of gypsum has been used to maintain a positive balance of sulfur where growers have recognised canola to have higher sulfur requirements than other crops in the south-western agricultural region of Western Australia.

A shift from superphosphate containing sulfur, to diammonium phosphate (DAP) and monoammonium phosphate (MAP) has caused a greater area of land to change to a negative sulfur balance. While gypsum (calcium sulphate) can be used as a relatively cheap form of sulfur to meet requirements, its use must be matched to other soil characteristics such as soil pH. Sulfur-enriched fertilisers are the other main sources of sulfur.