Sulphur

About Sulphur

Sulphur (S) is one of the macronutrients, the others being Nitrogen, Phosphorus and Potassium. Sulphur is a constituent of 3 of the 21 amino acids which form proteins, and thus it plays an important role in both plant and animal nutrition.

It has been demonstrated that sulphur is particularly import in the nutrition of some plant species, such as canola. Research also indicates that sulphur (along with phosphorus) are important in the development of improved pastures throughout Australia.

Functions of sulphur in plants

• Constituent of enzymes and vitamins (thiamine and biotin)
• Essential for nitrogen fixation by legumes
• Sulphur is essential for satisfactory uptake of Nitrogen regardless of whether this uptake is from fertilisers, released from the soil organic matter or fixed by legumes.
• Necessary for chlorophyll formation (although it is not a constituent of chlorophyll)
• A constituent of several volatile organic compounds which give the characteristic odours to garlic, mustard and onion.

Plant deficiency symptoms

Sulphur deficiency symptoms can be likened to those of nitrogen (which is also a constituent of protein). Cholorosis (yellowing) of the whole plant is a typical characteristic of sulphur deficiency. Unlike nitrogen deficiency where the older leaves are usually affected first, sulphur deficiency tends to affect the younger leaves first. Plants deficient in sulphur can be thin-stemmed and spindly. In severe cases the entire plant can be pale green and stunted.

Sulphur Containing Fertilisers

Sulphur deficiency can be avoided or corrected by utilising fertilisers containing significant quantities of sulphur such as: Single Superphosphate (SSP), Sulphate of Ammonia (SOA), Potassium Sulphate (Sulphate of Potash), or fertiliser mixtures based on these compounds. The sulphur in these products is in the sulphate (SO₄^2-) form which is readily available for plant uptake.

Single Superphosphate - SSP (0-9-0-11)

SSP is ideally suited to top-dressing perennial pastures, where both phosphorus and sulphur are required.

Sulphate of Ammonia - SOA (21-0-0-19)

This is commonly used in combination with other nitrogen sources, such as DAP, MAP and Urea, rather than applied on its own as the sole source of nitrogen.

Sulphate of Potash - SOP (0-0-42-17)

This product supplies potassium as well as sulphur. As it is considerably more expensive than MOP (muriate of potash), unless there are agronomic reasons (such as high soil chloride) as to why MOP should be avoided, then there are likely to be more economical sulphur containing fertilisers.

Brimstone 90

This sulphur fertiliser is 90% elemental sulphur plus a carrying or swelling agent, usually bentonite clay. Brimstone 90 is a dispersible elemental sulphur fertiliser. The granules disperse on wetting after application, to release fine sulphur particles.

Plants can only take up nutrients in an Inorganic (chemical) form. Elemental Sulphur, being in the organic chemical form, is not immediately available for plant uptake. It must first be oxidised to the sulphate (SO₄^2-) form, through a process called mineralization which involves soil bacteria (Thiobacillus).

For this process to occur effectively the soil needs to be:

• Warm
• Moist
• Contain correct bacteria (Thiobacillus)
• In Preferred pH range (low)
• Aerated

Elemental sulphur may be suitable for long term use or as a maintenance product. As elemental sulphur can reduce pH, it may be useful lowering pH for crops like Blueberries, where a pH (w) <5.5 is desirable.