Sulphur's role in growing healthy lucerne

Sulphur is present in all plants and is involved in the production of chlorophyll, the process of photosynthesis, overall energy metabolism and carbohydrate production.

In lucerne, sulphur (S) plays an important role in its vigour and potential dry matter production. It is also crucial for protein formation and influences the crude protein content of lucerne silage and hay by providing animals with a protein source when it is required. Sulphur is also required for nitrogen (N) fixation in lucerne.

Sulphur deficiencies can reduce the potential total dry matter production on a lucerne stand, so it is necessary to understand if S is limiting production so any deficiencies can be corrected.

One of the challenges of visually identifying an S deficiency is that at times it is obvious to the eye, while at others, too subtle to see.

Visual symptoms may be seen when comparing plants in sections of the paddock. In areas with no deficiency, plants are noticeably taller and darker green, while smaller plants with yellowing indicate a deficiency is present.

Deficiency indicators are similar to nitrogen (N), though an N deficiency presents on the older leaves, whereas S is usually found on the younger leaves. An S deficiency in younger leaves shows an overall chlorosis. With a continued deficiency, interveinal yellowing occurs on the foliage.

While an S deficiency in lucerne can be observed in the field, it can only be confirmed by a herbage test. The critical herbage level for lucerne at the hay stage at the commencement of flowering is approximately 0.20 percent of S.1 Remember, herbage testing should always be used alongside soil testing when interpreting test results. When taking a herbage test, take samples from the top 15 cm of the plant before flowering and avoid using diseased or insect-damaged tissues.

Two standard soil tests provide the S status. Sulphate-S (SO4-S) measures the immediate plant available S and the Extorganic-S (Org-S) measures the long-term supply of S. Along with these two soil S tests, there are two types of S fertiliser available for lucerne, sulphate-S which is provided in products such as superphosphate and serpentine super, and elemental-S offered in, for example, the SulphurGain range.

Elemental-S must first be oxidised by soil microorganisms to become sulphate-S before it is plant-available. In contrast, sulphate-S is in a readily available form for the plant and is fast-moving through the soil. Choosing which type of S product is suitable is dependent upon S fertiliser requirements, annual rainfall, soil drainage and type, and timing of applications. Soluble S is not recommended to be applied in winter due to a high risk of leaching loss.

To understand which sulphur fertiliser is best for your lucerne stand, contact your local PGG Wrightson Technical Field Representative.

1 McNaught, K. J., & Pieternealla J. E. Chrisstoffels. (1961). Effect of sulphur deficiency on sulphur and nitrogen levels in pastures and lucerne. New Zealand Journal of Agricultural Research 4 (1-2).