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Farmnote 64/1985 [Reviewed May 2006]

By JS Yeates, Research Officer, Plant Nutrition Branch

Sulphur is a major plant nutrient. It is a constituent of amino acids, essential in the formation of proteins. Sulphur deficiency restricts growth and reduces protein levels. Deficiency symptoms are usually seen as both young and old leaves because sulphur is partially mobile within the plant.

Symptoms

Sulphur deficient subterranean clover plants are usually small and pale green to lemon-yellow with both old and young leaves equally affected. In severely deficient plants the leaves tend to fold and stand erect and stems may redden slightly (Figure 1). Pasture production may be greatly reduced (Figure 2).

Plate 1. Severely sulphur deficient sub.clover plants. Leaves fold and stand erect and stems redden.

Plate 2. In severe sulphur deficiency (left half), pasture production may be severely reduced

Sulphur deficiency in sub. clover pastures is often patchy with clumps of deficient and healthy plants close together. In mixed pastures, usually only legumes respond to sulphur application, as the growth of other species is restricted more by nitrogen supply.

Occurrence

Sulphur deficiency in sub. clover commonly occurs in the high rainfall areas (above 750 mm per year) in Western Australia on sandy soils with more than 20 cm to gravel or clay. Occasionally, deficiencies have been found on other soil types and in lower rainfall areas.

The deficiency may be particularly severe on deeper sands and on areas flooded in winter. Symptoms are usually most noticeable during the spring growth flush.

Only the sulphate form of sulphur - the form present in most sulphur fertilisers - is taken up by plant roots. Most sulphur in soil is not in the sulphate form, but is organically bound and not available for plant uptake. However in the high and medium rainfall areas of Western Australia, heavy soils, such as gravels and clays, contain significant quantities of naturally occurring sulphate bound onto clay minerals. Sulphate from rainfall and the breakdown of organic matter is held by these soils and little is leached. Uptake of this sulphate by plants is usually enough to meet their needs and sulphur deficiency is rare on heavy soils - even without applications of fertilisers containing sulphur.

In contrast, sands have little ability to stop sulphate from leaching and as a result contain only small amounts of sulphate. Soluble sulphate added in fertilisers to these soils can be leached beyond the reach of plant roots. This may happen in less than two months in high rainfall areas, or may take several years in low rainfall areas. In the higher rainfall areas rapid leaching of sulphate from autumn-applied superphosphate may cause sulphur deficiency by spring in wet years. Unlike phosphorus, a significant "bank" does not build up from past sulphur applications.

However, even without superphosphate application to sandy soils, sulphur deficiency in sub clover rarely occurs before July. A small amount of sulphate from the breakdown of the previous year's organic matter and the residual from previous fertiliser applications is taken up by the plant early in the season, before leaching is severe. This is usually enough to meet the plant's needs until depletion by grazing and rapid spring growth cause the deficiency to develop. Deficiency appears earlier and is more severe when heavy rains, early in the season, leach the small amount of sulphate from organic matter breakdown before the seedling can take it up.

In areas with average annual rainfall below 750 mm, many soils, such as the red-brown sandy loams of the Avon Valley and the West Midlands sands contain only small amounts of sulphate. When superphosphate is not applied on these soils, sulphur deficiency is likely in years of high rainfall.

Treatment

The sulphur content of commonly used fertilisers is shown in the Table.

Because the occurrence of sulphur deficiency depends on seasonal rainfall and plant growth patterns the severity of deficiencies on susceptible soils is extremely difficult to predict. For this reason, only "best-bet" recommendations are possible to ensure sub. clover growth is not limited by sulphur deficiency.

Low rainfall areas (less than 425 mm per year)

Annual application of 50 kg/ha or more of superphosphate (5kg/ha or more of sulphur) to sandy soils will prevent sulphur deficiency occurring, though deficiencies are rare even without these application. Higher rates of sulphur applied less frequently (for example 100 kg/ha superphosphate applied every 2 to 3 years) can also be used. Other sulphur-containing fertilisers such as Agras are also suitable. A fertiliser which contains little sulphur can be used in intervening years (for example TSP or DAP on crops).

Medium rainfall areas (425 to 750 mm per year)

Annual application of 100 kg/ha superphosphate (10 kg/ha sulphur) will prevent sulphur deficiency in sub. clover on sandy soils. In drier areas a single application is likely to be effective for two or more years. There is little information on yield losses of sub. clover which may occur if fertilisers without sulphur are used for extended periods.

In a crop-pasture rotation on sandy soils a fertiliser high in sulphur should be used in place of TSP or DAP every two years. This is a practical way of preventing sulphur deficiency in pastures and crops, except in the wetter cropping areas, where sulphur requirements are higher and annual applications may be necessary for both crops and pastures.

High rainfall areas (more than 750 mm per year)

Annual application of sulphur to sands in the high rainfall areas is essential to prevent sulphur deficiency in sub. clover. In these areas, soils with reactive iron levels less than 1,600 ppm can develop some degree of deficiency without sulphur applications. Sub. clover yield losses of 30 per cent or more may occur.

There are a number of options to prevent sulphur deficiency:

• Autumn application of superphosphate.
• Autumn application of Coastal Superphosphate.
• Spring application of low rates of superphosphate (50 to 100 kg/ha)
• Spring application of gypsum, or
• Spring application of sulphur with nitrogen on hay crops (ammonium sulphate, Agras).

The most economic fertiliser type and rate depends on the profitability of the grazing enterprise and the phosphorus soil test level because superphosphate, Coastal Superphosphate and potash mixes contain phosphorus in differing proportions and forms, in addition to sulphur. If only sulphur is required, gypsum, though difficult to apply, is the cheapest source of sulphur.

Generally for pastures on the better sandy soils in the high rainfall areas, autumn applications of 150 to 200 kg/ha superphosphate (15 to 20 kg/ha sulphur) or 90 to 120 kg/ha Coastal Superphosphate (25 to 35 kg/ha sulphur) are recommended; or 80 to 100 kg/ha superphosphate (8 to 10 kg/ha sulphur) or 100 kg/ha gypsum (16 to 18 kg/ha sulphur) in spring.

A recently developed computer model (PHOSUL K) gives precise recommendations for particular paddocks based on soil type, phosphorus requirement and the particular economic circumstances. Details can be obtained from Agriculture Western Australia advisers.

Superphosphate and gypsum contain sulphur in the sulphate form, which is prone to rapid leaching. Leaching losses from superphosphate are reduced by its granular nature but sulphur deficiencies can still occur in spring in wet years despite application in autumn. Gypsum should not be applied in autumn because leaching can remove most of the sulphur before the peak plant demand in spring. Application of gypsum just before this demand (July/August) will ensure plants take up the applied sulphur, and growth is not reduced by sulphur deficiency. However on wet areas, topdressing of fertiliser in spring may be impractical.

Sulphur in Coastal Superphosphate is in the elemental form which is not soluble in water and has to be converted to sulphate by soil bacterial before it is available to plants. The particle size of elemental sulphur in Coastal Superphosphate ensures that sulphate is released fast enough for plant growth, but slowly enough to prevent rapid leaching. For this reason Coastal Superphosphate is suitable for topdressing in autumn.

Soil and plant tests

Used carefully, soil and plant analysis can predict sulphur requirements and diagnose deficiency in sub. clover. Analysis of soils for total sulphate-sulphur are not generally considered useful. Modified soil tests have been developed by Agriculture Western Australia for sub. clover pastures in the high rainfall areas of WA, but these are not yet commercially available.

Plant analysis for sulphur on young tissue (youngest open leaf and all younger tissue) of sub. clover s useful for diagnosing sulphur deficiency. A total sulphur content of less than 0.20 per cent (of dry weight) on young tissue up to mid-flowering indicates deficiency. Results of analyses of samples of sub. clover leaves of various ages can be misleading, because the critical sulphur level for clover tissue changes markedly with age. Currently few laboratories offer analyses of sulphur in plant tissue.

Table - Sulphur content of fertilisers