Sulfur deficiency

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Characteristics and occurrence

Sulfur (S) deficiency may not be recognised easily in the field, as the symptoms are not obvious except in severely deficient plants. However, recent soil fertility studies have shown that sweetpotato production may be frequently limited by low sulfur supply. Deficiencies have been documented on ultisols from the Papua New Guinea Highlands, andesitic volcanic ash soils in Tonga and coralline soils from coastal Papua New Guinea.

On the other hand, it appears that sweetpotato may be more tolerant of low sulfur than are a number of other crops. Sulfur deficiency is much more frequently reported in legume, cereal and tree crops than in sweetpotato. On low-sulfur soils of the North Guadalcanal Plains in the Solomon Islands, it was noted that, although a number of pasture grass, legume and tree species responded to sulfur fertiliser, sweetpotato did not.

Sulfur (dissolved as sulfate) is readily leached from the soil, and weathered soils in high rainfall areas frequently have a low sulfur status. Burning of vegetation results in gaseous losses of sulfur (as SO2) from the farming system. Regular high levels of phosphorus application may displace sulfur from the soil matrix and contribute to sulfur depletion. In areas close to industrial or volcanic activity, rain will take sulfur from the air and may provide sufficient sulfur to maintain crop production. Sea spray may also be a significant source of sulfur in coastal areas, but sulfur deficiency has been reported even on small islands.


Sulfur deficiency results in a uniform pale green chlorosis throughout the plant. Veins do not retain a green colour, and in many cases, they may be even paler than the interveinal tissue. In cultivars in which young leaves are normally green (ie. lacking red pigmentation), the youngest leaves may appear pale earlier or more severely than mature leaves. This is not obvious in cultivars with red or purple tips.

A considerable reduction in growth may be suffered without the appearance of any visible symptoms. Clear symptoms are associated with severe stunting, accompanied by reduced leaf size, and reduced activity of axillary buds, resulting in less branching.

Purple or red-brown pigmentation may develop on both young and old leaves. In cultivars with normally green shoot tips, the petiole and margins of young leaves may become red, and this may extend in a mottled pattern over interveinal areas of the upper leaf surface. A similar pattern is seen on the oldest leaves, although it is often confined to the tips of lobes rather than extending around the entire margin. Leaves of intermediate age generally lack purple pigmentation. In plants which normally have purple-pigmented shoot tips, pigment is lost in the normal manner as leaves mature (in contrast with nitrogen deficiency).

Possible confusion with other symptoms

The general growth reduction and yellowing of the whole plant closely resemble the symptoms of nitrogen deficiency. A useful distinguishing character is the red pigmentation of veins of young leaves in nitrogen-deficient plants, but no pigmentation on the oldest leaves. While sulfur deficiency may increase pigmentation of young leaf margins and petiole, a strong veinal pattern is not typical, and the oldest leaves are usually also pigmented.

When no purple pigmentation is present, the problem may be solved using fertiliser test strips. Two fertilisers, sulfate of ammonium (a fertiliser containing both sulfur and nitrogen), and urea (which contains only nitrogen) are applied to one row each of sweetpotato plants, with an untreated row between. After a week or two, if plants green up with sulfate of ammonium, but not with urea, it is likely that they were suffering from sulfur deficiency; if both treatments are equally effective, the problem is probably nitrogen deficiency.

Diagnostic soil and plant tissue tests

A critical concentration of 0.34% S in the 7th to 9th youngest leaf blades was estimated in solution culture studies using the cv. Wanmun. In healthy plants, concentrations were generally between 0.35 and 0.45% S.

The chemistry of sulfur in soils is complex and there is no reliable measure of plant-available sulfur which can be applied over a wide range of soil conditions. Total sulfur measurements may be related to crop response to sulfur fertiliser only when the soil sulfur reserves are very low. Calcium phosphate extraction is used to estimate the soluble plus adsorbed sulfate fraction (Rayment and Higginson, 1992). This may be used to predict sulfur deficiency in soils with a low organic matter content, but in most humid tropical soils, sulfur is predominantly supplied from the degradation of organic material. Approximate critical levels are 200 mg/kg total S, or 6 - 12 mg/kg extractable S, below which S deficiency is likely in soils with a low organic matter content.

In the field, the most practical way of confirming a suspected sulfur deficiency may be to apply a sulfur-containing fertiliser to a small area, and observe the response over the following 1-2 weeks.


Cultural control

Sulfur deficiency is usually corrected by the addition of a sulfur-containing fertiliser. The fertiliser chosen may be primarily for supplying S, such as gypsum or elemental sulfur, or sulfur may be added as a constituent of fertilisers intended to supply other nutrients, such as ammonium sulfate (24% S, but also contains nitrogen) or single superphosphate (11% S, but also contains phosphorus). It should be noted that many “high-analysis” fertilisers such as triple superphosphate, monoammonium phosphate, diammonium phosphate and urea, contain only trace amounts of S.

The maintenance of a high organic matter content in the soil will also increase the availability of sulfur to the roots, and decrease the rate of nutrient leaching. Burning of crop and fallow residues leads to direct losses of sulfur to the atmosphere, and loss of organic matter inputs to the soil. This practice not recommended where sulfur supply is poor.


Chase, L.D.C. and Widdowson, J.P. 1983. Sulfur in the agriculture of Pacific High Islands. In: Blair, G.J. and Till, A.R. (eds.) Sulfur in South East Asian and South Pacific agriculture. Commonwealth of Australia. pp 206-217.

Dowling, A.J., Konabe, B. and Tigat, R. 1994. Nutritional assessment of steeply sloping soils from Aiyura in the Eastern Highlands of Papua New Guinea. Papua New Guinea Journal of Agriculture, Forestry and Fisheries, 37(2), 23-29.

Fox, R.L., Asghar, M. and Cable, W.J. 1983. Sulfate accretions in soils of the tropics. In: Blair, G.J. and Till, A.R. (eds.) Sulfur in South East Asian and South Pacific agriculture. Commonwealth of Australia. pp 39-53.

Halavatau, S., Asher, C.J. and Bell, L.C. 1996. Soil fertility and sweet potato research in Tonga - Nitrogen and Phosphorus. In: Craswell, E.T. Asher, C.J. and O’Sullivan, J.N. (eds.) ACIAR Proceedings No.65: Mineral nutrient disorders of root crops in the Pacific. pp 58-64.

Landon, R.J. 1991) (ed.) Booker tropical soil manual: a handbook for soil survey and agricultural land evaluation in the tropics and subtropics. Booker Tate Ltd; Longman, London.

O’Sullivan, J.N., Asher, C.J. and Blamey, F.P.C. 1997. Nutrient Disorders of Sweet Potato. ACIAR Monograph No. 48, Australian Centre for International Agricultural Research, Canberra, 136 p.

Rayment, G.E. and Higginson, F.R. 1992. Australian laboratory handbook of soil and water chemical methods. Inkata Press, Australia.

Vance, P.N., George, S. and Wohuinangu, J. 1983. Sulfur in the agriculture of Papua New Guinea. In: Blair, G.J. and Till, A.R. (eds.) Sulfur in South East Asian and South Pacific agriculture. Commonwealth of Australia. pp 180-190.

Contributed by: Jane O'Sullivan

Characteristics and occurrence


Confusion with other symptoms

Diagnostic tests



Reduction in  growth, branching and leaf size and general paling of leaves on S-deficient plant (right) (J. O'Sullivan). 


Light green chlorosis affects the whole plant, but in some varieties young leaves may be palest (J. O'Sullivan).


Chlorosis on and adjacent to veins of mature leaves, varying with variety (J. O'Sullivan). 

Purpling of margins and interveinal areas is common on both young and oldest leaves (J. O'Sullivan).