SULPHUR: deficiency & toxicity

Sulphur deficiency in tomato leaves

In tomatoes, chlorosis starts from the younger leaves and proceeds to the older leaves with ongoing deficiency. Leaves are uniformly light green or yellow. (Image Yara)

Plant deficiencies or excesses of mineral elements show in a number of ways: in colour, density, size and shape of leaves; in the thickness and colour of stems and the length of internodes; in the colour, fibrousness and thickness of roots; in the abundance and timing of flowers; and in the size, colour, hardness and flavour of fruit. Recognising those particular effects is the key to diagnosing nutritional disorders. 

By STEVEN CARRUTHERS

Sulphur (chemical symbol S) is a secondary macronutrient and used in small amounts in the hydroponic nutrient solution compared to the major elements. In the hydroponic nutrient solution, it is mainly supplied in the form of magnesium sulphate (MnSO4), usually added to the ‘Part B’ nutrient concentrate to avoid precipitation – this is because calcium ions (positively charged when dissolved) in the ‘Part A’ concentrate react with sulphate ions (negatively charged when dissolved) to form insoluble calcium sulphate. It is only in the concentrated form that this precipitation becomes a problem – it remains soluble in a working solution. Sulphur can also be supplied in smaller quantities in micronutrients such as copper sulphate, manganese sulphate and zinc sulphate, also added to the Part B concentrate.

The plant requirement for sulphur equals or exceeds the requirement for phosphorus for most plants. In legumes, forages and some vegetable crops, sulphur is required in considerable amounts.

The ideal pH range for sulphur availability to plants is from pH 6 (slightly acidic) to strongly alkaline conditions. It becomes unavailable in very acid solutions.

Functions of sulphur
Sulphur has various functions in plants. It is found in some amino acids, the building blocks of proteins. In fact, most of the sulphur absorbed by plants, about 90%, is used for that purpose.

Sulphur plays an important role in chlorophyll formation – it is a major constituent of one of the enzymes required for the formation of the chlorophyll molecule.

Sulphur is necessary to maintain dark green colour, stimulate seed production, and promote root and general plant growth. It is important in respiration. Sulphur is also essential in the synthesis of oils, especially in oil crops, and is active in the metabolism of nitrogen.

Sulphur deficiency
Sulphur deficiency is similar in appearance to nitrogen deficiency. It begins in younger leaves because sulphur is not as mobile as nitrogen within the plant. Leaves are light green to yellowish in colour, but unlike nitrogen deficiency, the yellowing is much more uniform over the entire plant including young leaves. Sulphur deficient plants do not lose the lower leaves as in the case of nitrogen deficiency.

Sulphur deficiency results in spindly and stunted plants, but they are not as dark-coloured as in phosphorus or potassium deficiency. Stems turn hard and are woody. Deficiency can lead to delayed plant
development and maturity, and if deficiency occurs at vegetative stage, it can affect yield.

Advanced sulphur deficiency can be recognised by brown lesions and/or necrotic spots along the petiole, and the leaves become more erect and are often twisted and brittle.

Symptoms may vary between plant species. For example, in corn, sulphur deficiency shows up as interveinal chlorosis; in wheat, the whole plant becomes pale while the younger leaves are more chlorotic; in potatoes, spotting of leaves may occur.

In tomatoes, cucumbers, eggplants and chilli, the veins and petioles exhibit a very reddish colour, more distinct than in nitrogen deficiency – the underside veins and petioles appear pinkish. 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 sulphur 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.

In strawberry plants, middle to upper leaves develop a light green colouration. Over time the leaves become more uniformly yellow in colouration. With severe deficiencies the pale yellow leaves can develop necrotic spotting due to sunburning. The fruit can also be smaller in size, but have normal colour.

Sulphur toxicity
Sulphur toxicity usually occurs as a result of air pollution, which is difficult to control in open air cultivation areas close to industrial and volcanic activity, when rain will take the sulphur from the air to form what is commonly known as acid rain. Sea spray may also be a significant source of sulphur in coastal areas – toxicity has been reported on small islands. Sulphur toxicity usually occurs in saline conditions, especially in soil cultivation, resulting in plants wilting.

Management practices
It’s rare to experience sulphur deficiency in hydroponic/greenhouse crops. If you suspect a deficiency, problem. Alternatively, a nitrogen test kit or tissue tests will determine if it is N or S.

Sulphur deficiency is usually corrected by the addition of a sulphur-containing fertiliser. Sulphur deficiencies can be corrected using foliar sprays containing sulphur, usually potassium sulphate.

As for most nutrient deficiency problems, growers are generally unconcerned about leaf symptoms because they do not directly influence the market value of the crop. However, this ignores the fact that the same problems that cause the leaf symptoms can also reduce fruit production and quality. This is because leaves manufacture the food needed by the plant to produce fruit, so if they are not healthy, then yield and quality can be reduced, and the cropping season can be shortened.

An important fact is that plants produce leaf symptoms only when a nutritional problem has become serious. Often, yield or quality has been significantly reduced before symptoms appear. Therefore, at the first sign of a problem growers should identify and treat the disorder. Although growers will not prevent production losses (and hence profit losses) they can minimise the extent and severity of fruit losses.

References
Bromley, B.J., Nutrient deficiency symptoms. http://mgofmc.org/docs/nutrientdeficiency.pdf retrieved 20 Mar 2017.

Donnan, R., What are hydroponic fertilisers, Practical Hydroponics & Greenhouses, March 2014. https://www.hydroponics.com.au/what-are-hydroponic-fertilisers/

Farm.com, Sulphur Deficiency in Plants, http://www.farms.com/field-guide/crop-diseases/sulfur-deficiency-in-plants.aspx retrieved 20 March 2017 retrieved 20 Mar 2017.

Haifa Group, Crop Guides, http://www.haifa-group.com/knowledge_center/crop_guides/tomato/plant_nutrition/nutrient_deficiency_symptoms/#{62EFEBD2-0308-46D3-
A77F-6A94EB008C78} retrieved 20 Mar 2017.

Hydroponics BC, Nutrients are an essential part of healthy plant growth. http://www.hydroponicsbc.com/nutrients.html
retrieved 20 Mar 2017

Sela, G., Sulphur in plants and soil, Smart Fertilizer Management, http://www.smartfertilizer.com/articles/sulfur
retrieved 20 Mar 2017

Ward, G.M., Sulphur deficiency and toxicity symptoms in greenhouse tomatoes and cucumbers (1975).

Whipker, B.E., NC State University, Cooperative Extension
Resources. Strawberry sulfur (S) deficiency. https://content.ces.ncsu.edu/strawberry-sulfur-deficiency
retrieved 20 Mar 2017  Ω

PH&G April 2017 / Issue 178


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