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Everything you ever wanted to know about sulphur

Omnia recently hosted a series of farmers' days across South Africa to inform our clients of the importance of sulphur nutrition. The guest speaker was Dr Elke Bloem, a scientific director at the federal research centre for cultivated plants (Julius Kühn-Institute) at the Department for Crop and Soil Science in Germany. She is responsible for research on the biochemistry of plants, bio-active metabolites in agricultural crops, vegetables and medicinal plants, plant stress metabolism, soil biology and plant nutrition.

Dr Bloem has more than twenty years experience in research, especially in the field of sulphur nutrition. In her PhD she worked on a model for the prediction of sulphur deficiency on agricultural crops. She is involved in collaborative projects with several research organisations in Scotland, the Netherlands, Sweden, Finland, Austria, Brazil, Egypt and China, among others.

The reasons for sulphur deficiency

Dr Bloem opened with describing the reasons for the increase in sulphur deficiency symptoms experienced around the world. Her findings correspond with those of Dr Koos Bornman, Omnia's General Manager: Strategic Agricultural Services (see his article on page 10), and include the following:

  • a reduction in industrial sulphur emissions due to clean air legislation;
  • lower sulphur levels in fuel oil and diesel;
  • a change in fertilizer consumption from fertilizers with high levels of "impurities" such as sulphur, to high analysis fertilizers;
  • a reduction in sulphur based pesticides; and
  • an increased crop demand for sulphur due to the breeding of higher yielding and shorter growing varieties.

What is sulphur required for?

Sulphur is a necessary element in the physiology of the plant. It is taken up from the soil in the form of sulphate and reduced in the chloroplast of crop plants. Sulphur can also be taken up in small amounts from the ambient air in the form of sulphur dioxide, carbonyl sulphide or hydrogen sulphide.

Sulphur is needed in the formation of essential amino acids. These amino acids are important in the structure of proteins, as well as in certain enzymes, vitamins, secondary compounds and are also key ingredients of chlorophyll.

Sulphur and crop quality

Different quality aspects of crops are directly influenced by the sulphur content in plants:

  1. Sulphur influences the baking quality of wheat grain. Figure 1 shows how sulphur deficiency can negatively affect baking quality.

    Figure
    Figure 1: The effect of sulphur nutrition on baking quality

  2. The supply of sulphur also affects bio-active sulphur-containing metabolites in onion and garlic. This quality aspect will have a direct influence on the taste of these vegetables.
  3. Sulphur deficiency causes a build-up of nitrate in vegetables and contributes to nitrogen losses. On average, each kg of sulphur unavailable to satisfy the S demand of the plant, results in a potential loss of 10-15 kg of N to the environment. Figure 2 shows this correlation.

    Figure
    Figure 2: The correlation between sulphur content and nitrogen use efficiency (NUE) in canola

  4. Sulphur fertilization promotes insect diversity and increases the attractiveness of the crop to specialist insects. In research by Brauer (2007), the influence of the colour of flowering canola on the attractiveness for honey bees was studied (Figure 3). He found that severe sulphur deficiency coincides with a reduced number of visiting honey bees, because the major attractants: colour, scent and petal morphology alter significantly.

    Figure
    Figure 3: The effect of sulphur levels in canola on the number of visiting bees

Sulphur and plant health

Justus von Liebig (1873), the “father of fertilizers”, identified the nutritional status of plants as one of the key factors regulating their susceptibility to diseases. Sulphur and sulphur compounds directly and indirectly affect plant diseases in the following ways:

  • elemental sulphur has a direct fungicidal effect on plants;
  • sulphur dioxide is used as a postharvest preservative for dried fruits, vegetables, spices, grain and wine barrels;
  • carbon disulphide is used as a grain fumigant for the postharvest control of diseases as an alternative to methyl bromide; and
  • the increased resistance of plants to fungal pathogens by activation of the plant sulphur metabolism by soil-applied sulphur is known as sulphur induced resistance (SIR).

A good sulphur supply helps the plant to develop a higher resistance to fungal pathogens. Figure 4 shows how sulphur nutrition affects SIR. These are the results of pot experiments conducted by Wang et al. (2003). The potential efficacy of SIR was measured in greenhouse experiments at between 5 and 50%, and in field experiments between 17 and 35%.

Figure
Figure 4: Effect of sulphur application on the resistance of crops against several plant pathogens

How to identify the sulphur status of your crops

The sulphur status of crops can be determined through a visual diagnosis of deficiency symptoms. Figure 5 shows different crops suffering from sulphur deficiency.

Figure
Figure 5: Sulphur deficiency symptoms on (1.) canola; (2.) wheat; (3.) soya; (4.) tomato; (5.) tobacco; (6.) sugarcane and (7.) citrus

Sulphur status can also be determined by soil analysis. The desired value for canola is 60 kg sulphur per hectare in 0 – 60 cm soil depth. However, because of the high mobility and variability of soil sulphate, it is best not to use soil analyses as the only tool to determine sulphur status.

Plant analysis is a third tool that can be used to determine the sulphur status of the plants. If this tool is used, it should be kept in mind that the critical nutrient values will differ in relation to the growth stage at sampling, the sampled plant part, the targeted crop yields, the targeted crop quality and the experimental conditions (greenhouse, field, etc.). However, it is also possible to analyse plants using non-destructive measurement of plant parameters, such as SPAD meters and field spectrometers.

A rather complicated way of determining sulphur status is to calculate the balance of the sulphur inputs and outputs. But to do this, various factors have to be considered, such as the contribution of atmospheric deposition, fertilizer and irrigation water and ground-water inputs, as well as plant uptake, leaching and gaseous losses. As it is difficult to determine exact values for all of the above, this method is not always practical.

Another tool in determining sulphur status is to model the sulphur supply. An evaluation sheet is used as a simple modelling tool where certain site characteristics such as soil type, organic content and available root zone is assessed using a scale from one to five. The totals are then added up. Based on that, an assumption can be made regarding potential sulphur supply. The only disadvantage of this tool is that hydrological parameters from irrigation and capillary rise are not considered.

Sulphur supply can also be modelled using existing soil data in digital form. Maps can then be drawn up to show different levels of sulphur across a certain field. This type of modelling is also used to predict potential sulphur deficiency under actual and future climatic conditions. According to findings by Hartmann (2010), the risk of sulphur deficiency will increase in future because of higher precipitation during winter and consequently increased sulphate leaching.

Fertilizer recommendations

Dr Bloem ended her talk by giving some fertilizer recommendations for selected agricultural crops (Figure 6). According to her, soil application of sulphur has to be high enough to satisfy the sulphur demand of the crop. Foliar application is only complementary, but is very efficient. The time of application is also important. For example, canola removes 12 kg/ha sulphur 4 t/ha of seed yield, but takes up more than 80 kg/ha during the vegetative period!

CropS rate (kg/ha S)Optimum application timing
Canola 15 - 30 Before winter
  40 - 80 During the main vegetative period
Grain crops 10 - 15 Before winter
  20 - 40 At the start of vegetation
Maize 10 - 20 Before planting
Pastures 20 - 40 At the start of growth and proportionate after each cutting

Figure 6: Sulpur recommendations on selected crops

Dr Bloem's presentation confirms the trends in sulphur deficiencies observed by statistical data which Omnia has gathered from soil analysis and OmniSap® data over many years. It also highlights the importance of including sulphur nutrition in your fertilizer programme.

Speak to your Omnia agronomist for specific crop recommendations and more information.

By Hantie Jansen van Vuuren (Editor)
a summary of a lecture by Dr Elke Bloem

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