The hidden sting in sulphur fertilization
Some perhaps unknown aspects of sulphur crop nutrition that could lower risk of loss of nutrient use efficiency, yield and quality
This article should be read in conjunction with “Everything you ever wanted to know about sulphur”.
The world consumes approximately 55 million tonnes of sulphur for fertilizer production (IFA, 2015). Unfortunately only about 10 million tonnes finds its way to agricultural land (The Sulphur Institute, 2010). Ninety three percent of sulphur is used as sulphuric acid, mostly for the production of phosphate fertilizers, and the sulphur byproduct ending up on gypsum dumps. There was a steady decline in sulphur use for agricultural purposes until 2010, but since then, world demand has escalated. Calculating current crop demand for sulphur it seems that there is a need for approximately 25 million tonnes of sulphur worldwide as a plant nutrient. Unfortunately, it would seem that according to current trends, 12.2 million tonnes less is actually applied as fertilizer (Messick, from The Sulphur Institute, 2015). What is more disturbing is that the use of sulphur in fertilizer has almost halved in South Africa since 2008 while the rest of the world has increased consumption by 14% (Web accessed statistics, The International Fertilizer Association (IFA), 2015). Refer to Figure 1.
Figure 1: The global deficit of sulphur use on agricultural crops
Without the sulphur containing amino acids, cysteine and methionine and other sulphur containing organic compounds, there would be no life as we know it. It plays a major and essential role as a plant nutrient and is often called the fourth macro element after nitrogen, phosphorus and potassium due to its importance in plant physiology and metabolism. In fact, in the maize and soybean producing areas of the USA, sulphur is regarded as the third limiting major plant nutritional element after nitrogen and phosphorus.
The absolute importance of sulphur as a plant nutrient has been emphasised even more recently, because of three main factors;
- The focus on environmental protection and in particular the release of sulphurous gases from coal fired plants has been drastically reduced, especially in Europe. The problem has been drastically addressed since the early 1980's. Modern air scrubbers now remove such gases most efficiently. Coal-fired power stations distributed sulphur over agricultural land in the past, but also caused severe pollution in the form of sulphuric acid, earning the name “yellow poison”.
- The demand for sulphur has escalated exponentially due to higher yields being produced per unit of land. Advances in genetic material and modern cultivation methods have contributed to this phenomenon.
- The fertilizer industry, for various reasons, has moved to highly concentrated products containing no or very little sulphur. Classic low concentrate products such as superphosphate are scarce in world markets.
From the above, Omnia Fertilizer realises the importance of this plant nutritional element, not only for crop production but also for the feeding of people and animals.
Symptoms of sulphur deficiency in crops
Figure 6 shows the typical symptoms of sulphur deficiency of various crops which in general is the pale yellowing of the younger leaves due to the lack of chlorophyll formation.
Figure 6: Response of maize to nitrogen with and without sulphur applied. De Gomez, 2002 (Argentina).
The irony is that sulphur is needed in the early season, but by the time sulphur deficiency symptoms are identified by expert observation, little can be done to rectify the situation.
The fact that younger leaves are low in chlorophyll due to sulphur deficiency, while nitrogen deficiency induces chlorophyll deficiency in the older leaves, offers an opportunity to effectively use the SPAD 502 chlorophyll meter to identify sulphur deficiency in-field. Omnia has developed related norms for a number of crops.
Symptoms of sulphur deficiency may also include the chlorosis of flower petal colour, cupping of leaves and the red tinting of leaves. The latter is not distinct to sulphur deficiency as the presence of anthocyanin responsible for the red colouration of leaves may also be related to phosphorous deficiency, mechanical damage and cold stress.
Crop response to available soil sulphur and related economics
Since the discovery of sulphur as a plant nutrient a magnitude of response data have been published for various crops in many countries.
More than 100 such response functions (site years) across the world for various crops have been summarised in Figure 3. It is important to note the values on the X-axis. Available sulphur in the sulphate form is reported as kilogram per hectare. This value is calculated by adding fresh applied sulphur (as sulphate) to the soil analysis value reported as kilogram (kg) sulphur, again as sulphate, per hectare (ha). The kg per ha sulphur value from a soil analysis value is calculated from the sulphur concentration extracted with mono calcium phosphate (mg/kg soil) and taking the soil density into account. The effective depth of analysis and application (20 cm depth in this case) is also important in the calculation.
Figure 3: Potential margin loss, using average RSA yields and current price ratios, for grain crops if sulphur is not applied on sulphur deficient soils
Figure 2 shows that, for the crop responses studied, all would give a relative yield of more than 95% if a level of available sulphur (as sulphate) of more than 40 kg/ha is maintained. For most grain crops and pastures this value is 25 kg/ha.
Figure 2: Available soil sulphur affects relative yield of several crops. Data compiled from more than 50 site years from five continents (including RSA/Omnia data). Graphs show median response
It is clear that crop responses differ in magnitude depending on the crop type. Wheat and soybean do not seem to respond as well as maize and canola, while pastures respond dramatically to sulphur. It is documented that on average a 25% yield response to sulphate application over all crops may be expected if this element is highly deficient.
Canola response to bentonite sulphur pastels (left) and a chemically granulated sulphate containing product (right). (McKenzie, 2013)
Figures 3, 4 and 5 show the potential margin loss per ha per crop if sulphur fertilization is not effectively managed. Average South African yields and current price ratios (sulphur cost vs crop value as Rand per kg) were used in compiling this graph. It is interesting that if most grain crops and pastures are produced on soils with more than 35 kg sulphur available per ha (in sulphate form) the risk of monetary loss due to sulphur nutrition would be minimal. Crops like potatoes and sugarcane justify economic applications of up to 60 kg/ha and 80 kg/ha respectively. The fact remains that with current price ratios, a potential margin loss of between R1 000 to R40 000 per ha is possible without adequate sulphur nutrition, depending on the crop.
Figure 4 and 5: Potential margin loss, using average RSA yields and current price ratios, for sugarcane and potatoes respectively if sulphur is not applied on sulphur deficient soils
The best way to ensure efficient sulphur nutrition is by soil application. Early season application while the crop is still growing actively, and before the reproductive stage, is the best time to apply sulphur, preferably in the sulphate form. Omnia has developed a comprehensive model to calculate the sulphur requirement that takes many factors into account, among others the soil carbon to sulphur ratio which needs to be less than 200 to initiate mineralisation of sulphur from organic material.
It is important to note that any root restriction has a major influence on sulphur availability and uptake, similar to potassium. This aspect needs specific attention before sulphur application is considered.
The interaction of sulphur with other nutrients
The enhancement of nitrogen use efficiency is well documented on many crops and the effect of sulphur is quite substantial in this regard (Figure 6). Enhancement of nitrogen use efficiency by up to 30% is not uncommon in grains.
In acidic soils, sulphate may neutralise the increased positive character of the clay minerals and hydrated complexes of iron and aluminium oxides. This lessens the sorption of anions such as phosphorus, molybdate and borate. Figure 7 shows how sulphate application may enhance phosphorus uptake by soybean.
Figure 7: The distinct interaction between sulphate and phosphate nutrition of soybean. Pasricha and Aulakh, 1990. in Kleinhenz, 1999
Sulphur may also enhance the uptake of micronutrients directly or indirectly. The lowering of soil pH as a consequence of oxidation of sulphur to sulphuric acid enhances the solubility and thus the uptake of various micronutrient cations. The same effect is achieved for the uptake of phosphorus. The physiological acidification of soil by means of differential uptake of anions versus cations as in the case of ammonium sulphate has the same consequence.
Excessive application of sulphates, especially in the form of gypsum, may cause anion competition. Such suppressive effects have been noted for microelement anions such as borate and molybdate, as well as for phosphate and even nitrate.
Sulphur nutrition for crop quality
Sulphur has a significant impact on the quality aspects of several crops. The quality aspects influenced are quite diverse and includes the bread baking quality of wheat dough, increases in pungency in allium and brassicae species, the increase in protein levels in grains and legumes and the increase in oil content in oil producing crops. Applying sulphur for quality purposes usually far exceeds the requirement for optimum yield. Refer to Figures 8 and 9.
Figure 8: Sulphur nutrition enhances the protein content of maize. 150 kg of nitrogen were applied per ha. The maximum yield was 8 ton maize grain per ha. Rasheed and Mahood, 2004
Figure 9: Sulphur fertilization increases canola oil content. Mansoori, 2012
The impact on quality is often subtle, for instance the fact that sulphur content in forage enhances ruminants' ability to digest it. The nitrogen to sulphur ratio is an important indicator and larger than normal sulphur applications are usually necessary to increase forage quality. Antagonism between sulphate and selenate as well as molybdate may be an aspect to manage in pasture production (Figure 10).
Figure 10: Impact of soil free anion concentration on Mo uptake by plants, Stout et al, 1951. A 40-60% inhibition of Mo uptake in maize by high sulphate concentration in unbuffered soils was reported by Bornman, 1993 confirming work done by Haynes, 1983
Sulphur feeding enhances water use efficiency (WUE)
Previously, not much research was available on the contribution of sulphur nutrition to crop WUE. However, over recent years more and more facts have been published supporting the significant contribution of sulphur to WUE. A classic example is work documented with grass species in the USA where it has been shown that WUE almost doubled when sulphur is applied at adequate nitrogen levels.
Even local research has confirmed the concept. Work done in the Western Cape at different localities with canola has shown increases of WUE of between 10% and 14%. Refer to Figure 11. What is important, though, is that adequate levels of nitrogen need to be applied.
Figure 11: South African research confirms the increase of water use efficiency using the right source of sulphur on canola in the Western Cape (Ngezimana and Agenbag, 2015)
Sulphur containing products and their efficacy
Perhaps the sting that hurts most lies in the use of inefficient sulphur sources for plant nutrition. It has been mentioned that the tendency in current fertilizer products is towards the production of high concentrates such as urea and diammonium phosphate (DAP), primarily because of economy of scale and the transport benefit. Such materials are then blended closer to point of use. The problem, however, is that such materials usually do not contain sufficient secondary elements like sulphur. The cheap “quick fix” route for blenders is to add elemental sulphur because of its concentration, availability and favourable price. There are, unfortunately, quite a few hidden drawbacks to the use of these products.
Plants can only use sulphate as their source of sulphur in the soil. This means that elemental sulphur needs to be oxidised to sulphate before it is available. The result of sulphur oxidation is sulphuric acid, which is a highly aggressive acid. The use of elemental sulphur is therefore well-known under saline and sodic conditions. Unfortunately, under neutral and acid soil conditions, especially where the sulphur is applied in concentrated rows, the negative effects of acidification far outweigh the possible contribution of sulphur nutrition.
The second aspect, which is not always com-municated, is the inefficiency of conversion from sulphur to sulphate in most soils. The rate of oxidation of sulphur is dependent on a number of factors, among others, fineness of product, presence of microbes such as Thiobacillis species in the soil, adequate but not too much moisture, near neutral soil pH and moderate soil temperatures. The fineness of commercial products is not always acceptable and oxidation rates of typical products could take up to two years and more (Figure 12). Under dry, cold conditions with low organic matter, the periods needed for reasonable oxidation can be extensive.
Figure 12: Oxidation rate of elemental sulphur is highly dependent on particle size, but available water, microbes and temperature also plays a key role (Elmeades et al., 1994)
The third aspect, also not usually quantified, is the inefficient distribution of highly concentrated sulphur-containing products when applied to crops. Elemental sulphur-based products usually contain more than 90% sulphur. Therefore, the quantity that is added to bulk blends is minimal. From the densities of the raw materials it is calculated that a 3% sulphur containing bulk blend of common raw materials such as urea, DAP and potassium chloride (KCL) will only contain 1.4% sulphur (S) on a volume basis of a 90% sulphur containing raw material. In a one meter row spacing only five granules containing sulphur will be applied per meter when fertilizing 10 kg of sulphur per hectare. If a 3% sulphur containing chemically granulated product is applied there will be 250 granules per meter containing sulphur in readily available sulphate form.
The above aspects lead to poor agronomic efficiency of elemental sulphur products. Extensive work done in Canada (Figure 14) and New Zealand shows that a commercial sulphur containing bentonite clay product only has a sulphur use efficiency (SUE) of on average 19% while the SUE of sulphate containing granules exceeds 50%.
Figure 14: Response to sulphur containing compounds (Stefan de Jager, Omnia, 2007, Schweizer Reneke)
Table 1 shows some commonly available sulphur containing products that will ensure sulphur availability. Omnia markets several flagship sulphate containing products such as Greensulf™ and Maxiphos™, both containing 4% sulphur, that may also be used in fertilizer blends. Recent trials with maize as test crop have proven that at least a quarter tonne of additional maize grain may be expected per ha versus plain limestone ammonium nitrate (LAN) at a probability of 90% if Greensulf™ is used as nitrogen fertilizer.
|Ammonium Sulphate (21-0-0-24)||24|
|Ammonium Thiosulphate (12-0-0-26)||26|
|Potassium Sulphate (0-0-42)||18|
|Low concentration granules (e.g Omnia)||3-12|
|Omnia liquid fertilizer||1-3|
Table 1: The sulphur content of some common sulphate containing fertilizers
Figure 15 shows the benefit of using an Omnia chemically granulated product versus a highly concentrated bulk blend.
Considering the above, there is no doubt that there is a huge risk in not considering sulphur in the right form in a fertilization programme on any crop. Avoid the painful sulphur sting and consult an Omnia specialist when considering such a fertilizer programme.
By Dr Koos Bornman (General Manager - Strategic Agricultural Services)