Sulfur, or sulphur, is widely recognized as one of the six macronutrients for plant nutrition but its applications go far beyond this – to vulcanise black rubber, as a fungicide, as well as numerous applications through sulphuric acid. As detailed in this article by Tessenderlo Kerley’s Dr. Nicolas White, there are different forms of sulphur, which can be utilized to deliver sulphur to the crop over an extended period of time.
Dr. Nicolas White
Sulphur is an essential nutrient that is required for the growth and development of all living organisms. It is a component of the amino-acids methionine and cysteine, which are in turn important building blocks for proteins. Sulphur is also present in glutathione one of the most important antioxidants in human health.
Sulphur is present in glutathione one of the most important antioxidants in human health.
A non-metal, sulphur makes up around 0.03% to 0.04% of theearth’s crust, making it the 17th most abundant element, and inits solid elemental form it occursas bright yellow orthorhombic crystals, which means it has three mutually perpendicular axes of different lengths.
As well as elemental sulphur, the other most common naturally occurring forms of sulphur include metal sulphides, such as iron sulphide, commonly known as pyrite and often known as ‘fool’s gold’, and sulphates, such as gypsum (calcium sulphate), the most common sulphate mineral in the world. But what makes sulphur so special?
The many faces of sulphur Sulphur is perhaps unique amongst the elements in its ability to exist in a wide range of oxidation states. The oxidation state of an atom within a molecule is a positive or negative number that is given to that atom to indicate its degree of oxidation or reduction. It essentially measures an atom’s tendency to either attract or lose electrons within the molecule.
Sulphur can exist in a whole rangeof different oxidation states giving this element a very versatile and varied chemistry.
The ability of certain sulphur containing molecules to ‘mop’ up free electrons (associated with potentially harmful molecular ‘free radicals’), thanks to sulphur redox (reduction-oxidation) flexibility, can impart ‘antioxidant’ properties, beneficial for both plant andhuman health.
Allicin is a sulphur containing compound that gives garlic its distinctive odour and flavour and is also responsible for garlic’s antioxidant effect. Glutathione, which contains sulphur (derived from cysteine), has also been reported to have antioxidant properties and can be found in asparagus, avocados and spinach.
Sulphur as a key plant nutrient In many crops today sulphur is being talked about as the ‘fourth primary crop nutrient’ alongside nitrogen, phosphorus and potassium. In certain crops, the sulphur requirement is similar to that of phosphorus, particularly in oil-producing crops such asoilseed rape.
As the world’s population continues to grow, securing sufficient food production, whilst also farming in a sustainable manner means that growers must regularly replenish nutrients drawn from the soil, including sulphur. Obtaining the right balance between nitrogen and sulphur is particularly important to ensure optimal crop yield and quality. Cases of sulphur deficiency are becoming more widespread in many parts of France, as well as in other European countries and North America, particularly in crops such as maize and oilseed rape.
Increased sulphur deficiencies have catapulted the demand for sulphur in recent years, which has led to increased use of sulphur in agriculture.
Nutrient absorption in winter oilseed rape. Source:IFA World Fertilizer Use Manual
Visual symptoms usually give the first indication of sulphur deficiency, but they can sometimes be confused with nitrogen deficiency and often, by the time they appear, it can become difficult to correct the deficiency in time to prevent yield loss. Since sulphur is largely immobile in crops it is the younger leaves that will show the first signs of sulphur deficiency, turning a paler green or yellow colour. As deficiency worsens, signs of interveinal chlorosis may occur. Other symptoms include the leaves starting to roll inwards with purpling on their edges or underside. Deficiency of sulphur in plants impairs protein formation, slowing the growth rate and yield formation.
It is also possible that the crop could be suffering from a ‘hidden hunger’ for sulphur. In such cases an under supply of sulphur will impact the crop yield, but without any visible sign of deficiency symptoms. In such cases leaf analysis can help provide growers with an early warning that the crop may be lacking sulphur, or other nutrients.
Pale foliage on an oilseed rape crop caused by sulphur deficiency.Image ID: APFAJ9
In sulphur deficient soils and crops, application of sulphur fertilizer in a form that is rapidly available to the crop is a very effective way to boost yields. The long-term fertilization strategy must however aim to keep soil sulphur levels, as well as those of other nutrients, sufficiently high in order to meet the yield expectations for the crop. Soil-applied sulphur can also help improve soil quality, particularly on higher pH soils. The sulphate (SO42-) form is most abundant in well-drained soils, being the chemical form of sulphur most readily available to plants.
Sulphur in plants can be found in many organic compounds, and it is an important element because of its ability to form chemical bonds in complex organic molecules. It is a component in cystine, cysteine and methionine, the most important sulphur containing amino acids in plants that are building blocks of proteins. Because of its role in the development of proteins, sulphur compliments nitrogen in the protein forming function. Plant enzymes are another important group of organic compounds containing sulphur. The role of enzymes is to stimulate most internal biochemical reactions related to plant growth and development. An important enzyme, Coenzyme A, is involved in the oxidation and synthesis of fatty acids, the synthesis of amino acids and the oxidation of intermediate compounds in the citric acid cycle.
Sulphur is also a vital component of ferredoxins, iron-sulphur organic compounds occurring in the chloroplasts. Ferredoxins participate in the light reaction in photosynthesis and also have a significant role in the assimilation of nitrogen by root nodule bacteriaand free-living nitrogen fixingsoil bacteria.
Not all sulphur sourcesare the same Sulphur has always been recognized as an important crop nutrient, but in industrialized countries its use in regular fertilization practices was not regarded as necessity before the 1980s, since up until then, sulphur dioxide emissions of industrialorigin had automatically provided sufficient sulphur to soils and cultivated crops.
The sulphur balances in most cropping systems of industrial regions were historically positive. Surveys carried out in Europe indicated an annual sulphur deposition of about 45 kg S/ha in the 1970s, above the sulphur uptake of highly S-demanding crops such as oilseed rape. However, since the beginning of the 1980s the implementation of new stricter European environmental regulations resulted in a reduction of atmospheric sulphur dioxide pollution by industry, households and traffic. Such emissions have now dropped to well below 10 kg/ha, which is comparable to the quantity deposited at the beginning of the industrial revolution in Europe. During the same period, modernisation and the improvement of agricultural production led to more and more use of highly concentrated "high analysis fertilizers". The substitution of ammonium sulphate (24% S) as the dominant nitrogen source by urea and ammonium nitrate, and single super phosphate (12% S) by triple super phosphate (1-2% S) dramatically reduced the amount of sulphur applied through mineral fertilization during the same period.
At the same time, technical advancements in crop breeding, resulting in the development of high yielding varieties, led to a simultaneous rise in yields with a higher nutritional requirement and thus with an increased need for sulphur. Due to both the effective measures in reducing air pollution in industrialized countries and the resulting low inherent sulphur deposition in many of the developing countries, an ever-increasing gap between sulphur supply to and demand by cultivated crops developed. Thus, from the 1990s onward, cases of sulphur deficiency in crops started to become more widespread, and growers had to start to also integrate sulphur fertilizer into their nutritional programs for crops.
Sulphur content in soils is generally in the range of 0.02-0.2%. The soil parent material can be a source of sulphur for crops, supplying metallic sulphides that are transformed over time into sulphate compounds in well aerated soils. Plant and animal residues and soil organic matter are also important sources of soil sulphur. In today’s more intensive crop production it is invariably necessary to apply a sulphur fertilizer to the crop so as to ensure a balanced fertilization and to avoid the ‘mining ‘of soil sulphur reserves. Whatever the form of sulphur, it has to be mineralized and solubilized to the sulphate form (SO42-), that which is most readily taken up by the crop. But which sulphur fertilizer to choose?
The mineral based sulphur fertilizers available in the market today generally fall into two categories: elemental sulphur and sulphate salts. Elemental sulphur is itself insoluble in water. But now there is also a third option: Sulphur in the active thiosulphate form which is present in liquid fertilizers such as Thio-Sul® (ammonium thiosulphate), marketed by Tessenderlo Kerley, part of Tessenderlo Group.
Different sulphur-containing fertilizers
Elemental sulphur applied to the soil is at the mercy of time, temperature and moisture, to be transformed (oxidized) by soil microbes and made available to the plant in the sulphate form. In dry conditions or poor quality soil where the microbial activity is subdued this may be a problem. Granular elemental sulphur requires dispersion of the granules within the soil for the oxidation process to take place. This process is accompanied by an acidification effect, which is often beneficial in alkaline soils. The gradual transformation of sulphur into the sulphate form also helps to protect against sulphate leaching particularly on light sandy soils. The coating of urea with sulphur has been one route to producing a slow-release N source using the degradation of the sulphur coating by microbes.
In sulphur bentonite products, the bentonite clay absorbs moisture, causing the granules to disperse into finely divided particles, speeding up the conversion to plant available sulphate by microbes present in the soil. During this process sulphur bentonite products such as Major 90TM from Tessenderlo Kerley work as a soil amendment by generating acidity during the conversion process. Additionally, elemental sulphur resists leaching until converted into the sulfate form.
Thus, in these products, the sulphur becomes available to the crop slowly, albeit more rapidly than for elemental sulphur alone. In cool soils, the conversion of elemental sulphur to sulphate sulphur can take several weeks to months. Consequently, applications of sulphur bentonite fertilizers can be made in the autumn or early spring to allow time for the sulphur to become available to the plant. As soil temperature increases in the spring, conversion to sulphate, through oxidation by bacterial action in the soil, will increase, but nevertheless may not be fast enough for early season crop requirements particularly those with a high demand for sulphur during their rapid spiring growth. For this reason, sulphur-based fertilizers may in some situations be better suited to a fertilization strategy to build up available sulphur levels in the soil for subsequent cropping season.
Traditional sulphurbentonite pastilles.
and Tessenderlo Kerley’s sulphur bentonite prills.
Sulphate salts in solution are immediately available to the crop and are ideal for supplying the crops’ instantaneous need for sulphur. But under certain conditions (light soils and heavy rainfall) there is a risk of sulphate leaching. Sulphate salts do not have an acidifying effect on the soil (with the exception of ammonium sulphate which is slightly acidic in solution due to the ammoniacal nitrogen present). Since plants take up sulphate sulphur in solution, fertilizers such as gypsum and polyhalite which have low solubility may be less readily available to the crop.
With active thiosulphate liquid fertilizers, such as Thio-Sul® (ammonium thiosulphate) marketed by Tessenderlo Kerley, growers get the best of both worlds. But in what sense? Thiosulphates contains sulphur in the active thiosulphate form, in which sulphur is present in two distinctly different forms – the central sulphur atom (in yellow) is in an ‘oxidized’ form that more closely resembles sulphate sulphur and thus becomes plant available very rapidly. The peripheral sulphur atom (also yellow) is in a reduced form that more closely resembles elemental sulphur. This form is transformed into plant available sulphate sulphur more gradually. And yet thiosulphate is a liquid formulation that therefore gets to work as soon as it gets into the soil. These two complimentary forms of sulphur in the molecule means that the sulphur is made available to the plant over an extended period of time via a regulated release, in which all the sulphur will be eventually transformed (oxidized) by the soil microbes into plant available sulphate, part rapidly and the other more slowly. This provides prolonged sulphur nutrition, improving sulphur use efficiency to give more cost-effective and sustainable crop production. This transformation once again delivers a soil acidification effect (not only on the surface) helping to reduce soil alkalinity and reduce the pH in the rhizosphere. This effect enhances the uptake of nutrients such as phosphorus as well as micronutrients in the soil and ensures optimal availability and valorisation of these nutrients. The benefit is enhanced nutrient use efficiency and more cost-effective crop production. During this oxidation process the active thiosulphate itself transforms (reduces) micronutrients from higher oxidation (plant unavailable) forms to lower oxidation state available forms. The thiosulphate donates electrons to lower the oxidation state of the micronutrient. This too helps enhance nutrient use deficiency. There is also now scientific evidence that under certain conditions plants can take up sulphur in the thiosulphate formed directly. Tessenderlo Kerley International also markets the liquid fertilizers potassium thiosulphate (KTS®) and calcium thiosulphate (CaTs®).
Thiosulfate containstwo complimentaryforms of sulphur.
Thiosulphates can help improve nitrogen use efficiency Research has shown that thiosulphates can act as nitrification inhibitors when liquid nitrogen fertilizers resulting in more nitrogen being available to the crop. Thiosulphates, when added to nitrogen solutions in sufficient volumes, can help delay nitrification, resulting in a decrease of potential losses from nitrate nitrogen leaching. The scientific literature also suggests that thiosulphates can help reduce ammonia volatilization. Consequently, when they are added to nitrogen fertilizer solutions, they allow the crop to better utilize the applied nitrogen. The resulting mixtures are simple to use and for optimal results the ratio of thiosulfate within the nitrogen needs to be tailored to the crop, the soil and the fertilization program.
In crops that requires an important supply of sulphur throughout spring Thio-Sul’s concentrated liquid formulation is compatible with a wide range of liquid nitrogen fertilizers, particularly urea ammonium nitrate (UAN). This means it can be used in blends, to deliver sufficient quantities of liquid sulphur, thus optimizing the nitrogen-sulphur balance without any reduction in the nitrogen concentration in the mixture. The wide band width of application means the field can be treated rapidly and uniformly, saving time and effort. For growers using liquid UAN, Thio-Sul® is the perfect complementary liquid sulphur source, ideally suited for a more a sustainable, profitable and precise agriculture. ●
