Sulphur is essential for the production of plant proteins, chlorophyll, enzymes and vitamins. It promotes nodule formation in legumes, aids seed production and helps seedlings survive cool, moist soil conditions. International soil expert Neal Kinsey recommends that soil should contain at least 20ppm (parts per million) sulphur for optimal production of most crops. Woody plants and root crops will perform best at a sulphur level of 50ppm when all other plant nutrient requirements have been met. S ulphur is essential for root development.
Research indicates that wheat with sufficient sulphur shows a 50% better root development than wheat with a shortage. Similar results were found in citrus, wine grapes, cotton and maize. Addressing a sulphur deficiency will enhance crop growth and help lengthen the shelf-life of vegetables. Sulphur also helps improve the palatability and sweetness of crops. “People always talk about how sweet watermelons tasted in the past,” Kinsey says,” and the reason why they don’t anymore is because farmers are unwittingly applying less sulphur through fertiliser mixtures to soil than before.
A nitrogen excess often ties up copper so that it becomes unavailable to the crop, decreasing its palatability.” deficiency usually shows up in a light or yellow discolouration of young leaves. However, a leaf sample should still be taken, as leaf necrosis has many causes. Nitrogen and potassium deficiencies also cause leaf discolouration, but this starts on older leaves. Kinsey warns that when sulphur deficiency is evident on the leaf, there’s already been a negative impact on the crop. ccording to Kinsey, soil was already sulphur-deficient in the 1970s.
Levels have dropped even more because of changes in fertiliser composition. He explains that in the past, a sulphur deficiency was partially addressed by the fact that many fertiliser mixtures automatically contained sulphur, but this has been phased out. Shortages are further exacerbated by the continuous push for higher yields, as well as a decline in atmospheric sulphur deposition. Most soil that Kinsey has sampled is sulphur-deficient. Most affected is soil with a sandy or light texture, well-drained soil, or soil low in organic matter. Also, the cooler the soil, the less available the sulphur. Kinsey advises that sulphur, as boron and nitrogen, must be supplied regularly at slightly higher levels than the plant requires for optimal production because it leaches easily.
How to address a sulphur deficiency
Compost and manure are not good sources of sulphur. “You’re not going to get something for nothing. If animals or compost don’t receive sufficient sulphur, then they can’t produce it.” dding commercial sulphur is the best way to address a shortage. Kinsey stresses that farmers must base the sulphur product on the soil’s nutritional needs. Elemental sulphur is ideal when a quick boost is needed, but as it’s water-soluble, it soon leaches away.
Another drawback is that it only addresses a sulphur deficiency and therefore might not be economically viable when compared to other sources such as copper sulphate that also addresses copper deficiency, gypsum that will additionally raise the calcium levels, ammonium sulphate that doubles as a rich source of nitrogen, or magnesium sulphate which also provides magnesium. In John Fair’s Guide to Profitable Pasture, the author explains that he used ammonium sulphate on pasture, based on the going price of elemental sulphur minus the purchase price of additional nitrogen.
Ammonium sulphate was always a more economical purchase. He later found that gypsum was an even cheaper source and that it also increased the calcium levels. Kinsey always emphasises the importance of maintaining the right percentages of calcium and magnesium in the cation exchange of the soil (see “Elements vital to soil” in Farmer’s Weekly, 8 August 2008). “The closer the calcium/magnesium balance is to ideal, the less fluctuation will be seen in sulphur levels,” he explains. “Once equilibrium is achieved, it will also become more difficult for sulphur to leach away.”
A huge advantage of sulphur is that, when applying more than is needed, farmers will be able to strip the soil of excessive elements such as sodium, magnesium and or potassium – provided there is sufficient calcium in the soil. However, too much is never good so farmers should be careful not to overapply sulphur, as it can burn plants, cause phytotoxicity symptoms and acidulate the formation of iron. Contact information: SA Biofarm: (012) 333 4222 or visit www.sabiofarm.co.za. |fw
