Sulphur (S), magnesium (Mg) and calcium (Ca) are essential plant nutrients. They are called “secondary” nutrients because plants require them in smaller quantities than nitrogen (N), phosphorus (P), and potassium (K). On the other hand, plants require these nutrients in larger quantities than the “micronutrients” such as iron, zinc and boron.
Secondary nutrients deficiencies are increasingly becoming an important limiting factor in intensive crop production systems, especially in soils fertilized only with N, P and K.
Sulphur (S) is absorbed primarily in the sulphate form (SO4-2) by plants. It is part of every living cell and required for synthesis of amino acids and proteins. Sulphur is also important in photosynthesis and crop winter hardiness. Leguminous plants need S for efficient nitrogen fixation.
Crops such as corn that have a high dry matter production generally require the greatest amount of sulphur. Also potatoes, cotton, sunflower, canola, Brassicas (cabbage, broccoli, cauliflower) and many other vegetables require large amounts of S.
In the past, the deposition of sulphur to agricultural land was well in excess of crop and animal requirements and sulphur was not discussed as a crop nutrient. However, over the last 20 years, the anthropogenic emissions of sulphur – usually as sulphur dioxide (SO2) from the burning of fossil fuels – had reduced to such an extent, that sulphur became of great potential significance as a nutrient to counter S deficiency in crops and grassland.
Many coarse-textured, sandy soils and low-organic matter are found to be sulphur deficient for crop production. Farmers usually apply S fertilizers before planting, so it is prone to losses by rains or irrigation that will leach out the S from the soil profile and will not be taken up by the crop.
Magnesium (Mg) is an essential component of the chlorophyll molecule, with each molecule containing 6.7% Mg. It is essential for photosynthesis, cell division, protein formation phosphate metabolism, plant respiration and the activation of several enzyme systems.
Due to its potential for leaching in highly weathered soils, Mg deficiency is a critical concern in acid and tropical soils.
Calcium (Ca) is responsible for proper plant cell division and for strengthening cell walls. Calcium improves the absorption of other nutrients by roots and their translocation within the plant. It activates enzyme systems, helps convert nitrate-nitrogen into proteins and contributes to improved disease resistance. Without enough Ca, roots become stunted with impaired activity.
Calcium deficiency can be found in tropical and in acidic soils. Calcium supply can alleviate aluminum toxicity symptoms which are common in those regions.
Polysulphate, a new fertilizer with secondary nutrients
Polysulphate is a new multi-nutrient fertilizer, available in its natural state, and mined in the UK. Polysulphate contains: 19.2% S as sulphate, 14% K2O as sulphate of potassium, 3.6% Mg as magnesium sulphate and 12% Ca as calcium sulphate. All its nutrients are available for plant uptake over the course of a season. Its chloride content is very low, thus can be applied to sensitive crops.
Polysulphate is natural and has a low carbon footprint and the Standard Grade is certified for organic use.
Being a natural crystal, it has a very unique dissolution pattern, which releases its nutrients gradually after being applied to the soil. While most sources of S have high rates of dissolution, releasing S immediately with the risk of losing S as leached sulphate – Polysulphate provides a prolonged availability of S. This prolonged release pattern of S from Polysulphate matches the uptake timing of S by crops and minimizes the risk of loss of sulphate by leaching.