The crucial role of the secondary nutrients in sustainable agriculture

global food security is especially of concern looking forward as world population is expected to increase by about 35% over the next 40 years. agricultural output will need to significantly increase to feed the growing population. most of the increase is expected to come from producing more on existing farmland. boosting crop yields can be achieved by the implementation of balanced fertilization practices, which will also insure the preservation of the soil fertility.

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. many soils worldwide are poor in fertility – specifically in secondary nutrients – as they have consistently been depleted of their native nutrients due to continuous cultivation. this has resulted in the soil becoming a poor food crop producer.
plants get their secondary nutrients from the soil, but natural processes don’t supply enough to sustain crop production. mineral fertilizers containing s, mg and ca replenish nutrients in the soil to ensure plants grow to their full potential. this results in crops that contribute enough mineral elements for adequate animal and human nutrition and improve global food security.
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 (rape seed), 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.

 

sulphur deficiency in maize (usa)

 

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.

magnesium deficiency in soybean (brazil)

 

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.

 

ca deficiency in grapes (china)

polysulphate®, a new fertilizer with secondary nutrients
polysulphate is a new multi-nutrient fertilizer, mined in the uk, and available in its natural state. uniquely, it has four nutrients: sulphur, magnesium, potassium and calcium. all its nutrients are in sulphate form and are readily available for plant uptake.

 

polysulphate is mined in the uk by icl boulby, from the polyhalite layer of rock deposited 260 million years ago over 3,300 feet below the north sea off the north yorkshire coast.
polysulphate contains 48% so3 (19.2% s) as sulphate, 14% k2o (11.6% k) as sulphate of potassium, 6% mgo (3.6% mg) as magnesium sulphate and 17% cao (12.2% ca) as calcium sulphate. its chloride content is very low which means it can be applied to sensitive crops. it does not affect soil ph and does not acidify the soil.

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.
additional benefits include that polysulphate is a natural product and has a low carbon footprint. it delivers dependable high value with low negative environmental impact and is certified for organic use.

 

agronomic and environmental benefits of polysulphate for farmers
the prolonged release of s from polysulphate helps growers to save money on fertilizer costs, by cutting back on the number of fertilizer applications necessary. in addition, there is an environmental benefit of reducing the risk of s leaching.
polysulphate is suitable as a source of sulphate for inclusion with multiple dressings of fertilizers over the season, but its strength is that it can be recommended as a single early dressing without causing a sudden high concentration of sulphate in the soil and with minimum risk of loss through leaching.

the steady, prolonged release of s over a longer period than other fertilizers is a key advantage of polysulphate. along with s, polysulphate provides three other nutrients (k, mg and ca) in one single application and has a positive effect on crop growth, yield and quality – many experiments worldwide show this steady yield response to polysulphate application. for example, in a trial in indonesia. rice paddy plants fertilized with polysulphate fertilizer are 8-10 cm taller than those grown according to farmer’s practice. importantly, the polysulphate grown plants are also more tolerant to lodging:

paddy rice trial in karawang, jawa barat, indonesia investigating the effect of polysulphate on the yield and quality parameters of rice.

in another trial with potato in poland, the application of polysulphate produced plants with significantly more tubers and of bigger size than the plants given the farmer’s usual crop nutrition treatment:

 

trial in popowo, poland investigating the effect of polysulphate on the yield and quality parameters of brook variety of potato.

polysulphate also improves the quality of produce. there is a growing body of evidence for this in crops in many parts of the world. for example, in an experiment with wheat carried out by southeast missouri state university, usa, polysulphate improved n:s ratio in grains and thus enhanced baking quality of the flour.
another key environmental advantage of polysulphate is the improvement of the nitrogen use efficiency (nue). by using polysulphate, n can be applied independently, separating s from n application. nitrogen can be applied after germination at the right time for the crop, in the right form, and in right weather conditions avoiding n overdosing or leaching. with polysulphate, higher n use efficiency can be achieved without waste and unnecessary cost to the farmer or the environment.

concluding remarks
fertilizers play a critical role in the world’s food security. targeted, balanced and precise crop nutrition, with secondary nutrient and gradual and prolonged release of adequate s, is one of the keys to unlocking crop performance and achieving both crop productivity and profitability.

 

for more info please contact: patricia imas

 

 

this article was published in the special issue for fwd 2019