Technical Bulletin: Polysulphate as a Magnesium Source

Granular Polysulphate® is a natural mineral from underneath the North Sea that contains four nutrients: sulfur (19.2%), potassium (14% K2O), magnesium (3.6%), and calcium (12.2%). It is a single complex crystal that is mined, crushed, then ready to use on agricultural fields. This multi-nutrient mineral (polyhalite) is sold by ICL as Polysulphate® and is OMRI certified, low in chloride and crop safe, suitable for use alone or in blends and compound fertilizers.

Compared to potassium magnesium sulfate (SOPM) with ~11% of Mg, Polysulphate contains 3.6% Mg. However, research trials at the University of Georgia found Polysulphate compared to SOPM applied to supply either Mg (14 lbs. Mg/A = 400 lbs./A of Polysulphate or 133 lbs./A of SOPM) or K2O (29 lbs. K2O/A = 208 lbs./A of Polysulphate or 133 lbs./A of SOPM) resulted in similar or higher plant Mg concentrations for Polysulphate (Figures 1 & 2). Higher Mg for crop uptake from the soil (higher % cumulative available Mg compared
to the amount applied – Nutrient Use Efficiency) was achieved with polyhalite (Figure 3 adapted from Barbier et al., 2017).

Increase in Mg Uptake with Polysulphate

1. As a slow-release mineral fertilizer for S, Mg, K and Ca, Polysulphate can reduce potential loss of Mg to leaching, while increasing Mg uptake. Compared to other cations like NH4, K and Ca, Mg is more mobile and susceptible to leaching (Gransee & Führs, 2013).
2. Polysulphate contains 12.2% Ca compared to other common sources of soil-applied Ca like gypsum (calcium sulfate) that is ~23% Ca. The presence of Ca in Polysulphate can enhance Mg uptake in plants. For example, increasing Ca in the nutrient solution resulted in increased Mg root uptake except at remarkably high Ca concentrations (Figure 4 from Fageria, 2009).

Impact of Polysulphate on Magnesium Availability

• Increasing the Ca concentration to a certain level increases the uptake rates of Mg. Higher Mg concentrations in corn and cotton plant analysis with approximately 200 lbs./A of Polysulphate.
• Magnesium is more mobile in soil compared to other cations (NH4 +, K+ and Ca++) making it more susceptible to leaching and more difficult to manage. Slow release of Mg by Polysulphate can increase Mg availability and reduce leaching loss.
• The high mobility of Mg makes it particularly challenging to relate soil test results to crop growth. Both soil and plant analysis are recommended to monitor and manage for Mg deficiency. Plant analysis shows sufficient supply of Mg in plants when using Polysulphate as the Mg source.

Granular Polysulphate Application

Polysulphate offers flexible applications (preplant and incorporated, banded, or broadcast in the spring), use across all crops, conventional or organic, and a release pattern that matches crop uptake. See Table 1 for baseline rates based on S requirements.

Polysulphate recommendation for different crops

Table 1. Polysulphate rates for various crops to meet the S fertilizer requirement.

* Polysulphate is a four nutrient mineral consisting of K2O (14%), Ca (12.2 %), Mg (3.6%), and S (19.2%). Therefore, these recommendation ranges are guidelines and should be adjusted based on soil test levels of K, Ca and Mg. Soil test sulfur (S) is not entirely accurate and is highly variable base on topography, soil texture, sample depth, and organic matter. Sulfur soil test results should be used with caution with the understanding that the soil texture, organic matter, recent rainfall events may be more predictive of future S needs than a soil test. These recommended ranges are based on estimated S removal.

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Source
• Barbier, Marcel, Yuncong C. Li, Guodong Liu, Zhenli He, Rao Mylavarapu, and Shouan Zhang. 2017. Characterizing polyhalite plant nutritional properties. Agric Res Technol 6, no. 3.
• Fageria NK. 2009. The use of nutrients in crop plants. CRC Press, Taylor and Francis Group, London.
• Gransee, A., & Führs, H. 2013. Magnesium mobility in soils as a challenge for soil and plant analysis, magnesium fertilization and root uptake under adverse growth conditions. Plant and Soil, 368, 5-21.