Polysulphate solubility and transport in soils and mineral uptake by plants
Dr. Uri Yermiyahu
Start: Dec 2015 – End: Dec 2018
Polysulphate is a fertilizer produced from polyhalite mineral. The mineral is a hydrated sulfate of potassium, calcium and magnesium with formula: K2Ca2Mg(SO4)4·2H2O. Polyhalite mineral has been identified in a number of locations, including Texas (New Mexico), Poland, former Soviet Union, Rajasthan (India) and Cleveland (UK). The mine in Cleveland is the first operational polyhalite mine in the word. In this mine the polyhalite mineral seam is at the depth of about 1,250 meters being approximately 170 meters below the potash seam. The purity of the product is very high (95% polyhalite) with <5% of NaCl and trace elements, including Boron and iron, 300 and 100 ppm, respectively. The declared minimum analysis of Polysulphate for S, K, Mg and Ca is 48% SO3, 14% K2O, 6% MgO and 17% CaO, respectively. After mining, the material is treated to produce different granola sizes.
Polyhalite may serve as an alternative fertilizer that supplies four nutrients and that conceivably may provide a slower release of nutrients since it is less water soluble than more conventional sources. A number of studies comparing polyhalite to other K, and Mg fertilizers have shown that polyhalite was at least as effective as K2SO4 as a source of K, and was as effective as KCl plus MgSO4 (Barbarick, 1991). Lately, a field and pot trials which was done in UK with Cleveland polysulphate products (done by Cleveland Potash Ltd.) demonstrated that the K, Mg and SO4 nutrients in the polyhalite are as available and effective as those from established standard fertilizer sources (K2SO4, KCl and CaSO4).
Because polyhalite is not a mixture of salts but a single crystal it would be expected that all of its constituents would be released into solution proportionally, and indeed this was found to be the case (Cleveland Potash report). This behavior is expected to be similar in polyhalite-soil system. However, after solubility each constituent will interact with the soil differently and it will be affected also by soil properties. For example, the adsorption strength of K+ to mineral soil surfaces is lower than Ca2+ and Mg2+ and the total adsorption capacity increases as the mineral (e.g. montmorillonite) content in the soil increases. Unlike the cations mentioned above, sulfate is an anion with relatively low interaction with soil mineral surfaces. Moreover, the chemical interactions in soil solution between constituents themselves (Ca and SO4) and with other soluble elements affect the constituents’ free concentration. These processes affect the mobility of the nutrients in soil and their availability to plants. Boron (B) is not a constituent of the polyhalite crystal, however the exact chemical form of B in the Polysulphate material is unknown.
Boron is an essential micronutrient for normal growth of most plants. At high environmental levels though, B is markedly toxic to plants. There is a relatively small range differentiating between soil solution B concentrations that are deficient and those that cause toxicity in plants (Keren and Bingham, 1985). Boron is taken up into plants from the aqueous soil solution and the rate of B uptake is largely a function of the solution’s B concentration. The concentration of B in soil solution is strongly affected by B-soil chemical interactions – primarily adsorption-desorption processes that are, in turn, affected by soil constituents and conditions (Yermiyahu et al., 2001).
