ISRAEL JOURNAL OF PLANT SCIENCES, 64 (2017) 3-4 http://dx.doi.org/10.1163/22238980-06401001 Polyhalite as a multi nutrient fertilizer – potassium, magnesium, calcium and sulfate Uri Yermiyahua*, Isaac Ziporia, Inna Faingolda, Ludmila Yusopova, Nitay Fausta, Asher Bar-Talb aGilat Research Center, Agricultural Research Organization, M.P. Negev, 85280, Israel; bInstitute of Soil, Water and Environmental Sciences, Agricultural Research Organization, Volcani Center, Derech HaMaccabim, Rishon LeTsiyon, 75359, Israel ABSTRACT KEYWORDS Polyhalite is a hydrated sulfate of potassium (K), calcium (Ca) and magnesium (Mg) with the formula: fertilization; polyhalite; potassium; calcium; K2Ca2Mg(SO4)4·2H2O. The main objective of the present study was to investigate and compare the ef- ficiency of polyhalite as a fertilizer supplying K, Ca, Mg and sulfur (S) relative to equivalent soluble magnesium; sulfate salts. The specific objectives were to investigate: 1. The release and transport of Ca, Mg, K and S in soil; 2. Uptake of these minerals by wheat plants and 3. Biomass production of wheat plants. To meet these objectives, two pot experiments (20 l pots filled with dune sand) were conducted in which the effects of four doses of polyhalite (0, 500, 1,500 and 2,500 kg/ha) and one dose of Ca, Mg and K sulfate salts (equivalent to the 1500 kg/ha polyhalite dose) were investigated. In a third experiment, residual effects of the fertilizers were studied using the pots from the first experiment. Different leaching fractions were used in the experiments (30% in the first one and 7–10% in the second and third experiments), to investigate the effect of water management on mineral transportation in the soil and plant uptake. Polyhalite was found to be a more efficient fertilizer for supplying K, Ca, Mg and S relative to equiva- lent soluble salts. To meet the plant required ratios for Ca, Mg and K, the polyhalite dose should be applied accordingly to provide sufficient Ca and Mg, and additional fertilizers should be used as a source of K. Transport and leaching of Ca, Mg, K and S in soil following polyhalite application was lower than following the application of the equivalent sulfate salts. The residual effect of polyhalite fertilizer on the subsequently grown crop was higher than the effect from the equivalent sulfate salts, especially regarding Ca, Mg and S. Irrigation management, as determined by the leaching fraction, has a strong impact on the efficiency of polyhalite as a source of K, Ca, Mg and S for plant nutrition. Introduction crystals. It is water soluble with precipitation of gyp- sum and syngenite (K Ca(SO ) *2H O) which is also Polysulphate fertilizer is the trade mark of the natu- 2 4 2 2 soluble, leaving behind a precipitate of gypsum. ral mineral polyhalite and is produced by ICL (North The purity of the Polysulphate product is very high York Moors National Park, UK) at the Boulby mine in (95% polyhalite) with <5% sodium chloride (NaCl) and Cleveland, UK. The mineral is a hydrated sulfate of po- traces of boron (B) and iron (Fe) at 300 and 100 ppm, tassium (K), calcium (Ca) and magnesium (Mg) with respectively. The declared minimum analysis of poly- the formula: K Ca Mg(SO ) ·2H O (Tiwari et al., 2015). 2 2 4 4 2 halite for S, K, Mg and Ca is 48% sulfur trioxide (SO ), The mineral was identified in a number of locations 3 14% potassium oxide (K O), 6% magnesium oxide including Cleveland (UK), Poland, Rajasthan (India), 2 (MgO) and 17% calcium oxide (CaO), respectively. Af- Texas (New Mexico) and Russia (Frafs, 1932; Lepeshkov ter mining, the material is treated to produce different & Shaposhnikova 1958; Tiwari et al., 2015). The mine in granule sizes (ICL Fertilizers, Cleveland, UK). Cleveland is the first operational polyhalite mine in the Polyhalite, which may serve as a suitable fertilizer to world. In this mine, the polyhalite mineral seam is at a supply four nutrients, is less water soluble than the more depth of about 1,250 m, approximately 170 m below conventional sources and may conceivably provide a the potash seam. Polyhalite is a single complex crystal slower release of nutrients (Barbarick, 1991). A number (triclinic, pseudo-orthorhombic). It usually appears as of studies comparing polyhalite to other K and Mg fertil- fibrous, massive pieces and rarely appears as tabular izers have shown that polyhalite is at least as effective as CONTACT Uri Yermiyahu [email protected] This paper has been contributed in honor of Professor Uzi Kafkafi © Koninklijke Brill NV, Leiden, 2017 146 U. YERMIYAHU ET AL. potassium sulfate (K2SO4) as a source of K, and at least The use of polyhalite as a fertilizer in comparison as effective as potassium chloride (KCl) plus magnesium to the equivalent soluble, commercial salts has been sulfate (MgSO4) as a source of K and Mg (Barbarick, 1991). studied in a number of experiments. Barbarick (1991) In pot experiments with corn (Zea mays, L.) and sor- found a better yield response of sorghum-sudangrass ghum (Sorghum bicolor, L.), Fraps (1932) showed that to increasing levels of polyhalite when compared to polyhalite is suitable for use as a potash fertilizer when the same application rate of soluble salts. Tiwari et al. ground to pass through a 20 mesh sieve. He showed (2015) found an advantage regarding oil production in that polyhalite provided a 96% equivalency of K avail- mustard and sesame when applying polyhalite com- ability compared to KCl and K2SO4 fertilizers. Similar pared to using equivalent amounts of soluble fertil- results were found for potato (Salanum tuberosm, L.), izers. In cabbage and cauliflower, increased quality flax Linun( usitatissum, L.) and beets (Beta vulgaris, L.) and higher yields were obtained when polyhalite was (Panitkin, 1967; Lepeshkov & Shoposhnikova, 1958). used as a source for Ca, Mg and S compared to fertiliza- Barbarick (1991) was the first to compare polyhalite tion with the equivalent soluble salts (Satisha & Gane- with the same rates of S, K, Ca and Mg sulfate salts. shamurthy, 2016). Field and pot trials recently carried out in the UK An important benefit of using polyhalite as a fer- on Cleveland polyhalite products (done by Cleve- tilizer, when compared to the equivalent salts, is the land Potash Ltd.) demonstrated that the K, Mg and S rate at which it releases minerals to the soil profile. The nutrients in polyhalite are as available and effective leaching of Ca, Mg, K and S from polyhalite appears as those from established standard fertilizer sources to be slower than the leaching of these ions from the (K2SO4, KCl, MgSO4 and CaSO4) (Tiwari et al., 2015). commonly used soluble salts (Barbarick, 1991; Vale, The molecular ratios of K : Ca : Mg : SO 4 are 2:2:1:4, re- 2016). In this regard, polyhalite has the potential for spectively and the weight ratios of these nutrients are longer-term effects compared to commercial fertil- 3.2:3.3:1:5.3, respectively. The desired ratios in fertiga- izers. The assumption was that uptake efficiency by tion are 5:2:1:1.5, respectively. This means that if, for wheat plants of Ca, K, Mg and SO 4 would be higher example, Mg is maintained at the optimal level, there when the nutrients were applied as polyhalite than as will be a K deficiency and over-fertilization with Ca their separate sulfate salts. and SO 4 . The main objective of the present study was to in- As polyhalite is not a mixture of salts but a single vestigate the efficiency of polyhalite as a fertilizer sup- crystal, it would be expected that its constituents plying K, Ca, Mg and S relative to equivalent soluble would be released into solution proportionally, and salts. The specific objectives were to investigate the indeed this was found to be the case (Barbarick, 1991). effect of fertilization with different doses of polyhalite This behavior is expected to be similar in polyhalite-soil relative to equivalent soluble salts on: 1. The release systems. However, after solubilization, each constituent and transport of Ca, Mg, K and S in soil; 2. Uptake of will interact with the soil differently and be affected by these minerals by wheat plants and 3. Biomass produc- soil properties. For example, the adsorption strength of tion of wheat plants. K+ to mineral soil surfaces is lower than Ca2+ and Mg2+ and the total adsorption capacity increases as the clay mineral (e.g. montmorillonite) content in the soil in- Materials and methods creases (Feigenbaum et al., 1991; Bar-Tal et al., 1991a; 2015 experiment Bar-Tal et al., 1991b). Unlike the aforementioned cations, sulfate is an anion and has a relatively low interaction The experiment was carried out in 20 l pots filled with with mineral surfaces of the soil. Moreover, chemical dune sand. At the bottom of each pot, a rockwool interactions in the soil solution between the constitu- sleeve was fitted to ensure water suction and avoid ents themselves (Ca and SO4) and with other soluble water logging (Ben-Gal and Shani, 2002). Each pot was elements, affect the constituents’ free concentrations. leached with 60 l (three pot volumes) of desalinated These processes affect nutrient mobility in the soil pro- water to remove all salts, although low concentra- file and thus, their availability to plants (Nye & Tinker, tions of some salts remained e.g. Ca and Mg, 10 and 1977; Addiscott & Wagenet, 1985; Barber, 1995; Bar-Tal 1 mg/l, respectively. After leaching, the upper 5–7 cm et al., 1991a; Bar-Tal et al., 1991b; Kolahchi & Jalali, 2006). layer of the soil in each pot was mixed with powder ISRAEL JOURNAL OF PLANT SCIENCES 147 Polysulphate (produced by ICL at Boulby mine in the sured.