Polyhalite is a naturally occurring potassium, calcium, magnesium sulfate mineral with a chemical formula K2Ca2Mg(SO4)4·2(H2O) corresponding to 13.0% K, 13.3% Ca, 4.0% Mg, and 21.3% S. Many horticultural crops including potatoes (Solanum tuberosum L.) require each of the nutrients in PH, often in significant quantities, and are commonly included in nutrient management programs. Polyhalite has a known potential as a fertilizer (Barbarick, 1989; Fraps, 1932) but has had limited use worldwide because it has not been commercially available. Consequently, literature on the performance of PH fertilizer for agronomic crops is very limited compared with common commercial fertilizer sources used to supply K, Ca, Mg, and S to plants. With the discovery of the largest high-grade known resource of PH in the world, the Zechstein deposit located in the southern North Sea Basin in the United Kingdom (Kemp et al., 2016), PH fertilizer is becoming available worldwide. Polyhalite fertilizers from the Zechstein deposit have purities approaching 90% and have a guaranteed analysis of 12% of K, 12% Ca, 3.6% Mg, and 19% S (Sirius Minerals, 2016). Polyhalite is a neutral salt with a solubility of 27 g·L−1 at 25 °C, lower than potassium chloride and potassium sulfate, and has <3% Cl, making it suitable for chloride-sensitive crops such as potatoes.
Potatoes make an ideal crop to evaluate PH as a fertilizer. Westermann (2005) reported K uptake in potatoes to exceed N, P, Ca and Mg with an average uptake of 4.2 kg of N, 0.55 kg of P, 6.0 kg of K, 1.6 kg of Ca, 1.1 kg of Mg, and 0.4 kg of S per ton of tuber yield. In addition, chloride or salt content can negatively affect dry matter percentage (Panique et al., 1997; Roy et al., 2017) and petiole nitrate contents (James et al., 1994) such that nonchloride forms of K are desired as potato fertilizers. Because potatoes are commercially grown on soils often inherently low in fertility and irrigated to achieve high yields and tuber quality, large quantities of fertilizers, particularly N, P, and K, are frequently applied.
Potassium influences both quantity and quality of potatoes (Karam et al., 2011; Lakshmi et al., 2012) through various mechanisms such as enzyme activation (Werij et al., 2007), stomatal conductance, photosynthesis, protein synthesis, and transport of sugars and starch (Mengel and Kirkby, 1987). Potassium application resulted in higher leaf area, increased plant height, prolonged bulking duration, enhanced tuber size, and a higher proportion of medium and large size grades and higher yields (Trehan et al., 2001). Potassium also affects dry matter percentage, increases ascorbic acid content, decreases reducing sugars, phenol contents, and enzymatic degradation (Werij et al., 2007). Calcium is important for potato growth, development, and yield (Chang et al., 2007; Kumar et al., 2007a). Calcium influences tuber grade index, dry matter content, and tuber quality (Tawfik 2001); and improves cell wall rigidity, firmness, plasma membrane structural stability, and tuber periderm calcium concentrations. Thus, plant resistance to diseases and tuber disorders such as brown center, hollow heart, internal brown spot, black leg, gangrene (Kondo et al., 2001), and soft rot (Abo-Elyousr et al., 2010) are affected by Ca nutrition. Magnesium is a constituent of chlorophyll molecules and affects photosynthesis (Peaslee and Moss, 1966), carbon allocation, and the level of reactive oxygen species at molecular levels (Cakmak and Kirkby, 2008). At the canopy level, Mg increases N, P, and K uptake and thereby, increases yield (Kene et al., 1990). Sulfur plays a significant role in amino acid, protein, and chlorophyll synthesis and influences N metabolism and tuber composition by decreasing sugar content (Muttucumaru et al., 2013) and by controlling diseases such as common scab and black scurf.
Potato is the fourth most important crop in the world with a total production of ∼374 million tons consumed by more than 1 billion people (CIP-International Potato Centre, 2014). An increasingly important crop in Brazil, 3.7 million tons are produced on 0.13 million ha with an average yield of 28 t·ha−1 (FAO–Food and Agriculture Organization, 2017). Potato production in Brazil is concentrated in the Southeast in two states where the soil and climate are most conducive to potato production; São Paulo and Minas Gerais produce more than 50% of the nation’s total potato crop with an average yield of 30 t·ha−1 (IBGE, 2017). Mean potato yield in Brazil is lower than in the United States where potato yields in 2015 averaged 47 t·ha−1 with a high of 68 t·ha−1 in Washington State (USDA, 2016), reflecting more difficult growing conditions in the humid subtropical climate. Because the soils in Brazil are deep and highly weathered, and frequently deficient in many plant nutrients (Bernardi et al., 2002), they may respond to the nutrients in PH fertilizer.
Fertilization rate for potatoes in Brazil is high with an average K application rate of 189 kg·ha−1 reported by Bernardi et al. (2002) and 166 kg·ha−1 reported by IBGE (2017) but varies considerably: 199–374 kg K/ha, 64–250 kg N/ha, and 196–371 kg P/ha (Schepers et al., 2015). Fertilizers are predominately applied in blends, such as 4–14–8, in which triple super phosphate (TSP) or SSP is used as the P source such that Ca, and S in the case of SSP, is provided in the blend. Additional N is applied at hilling, and ammonium sulfate (AS) is a common N source which provides S. Of interest is that there is a shift occurring in Brazil to lower costs by shifting to urea as the N source and MAP as the P source which would eliminate Ca and S in the blends. Bernardi et al. (2002) reported the N, P, K, Ca, Mg, and S removed per metric ton of tuber yield in Brazil to be 3.0, 0.3, 4.0, 0.2, 0.2, and 0.2 kg·ha−1, respectively, considerably less than that reported by Westermann (2005) for potatoes grown in the United States where yields are higher.
The response of potato crop to MOP or SOP depends on the soil fertility, climate, and crop variety grown (Bansal and Kumar, 1998). The effects of K in a fertilizer are associated with the way in which it is chemically combined in the fertilizer (Zehler et al., 1981). The accompanying anion may affect the uptake of nutrients, yield, and quality of potato. For example, Pavuluri et al. (2017) indicated the role of sulfur in PH in increasing corn (Zea mays L.) grain yields compared with MOP in the southern highland region of Tanzania. Similarly, enhanced yield from PH compared with MOP, SOP, and magnesium potassium sulfate (SOPM) was attributed to Ca by Mello et al. (2018) in tomato (Solanum lycopersicum L.) in the São Paulo region of Brazil under very low soil K conditions. In soils with S deficiency, SOP has been more effective in increased yield of potato than MOP (Bansal, 2003). However, studies also showed that MOP was similar or higher than SOP for potato quantity and quality, and yield (Davenport and Bentley, 2001; Khan et al., 2010, 2012; Panique et al., 1997). Lower solubility of PH relative to MOP or SOP could be advantageous for plant nutrient uptake in the tropical conditions of Brazil by reducing the leaching loss of nutrients and by improving the residual effect of fertilizers on subsequently grown crops (Yermiyahu et al., 2017).
We investigated how a common fertilizer blend in Brazil made with MOP as the K source and SSP as the P source compared with the same blend made with either PH or a synthetic PH created using a mixture of SOP, kieserite, and gypsum with MAP as the P source for commercial potato production. Our main interest in this paper is whether the three blends perform similarly with respect to tuber yield and quality in two different growing environments in the dry season and the wet season.
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