Potential of Polyhalite Fertilizers to Enhance Potato Yield and Quality in the United Kingdom

Abstract

Following three decades (1961-1990) of consistent increases, average potato yields in the UK have plateaued at 36-44 Mg ha^–1. Coincidentally, worldwide atmospheric sulfur (S) deposits have substantially declined, resulting in an emerging occurrence of S deficiencies in many crop species. It was hypothesized that S donor fertilizers might restart the trend of increasing potato yield in the UK. Polyhalite, available as a new commercial fertilizer marketed as Polysulphate^® by ICL, is a natural hydrated sulphate of K, Ca, and Mg with the formula: K₂Ca₂Mg(SO₄)₄•2H₂O. Polyhalite is comprised of 48% sulfur trioxide (SO₃), 14% potassium oxide (K₂O), 6% magnesium oxide (MgO), and 17% calcium oxide (CaO). Nevertheless, and due to the very high K requirements of potato crops, the relatively low proportion of this nutrient in polyhalite does not permit the use of this fertilizer as a sole K source. ICL PotashpluS^® (ICL UK) is a new granular fertilizer formulated using a combination of potash (MOP, KCl) and polyhalite, in the formula: 37% K₂O, 24% SO₃, 3% MgO and 8% CaO. Both fertilizers contain traces of boron. The objective of the present preliminary study was to evaluate the influences of various polyhalite rates and combinations with MOP on tuber yield and quality. A set of three separate experiments was carried out, in which MOP was compared to PotashpluS (Exp. 1), MOP was progressively replaced by polyhalite (Exp. 2), and PotashpluS was progressively replaced by polyhalite (Exp. 3). In all three experiments, the application doses of nitrogen (N), phosphorus (P), and potassium (K) were kept equal throughout treatments, whereas the application doses of calcium (Ca), magnesium (Mg), and S were modified by the polyhalite rates (100, 200, and 300 kg ha^–1, in Exp. 2 and 3). While the differences in crop performance between MOP and PotashpluS-applied plots were small, replacing MOP or PotashpluS by polyhalite resulted in significantly enhanced yields at the higher polyhalite rates. It appears that the combination of high application doses of all three nutrients, Ca, Mg, and S together promotes higher yields than when each nutrient applied alone. In addition, high Ca rates increased tuber Ca concentration at harvest, and reduced tuber weight loss during storage. In conclusion, the set of experiments carried out in the present study demonstrates the potential of polyhalite fertilizers to enhance potato crop performance and tuber yield and quality through a more balanced mineral nutrition. However, further research is necessary to elucidate the contribution of Ca, Mg, or S to this enhancement, and to establish precise fertilization strategies for various edaphic conditions.

Keywords: Calcium; magnesium; polyhalite; Polysulphate; Solanum tuberosum L.; sulfur; tuber quality.

Introduction

Potato (Solanum tuberosum L.) is among the five most important staple crop species cultivated worldwide. With 144,000 ha cultivated with potatoes, and production of 5.25 million tonnes in 2019 (FAO, 2021), the United Kingdom is among the 15 leading potato producing countries in the world.

Historical analysis of the UK potato industry from 1961 to 2019 shows a consistent decline in the potato cultivated area until year 2000, when it stabilized at about 140,000 ha (Fig. 1A). Annual production slightly decreased, exhibiting substantial fluctuations until year 2000, from when it consistently dropped from 6.6 to the present levels of 5.2 million tonnes (Fig. 1A). The average annual potato yield doubled from 22-44 Mg ha^–1 during the years 1961-2000 but since then it remained stable or even decreased (Fig. 1B).

During the 20^th century, industrially-generated atmospheric S deposition could meet agricultural requirements. Nevertheless, this situation changed remarkably with the declining S depositions during the beginning of the 21^st century. In Indiana USA, for example, soil enrichment by S pollution was cut by 45% from 2001 to 2015 (Camberato and Casteel, 2017). Similar processes were reported in Iowa, USA (Sawyer et al., 2015), as well as in Western Europe (Engardt et al., 2017) and in China (Chen et al., 2019).

Optimum management of crop nutrition supports high tuber yield (Koch et al., 2019a), however, it is also significant to primary tuber quality traits including dry matter and starch contents; skin integrity; and tolerance to various diseases (Zhang et al., 2018; Koch et al., 2019b; Naumann et al., 2020). While efficient potato crop nutrition management is well established in the UK with regard to N, P, and K, less attention has been paid to other essential macronutrients such as Ca, Mg, and S.

The significance of Ca nutrition to the development and yield of potatoes has been investigated in the last two decades, and the number of studies demonstrating Ca effects on tuber number and size is steadily increasing (Ozgen et al., 2003; Palta, 2010; Seifu and Deneke, 2017; Singh, 2018; Potarzycki and Grzebisz, 2020). In addition, Ca is pivotal to the process of tuber skin development and maturation, being a key element in periderm cell wall integrity (Ginzberg et al., 2012), which explains the contribution of Ca to the generally enhanced postharvest tuber quality (Palta, 2010; Murayama et al., 2016; Keren-Keiserman et al., 2019; Koch et al., 2019b; Naumann et al., 2020). Magnesium is part of chlorophyll in all green plants and is essential for photosynthesis and carbohydrate partitioning (Cakmak and Yazici, 2010; Farhat et al., 2016). The importance of adequate Mg nutrition for potato plant development, carbohydrate partitioning (Koch et al., 2019a), as well as tuber yield and disease tolerance (Singh, 2018) has recently been demonstrated. Potato crops exhibit no special S requirement compared to other crop species. Typically, 1 tonne of potato tubers will remove 4.5 kg of S (Burke, 2016). However, S-deficient plants develop short and spindly stems, smaller size, pale yellow foliage, and bright yellow young leaves. As a result, tuber yield and quality may decline, resembling N deficiency consequences (Burke, 2016).

Polyhalite, a new commercial fertilizer marketed as Polysulphate^® by ICL, is a natural hydrated sulphate of K, Ca, and Mg with the formula: K₂Ca₂Mg(SO₄)₄•2H₂O. The purity of the product is very high (95% polyhalite) with <5% sodium chloride (NaCl) and traces of boron (B) and iron (Fe) at 300 and 100 ppm, respectively. Polyhalite is comprised of 48% sulfur trioxide (SO₃), 14% potassium oxide (K₂O), 6% magnesium oxide (MgO), and 17% calcium oxide (CaO). Calcium, the least soluble nutrient in polyhalite (Yermiyahu et al., 2019), can provide available Ca at rates equivalent to those of gypsum. Polyhalite fertilizer, tested as an alternative source of Ca in potato, had a positive effect on tuber skin appearance and skin-related gene expression (Keren-Keiserman et al., 2019). When examined as a substitute K donor fertilizer in Brazil, polyhalite enhanced potato yield with a positive influence on various quality traits (da Costa Mello et al., 2018).

Nevertheless, and due to the very high K requirements of potato crops, the relatively low proportion of this nutrient in polyhalite does not permit the use of this fertilizer as a sole K source. ICL PotashpluS^® is a new granular fertilizer formulated using a combination of potash (MOP, KCl) and polyhalite. While primarily a potash and sulphate fertilizer, PotashpluS also contains essential Mg and Ca, and supplies all K and S crop requirements in a single application. The formula is 37% K₂O, 9% S (24% SO₃), 3% MgO and 8% CaO. In addition, PotashpluS contains boron.

In the context of the diminishing atmospheric S deposits and the recently expanding occurrence of S deficiency in various crop species, it was hypothesized that S donor fertilizers would restart the trend of the increasing potato yield in the UK. The objective of this preliminary study was to evaluate the influences of various polyhalite rates and combinations with MOP on tuber yield and quality.

Materials and methods

Three preliminary experiments were carried out at three locations in the UK. The soil properties of the experiment sites are detailed in Table 1.

The first experiment (Exp. 1) aimed to compare the effects of the fertilizer PotashpluS vs. MOP (usual farmers’ practice) with regard to crop performance, yield, and tuber size. The second experiment (Exp. 2) tested polyhalite at three different rates (100, 200, and 300 kg ha^–1) as a partial replacement of MOP as the K source, compared to the farmers’ practice of 100% MOP. In the third experiment (Exp. 3), three polyhalite rates were tested in combination with declining PotashpluS rates, from 400 to 310 kg ha^–1. About half of the K application dose in Exp. 3, 185 kg ha^–1, was applied through a composite NPK fertilizer. In each experiment, the rates of N, P, and K application were equal in all treatments, while the differences were focused on the K source, and on the rates of Ca, Mg, and S application, as described in details in Table 2.

Seed tubers were sown in April in all three experiments. Aerial NDVI screening of the fields was conducted in June, when the crop reached full coverage. At harvest, sample 2 m digs were taken from each treatment (5 replicates) for the determination of plant and stem counts, tuber yield, and tuber size distribution.

In Exp. 1 and 2, tubers were sampled for the determination of N, K, Ca, Mg, and B concentration. In Exp. 3, tuber samples of 10 kg from each polyhalite treatment (5 replicates) were stored under commercial conditions for 4 months, and weight loss was determined.

Statistical analyses were made separately for each experiment using ANOVA and JMP software.

Results

Experiment 1 – PotashpluS vs. MOP

In Exp. 1, the area treated with PotashpluS showed a darker color in NDVI images recorded in June (Fig. 3), which is usually associated with a better status of crop nutrition. However, no differences could be observed from visual assessments at ground level between the two treatments during site visits.

No significant differences between the fertilizer treatments were detected in the plants and stems counts at harvest (Table 3). Total and marketable tuber yields were slightly but not significantly higher under the PotashpluS treatment. The proportion of larger size (65-85 mm) tubers was significantly greater under the PotashpluS treatment. Tuber nutrient concentrations were unaffected by the fertilizer treatments in this experiment (Table 3).

Experiment 2 – polyhalite vs. MOP

In Experiment 2, the plots treated with polyhalite showed slightly darker colors in NDVI images made in June (Fig. 4). The polyhalite-applied strips looked greener throughout the season.

Polyhalite treatments resulted in progressive increases of tuber N, Ca, Mg, and B concentrations that also were significantly higher than under MOP (Table 4). Similarly, tuber K concentration was significantly lower under MOP application compared to the polyhalite-applied plants, however no differences occurred between the treatments with different polyhalite rates.

Experiment 3 – PotashpluS vs. polyhalite

In experiment 3, the polyhalite-applied plots were much darker than the PotashpluS-applied area in the NDVI image made in June (Fig. 6), and exhibited a slightly greener color during most of the season.

Discussion

In the light of plateaued potato yields during the last two decades, the impact of various polyhalite fertilizers that enrich the crop rhizosphere with four essential macronutrients (K, Ca, Mg, and predominantly S) was preliminarily evaluated. The diminishing atmospheric S deposits since the 1990’s have led to an increasing occurrence of S deficiency symptoms in numerous crop species (Camberato and Casteel, 2017). Accordingly, awareness of crop S requirements is steadily rising (Engardt et al., 2017; Chan et al., 2019; Chen et al., 2019; Courbet et al., 2019; Huang et al., 2019). The synergistic relationships between N and S in plant metabolism gave rise to the assumption that soils with poor available S might restrict N uptake and subsequently, inhibit potato crop establishment. No significant differences in the number of plants or stems were observed that could be related to the fertilizer type – MOP, PotashpluS, or polyhalite. Establishment differences observed in response to polyhalite application rate were inconsistent.

Tuber yield was positively influenced by the polyhalite fertilizers commensurate with rate of application. While the advantage of PotashpluS comparing to MOP with regard to tuber yield was small and insignificant statistically (Table 3), the progressive replacement of MOP by polyhalite was responded to by a clear increase of the tuber yield under the higher polyhalite rates (Fig. 5). In a similar way, the higher the polyhalite rate the higher tuber yield under a progressive replacement of PotashpluS by polyhalite (Fig. 7). In each experiment, the rates of N, P, and K application were kept equal between treatments and hence, the differences in crop performance could be attributed solely to the rates of the secondary macronutrients – Ca, Mg, and S. At this early stage of the present study, it would be difficult to elucidate the precise nutrient responsible. Nevertheless, a rough analysis of the results indicates a minimum threshold for Ca input of about 30 kg ha^–1, which would be required for achieving marketable yield levels higher than 50 Mg ha^–1 (Table 5), compared with the current 10-year range of marketable tuber yields in the UK of 36-44 Mg ha^–1 (Fig. 1C).

Increasing Ca input resulted in greater Ca concentration in the tubers (Tables 1 and 3). Recent studies have pointed to the key role Ca plays in the tuber development (Ozgen et al., 2003; Palta, 2010; Seifu and Deneke, 2017; Singh, 2018; Potarzycki and Grzebisz, 2020), and particularly in the periderm maturation and integrity (Ginzberg et al., 2012; Keren-Keiserman et al., 2019), and its subsequent positive effects on the duration and safety of potato tuber storage (Murayama et al., 2016; Koch et al., 2019b; Naumann et al., 2020). An indication for these positive influences is the clear reduction of water loss from tubers during long storage (Fig. 8).

However, high Ca inputs do not guarantee high marketable yields. High yields also coincided with high S inputs, but not always, and Mg inputs displayed no correlation with tuber yield at all. It appears that the combination of high application doses of all three nutrients together promotes higher yields rather than each nutrient alone (Table 5). Such combinations are made easily available to the crop using various polyhalite fertilizers and application rates that should be thoroughly adjusted to the properties of the local soil and to crop rotation.

In conclusion, the set of experiments carried out in the present study demonstrates, in agreement with several recent studies (da Costa Mello et al., 2018; Keren-Keiserman et al., 2019), the potential of polyhalite fertilizers to enhance potato crop performance and tuber yield and quality through a more balanced mineral nutrition. However, further research is necessary to elucidate the contribution of Ca, Mg, or S to this enhancement, and to establish precise fertilization strategies for various edaphic conditions.