Impact of crop production system on the content of macronutrients in potato tubers

ABSTRACT

The content of macronutrients in potato tubers arouses interest because of their substantial consumption in the world and significant role in elements' budget in human diet. The research objective has been to evaluate the content of macronutrients in tubers of potatoes grown in different systems of crop production in Poland. In 2012–2014, an experiment was conducted to test potato cultivation in three-crop production systems: conventional, integrated and organic. In each of the six-field crop rotation systems, there were five potato cultivars classified into different earliness groups. The following macronutrients in dry mass of potato tubers were determined: N, P, K, Ca, Mg, Na, S and Cl, and the various ratios between them were calculated. The research carried out proved that potatoes production systems affect the chemical composition of tubers, which is additionally modified by whether condition and genetic features of cultivars. Potato tubers from organic farming contained by about 20% less N than tubers from conventional or integrated systems. Potato tubers from organic production system displayed lower content of Ca and Na in comparison with the conventional and integrated systems. The least of P and S were detected in tubers of the potatoes grown conventionally.

KEYWORDS:

• Organic farming
• potato cultivars
• proportion of chemical elements

Introduction

Owing to its high yields and unique nutritional values, potato is among the staple crops grown worldwide (Burlingame et al. 2009). Macro- and micronutrients constitute about 1%–1.2% of the total content of potato tubers and play a building as well as a regulatory role in the plant (Gugała et al. 2012; Ezekiel et al. 2013). One of the most important factors which influence the volume and quality of potato tuber yields is fertilisation. Fertilisation doses are defined by the adopted crop production system. The conventional and integrated crop production systems are based on the application of mineral fertilisers, while organic systems rely on natural fertilisers (EC 834/2007).

In human diet, potato is one of the main sources of minerals (Flis et al. 2012; Nassar et al. 2012). Consumers presume that crops from organic farming have a higher nutritional value than ones grown according to the guidelines of conventional or integrated agriculture. However, the relevant literature does not provide an unequivocal opinion about the influence of crop production systems on the content of macronutrients in potato tubers (Hoefkens et al. 2009; Lombardo et al. 2014; Tömösközi-Farkas et al. 2016).

Different crop production systems, including crop rotation and fertilisation, can affect mineral composition of potato tubers (Sawicka et al. 2016). The content of nutrients can also be affected by weather condition during vegetation as well as by cultivars. The reason for undertaking the research was the lack of experimental data concerning the quality of potato tubers grown under various crop production systems carried out at the same time as well as in the same climatic and soil conditions.

The research objective has been to evaluate the content of macronutrients in the tubers of very early, early and medium-early maturing potato cultivars grown in the conventional, integrated and organic systems of crop production in Poland.

Material and methods

Description of the field experiment

In 2012–2014, an experiment was conducted at the Experimental Station in Bałycyny (53^o 35ʹ 49ʺ N, 19^o 51ʹ 20ʺE; Poland), to test potato cultivation in three-crop production systems: conventional, integrated and two variants (A and B) of organic systems (Table 1). In each of the six-field crop rotation systems, there were five potato cultivars from the Polish breeding resources, classified to different earliness groups: very early cv. Miłek; early cv. Cyprian and Etola; medium-early cv. Tajfun and Tetyda.

Table 1. Potato production systems.

Conventional	Integrated	Organic (A)	Organic (B)
Potato Winter wheat Spring wheat Oat Winter wheat Winter wheat	Potato* Spring wheat Spring barley + red clover Red clover Winter wheat Winter rye/winter triticale	Potato* Spring barley + red clover with alfalfa Red clover with alfalfa Red clover with alfalfa Winter wheat Oat + cath crop	Potato* Spring wheat Spring barley + red clover Red clover Winter wheat Winter rye/winter triticale + cath crop

*Manure 30 t ha⁻¹.

The field experiment in triplicate is located in a slightly undulating area, on Luvisols, underlain by silty light loam. Soil samples were analysed according to van Reeuwijk (2002). Soil reaction (pH) was determined potentiometrically in water and in potassium chloride solution (1 M KCl dm⁻³). Total organic carbon was measured with a Vario Max Cube CN Elementar analyser. The content of sulphate sulphur (S–SO₄) in soil was determined by the nephelometric method. In the arable layer, the clay fraction (ø < 0.002 mm) accounted for 2%–4%, the silt fraction (ø 0.05–0.002 mm) for 26%–39% and the sand fraction (ø 2.0–0.05 mm) for 57%–72% (Rychcik et al. 2006; Wierzbowska et al. 2018). The soil was slightly acidic in reaction and had a high and very high content of available phosphorus, while the content of available potassium was very high in the conventional and integrated production systems, and moderate in the organic system. Irrespective of the crop production systems, the content of magnesium was moderate, and that of sulphate sulphur (S–SO₄) was low and very low (ISSPC 1999) (Table 2).

Table 2. Selected soil properties.

Properties	Potato production systems
	conventional	integrated	organic (A)	organic (B)
pH KCl	5.48–5.74	5.57–5.83	5.23–5.56	5.32–5.62
g kg^−1
C-org.	9.90–10.40	9.40–10.20	10.70–11.30	9.60–9.90
S–SO4	5.00–5.87	5.22–6.74	4.35–6.09	4.35–5.22
mg kg^−1
P- available	81.3–95.8	79.5–134.6	43.7–88.9	68.9–71.4
K- available	210.7–228.9	165.8–184.6	82.3–85.2	106.2–108.9
Mg- available	53.3–63.8	55.3–69.8	49.2–72.8	52.3–65.2

Agronomic characteristics of potato cultivation

The soil tillage system included ploughing. Manure in the integrated and organic systems (A and B) was applied to soil in autumn, in a dose of 30 t ha⁻¹. Mineral fertilisers were used in the conventional system (N – 150 kg ha⁻¹ as 34% ammonium nitrate (90 kg N before seeding, 60 kg N in the 30–40 BBCH); P – 39.30 kg ha⁻¹ as 17.5% superphosphate and K – 120 kg ha⁻¹ as 50% potassium salt) and in the integrated system (N – 100 kg ha⁻¹ as 34% ammonium nitrate (60 kg N before planting, 40 kg N in the 30–40 BBCH); P – 39.30 kg ha⁻¹ as 17.5% superphosphate and K – 120 kg ha⁻¹ as 50% potassium salt). Weeds, diseases and pests were controlled using synthetic pesticides in the conventional and integrated crop production systems, while in organic farming weeds were removed mechanically and diseases and pests were eradicated using substances recommended in the Commission Regulation (CE) 889/2008. The potato was preceded by winter wheat in the conventional system, by winter rye or winter triticale in the integrated system, and by cereals in the organic systems, followed by stubble intercrop. The experiment consisted of three replications, and a surface area of a crop rotation plot was 100 m². Sprouted potato tubers were planted in the last 10 days of April, in the density of 0.625 × 0.400 m. The harvest fell on the first days of September.

Meteorological conditions

The weather conditions were presented using the Selyaninov’s hydrothermic coefficients (K), which was determined by the following equation: K = P/0,1 Σ t, where P is the monthly sum of precipitation and t is the monthly sum of temperature > 0°C. In the first and second years of the experiment the hydrothermic coefficients were similar to the means from many years (Table 3). In 2012, the wettest months were June and July, while August was a very dry month. In the subsequent year of the research, a rather dry start of the season (April–June) was followed by a considerable surplus of rainfall in July. In 2014, the hydrothermal conditions favourable to the growth of potato appeared only once, in June (K = 1.62), whereas the other months were characterised by rainfall shortages, and July was an extremely dry month (K = 0.32).

Table 3. Weather conditions during the experiment, 2012–2014.

Year	Month						Mean IV–IX
	IV	V	VI	VII	VIII	IX
Selyaninov’s hydrothermic coefficients, K*
2012	1.76	1.02	2.36	1.97	0.48	0.98	1.43
2013	1.18	1.03	0.87	3.05	0.47	2.02	1.44
2014	0.92	0.88	1.62	0.32	1.10	0.71	0.93

*K < 0,4 – extra dry; 0,4–0,7 – very dry; 0,7–1,0 – dry; 1,0–1,3 – rather dry; 1,3–1,6 – favourable; 1,6–2,0 – rather wet; 2,0–2,5 – wet; 2,0–3,0 – very wet; >3,0 – extra wet.

Methods of chemical analyses

Samples of tubers for chemical analyses were collected after harvest. The plant material was dried at 60°C in a dryer with forced airflow and ground in a laboratory grinder (IKA WERKE M20), after which it was dry mineralised in concentrated sulphuric acid (H₂SO₄) with added hydrogen peroxide (H₂O₂) as an oxidant (BÜCHI Speed Digester K-439). The mineralised plant material was submitted to the following determinations: content of nitrogen (N) by Kjeldahl’s method (a KjelFlex K-360 device); phosphorus (P) by colourimetry with the vanadate/molybdate method (on a Shimadzu UV 1201V apparatus); potassium (K), calcium (Ca) and sodium (Na) by the atomic emission spectrophotometric method (AES) (Jenway LTD PFP 7) and magnesium by atomic absorption spectrometry (AAS) (Shimadzu AA-6800). The content of sulphur (S) was determined by nephelometry, having first mineralised the plant material in a mixture of perchloric acid and nitric acid (HClO₄ and HNO₃ mixed in a 1:1 ratio) with added magnesium nitrate (Mg(NO₃)₂) (Khan 2012). Chlorions (Cl⁻) were extracted with 2% solution of acetic acid (CH₃COOH), and their content was determined via nephelometry (Ostrowska et al. 1991). The content of macronutrients was expressed in gram equivalents, and the following ratios were calculated: K⁺:Mg²⁺, K⁺:Ca²⁺, K⁺:(Ca²⁺ + Mg²⁺) and (K⁺ + Na⁺):(Ca²⁺ + Mg²⁺) in addition to weight ratios of N:S.

Statistics methods

The research results were processed statistically with the support of STATISTICA 13.3 software, and differences between the means were compared by Tukey’s test at significance p < 0.05. Principal component analysis (PCA) was applied to show the structure of relationships between studied variables as well as between the studied crop production systems and years. A set of eight original variables was transformed into a set of orthogonal variables (principal components). Due to the differences in units of the variables, the data were subjected to standardisation, and the principal components were calculated based on the correlation matrix.

Results and discussion

In the present study, the concentration of macronutrients in potato tubers was affected by the production system, variety’s genotype and weather conditions during the growing season (Table 4, Figures 1–3). Tubers of the potato plants grown in the conventional and integrated systems were characterised by the highest content of N (10.82 and 10.64 g kg⁻¹ d.m., respectively), K (25.64 and 26.13 g kg⁻¹ d.m., respectively), Ca (0.50 and 0.55 g kg⁻¹ d.m., respectively) and Na (0.35 and 0.38 g kg⁻¹ d.m., respectively) (Figure 1). Potato tubers from organic farming contained by about 20% less N than tubers from conventional or integrated systems. Two variants of organic crop production system led to a decrease in the content of K by 8% and 13% in comparison with the conventional system and by 10% and 17% in comparison with the integrated system. The content of Ca in organic potato tubers was about 20% lower and that of Na – 16% lower than in tubers from the conventional and integrated systems. The least of P (2.29 g kg⁻¹ d.m.) and S (0.89 g kg⁻¹ d.m.) were detected in tubers of the potatoes grown conventionally. Organic cultivation, especially the organic crop production system B, was beneficial to the accumulation of these two elements in tubers (by 8% and 13% more, respectively, than in the conventional system). The crop production systems did not affect the content of Mg in tubers. The content of Cl⁻ ranged from 4.82 to 5.64 g kg⁻¹ d.m., and the crop production systems had no effect on the content of this element.

Figure 1. Content of macronutrients (means and ± 0.95 confidence interval) in potato tubers depending on the crop production systems – means from 3 years of the experiment. Note: Explanation: con. – conventional; int. – integrated; org.(A) – organic (A); org.(B) – organic (B); * – data indicated with the same letters do not differ significantly at P < 0.05.

Figure 2. Content of macronutrients (means and ± 0.95 confidence interval) in potato tubers depending on a cultivar – means from 3 years of the experiment. Note: Explanation: Mił – Miłek; Cyp – Cyprian; Eto – Etola; Taj – Tajfun; Tet – Tetyda; * – data indicated with the same letters do not differ significantly at P < 0.05.

Figure 3. Content of macronutrients (means and ± 0.95 confidence interval) in potato tubers depending on the year of cultivation. Note: * – data indicated with the same letters do not differ significantly at P < 0.05.

Table 4. Content of macronutrients in potato tubers depending on the crop production system and cultivar (means from 3 years).

Crop production system	Cultivar	N	P	K	Mg	Ca	Na	S	Cl
		g kg^−1 d.m.
Conventional	Miłek	11.24^fg*	2.19^a	25.00^a–c	1.03^a–c	0.48^ab	0.27^a–c	1.02^d–f	4.14^ab
	Cyprian	10.26^e–g	2.05^a	22.70^a–c	1.01^a–c	0.47^ab	0.43^hi	0.84^ab	6.10^a–d
	Etola	9.99^d–f	2.49^a	25.70^a–c	1.07^a–c	0.49^ab	0.38^f–i	0.91^a–d	6.54^bd
	Tajfun	11.40^fg	2.30^a	27.78^c	1.01^a–c	0.52^ab	0.25^ab	0.83^a	4.39^ab
	Tetyda	11.20^fg	2.41^a	27.04^c	1.08^a–c	0.53^ab	0.42^g–i	0.87^a–c	6.21^ad
Integrated	Miłek	12.19^g	2.27^a	26.72^bc	1.02^a–c	0.52^ab	0.43^hi	0.92^a–d	5.67^a–d
	Cyprian	10.05^d–g	2.30^a	22.97^a–c	1.10^bc	0.54^ab	0.37^e–i	0.96^b–e	7.25^d
	Etola	10.25^e–g	2.31^a	27.18^c	1.10^bc	0.53^ab	0.41^f–i	0.98^c–e	5.45^a–d
	Tajfun	10.31^e–g	2.52^a	25.99^bc	1.07^a–c	0.53^ab	0.26^ab	0.88^a–c	5.14^a–d
	Tetyda	10.40^e–g	2.65^a	27.80^c	1.01^a–c	0.61^b	0.41^f–i	0.83^a	4.67^a–c
Organic (A)	Miłek	9.45^b–f	2.28^a	22.77^a–c	1.01^a–c	0.37^a	0.28^a–d	0.94^a–e	5.18^a–d
	Cyprian	9.29^b–f	2.20^a	22.29^a–c	1.07^a–c	0.44^ab	0.31^b–e	0.92^a–d	4.84^ab
	Etola	8.40^a–e	2.62^a	22.98^a–c	1.11^c	0.43^ab	0.28^a–d	0.99^c–f	5.16^a–d
	Tajfun	7.66^a–c	2.43^a	24.24^a–c	1.01^a–c	0.37^a	0.22^a	1.05^ef	5.17^a–d
	Tetyda	7.12^a	2.46^a	25.14^a–c	1.02^a–c	0.40^ab	0.34^c–f	0.91^a–d	3.76^a
Organic (B)	Miłek	9.82^c–f	2.46^a	21.89^a–c	0.97^ab	0.39^ab	0.35^d–g	1.02^d–f	4.52^ab
	Cyprian	8.42^a–e	2.48^a	19.78^a	1.03^a–c	0.39^ab	0.21^a	0.95^b–e	4.29^ab
	Etola	8.04^a–d	2.52a	20.93^ab	1.08^a–c	0.37^a	0.39^f–i	1.11^f	4.96^a–d
	Tajfun	7.62^ab	2.47^a	22.82^a–c	0.95^a	0.40^ab	0.36^d–h	1.02^d–f	7.08^cd
	Tetyda	7.12^a	2.50^a	25.42^a–c	1.04^a–c	0.46^ab	0.27^a–c	0.94^a–e	4.26^ab

* - data indicated with the same letters do not differ significantly at P < 0.05.

Płaza et al. (2017) maintained that the tubers of potatoes from integrated farming contained more macronutrients than organic ones. These authors noted the highest content of N in the tubers of potato which was grown in a crop production system including honey clover mulch. The content of P, K, Ca and Mg was the highest in tubers of potato from the integrated system after honey clover had been ploughed into the soil in autumn.

Gaj et al. (2018) observed the highest content of N and Ca in the tubers of control plants, while the highest concentrations of P, K and Mg were determined in the tubers of potato grown after the application of manure. These researchers suggest that intercrops can be an alternative to manure in potato fertilisation because they do not require high inputs, are a good source of macronutrients and organic matter, and improve the soil’s structure while preventing its degradation. Wszelaki et al. (2005) detected differences in the content of minerals in potato tubers depending on the system of crop production. More P and Mg, as well as K and S, were determined in tubers from organic than conventional farming. Tömösközi-Farkas et al. (2016) also demonstrated that the tubers of organically grown potatoes had more Mg than ones from a conventional plantation, but the crop production systems had no influence on the tuber content of K. According to these authors, the weather conditions had a stronger impact on the tuber content of Ca than the implemented system of crop production. Lombardo et al. (2014) maintained that K, Ca and Na appeared in smaller quantities in organic potato tubers than in tubers from potatoes grown in line with conventional agriculture. Worthington (2001) showed a higher Ca content in the tubers of potatoes grown organically, but Hoefkens et al. (2009) claimed that the crop production systems they tested had no influence on the tuber content of this element.

The content of P, K, Ca and Mg in the fresh matter of tubers was by 15.8%–34.5% higher, depending on an element, in the tubers of potatoes from organic production system than in the tubers of potatoes from the conventional system (Järvan and Edesi 2009). To some extent, this discrepancy might be due to the so-called dilution of minerals because of a much higher yield of tubers in the conventional system, where fertilisation supplied much higher quantities of nutrients than in the organic system.

Zarzecka et al. (2019) maintained that biostimulants and herbicides used to protect potato plantations had a significant effect on the content of macronutrients in potato tubers and, compared to tubers from the control variant, they raised the concentrations of P, Mg and Ca. These authors, same as Wierzbowska et al. (2015; 2016), noted the highest content of minerals in the tubers of potatoes treated with Asahi SL. In the opinion of Gugała et al. (2015), mechanical and chemical plant treatment contributed to an increase in the N-total content and to a decrease in the content of P and K in comparison with tubers harvested from potato plants provided only mechanical treatment. Rogóż and Tabak (2015) suggested that changes in the soil reaction did not have any considerable influence on the content of P, Ca and Mg in potato tubers.

The content of macronutrients in potato tubers was differentiated by the genotype traits of the analysed cultivars (Figure 2). Most N (10.68 g kg⁻¹ d.m.) was found in tubers of a very early cultivar Miłek, while the least of this element was contained in tubers of a medium-early cv. Tetyda. The content of P in tubers did not depend on the grown cultivars and ranged from 2.26 to 2.51 g kg⁻¹ d.m. The lowest content of K was determined in tubers of an early cultivar Cyprian (21.93 g kg⁻¹ d.m.), whereas the medium-early cv. Tetyda contained by about 20% more of this element. The highest content of Mg, S and Na (1.09, 1.00 and 0.37 g kg⁻¹ d.m.) was found in the early cultivar Etola. The content of Ca varied from 0.44 to 0.50 g kg⁻¹ d.m., and the traits of the tested cultivars did not affect the content of this element in the tubers. The least Cl⁻ was contained in tubers of cv. Miłek and Tetyda (4.72 and 4.88 g kg⁻¹ d.m.), while the tubers of cv. Cyprian had by about 30% more of this component.

Wierzbowska et al. (2015), in another study, found that the content of Cl in potato tubers, depending on a cultivar, varied from 2.78 to 4.25 g Cl⁻ kg⁻¹ d.m. According to Podleśna (2009), most of chlorine is accumulated in leaves. In turn, the content of chlorine in roots and aerial parts of plants decrease as the nitrogen fertilising doses increase. A good supply of plants with Cl improves their resistance to biotic and abiotic stresses. Despite the commonly held belief, Hütsch et al. (2018) claim that there is a lack of unequivocal scientific evidence that Cl⁻ ions in potassium fertilisers have a harmful effect on yields and the quality of potato tubers.

In our study, significant interactions were observed between the crop production systems and genotypes of the tested cultivars with respect to the content of macronutrients in potato tubers (Table 4). Significantly the highest content of N (12.19 g kg⁻¹ d.m.) was determined in the tubers of the very early cultivar Miłek, while the lowest content of this element (7.12 g kg⁻¹ d.m.) was accumulated in tubers of the medium-early cv. Tetyda grown in both organic production systems. No significant differences in the content of P in tubers of the analysed cultivars were detected. The highest content of K was determined in the tubers of the cultivar Tetyda grown in the conventional and integrated systems (27.04 and 27.80 g kg⁻¹ d.m., respectively) and cv. Tajfun from the conventional system (27.78 g kg⁻¹ d.m.) as well as cv. Etola (27.18 g kg⁻¹ d.m.) cultivated in the integrated system. The smallest content of Mg (0.95 g kg⁻¹ d.m.) was characteristic of tubers of cv. Tajfun from the organic crop production variant B, while the highest content of this element (1.11 g kg⁻¹ d.m.) was found in the tubers of cv. Etola from the organic crop production system A. The highest content of Ca (0.61 g kg⁻¹ d.m.) was recorded in tubers of cv. Tetyda grown in the integrated system, while the least (0.37 g kg⁻¹ d.m.) was determined in tubers of cv. Miłek and Tajfun in the organic system (A) and cv. Etola in the organic system (B). Significantly the least Na (0.21–0.22 g kg⁻¹ d.m.) was contained in tubers of cv. Tajfun from the organic system (A) and cv. Cyprian from the organic system (B), whereas tubers of cv. Cyprian (conventional system) and cv. Miłek (integrated system) contained around twice as much of this element. Demonstrably the highest S content (1.11 g kg⁻¹ d.m.) was determined in tubers of the early cultivar Etola grown in the organic crop production system B, while the least S was in the tubers of medium-early cultivars Tajfun (conventional system) and Tetyda (integrated system). The smallest content of Cl⁻ (3.76 g kg⁻¹ d.m.) was found in tubers of cv. Tetyda in organic crop production system (A), while the highest content of these ions (7.25 g kg⁻¹ d.m.) appeared in tubers of cv. Cyprian grown in the integrated system.

According to White et al. (2009), interactions between minerals in soil and within a plant can affect the content of these elements in potato tubers, regardless of the dilution effect in yield. Thus, cultivation of potato cultivars with a naturally higher content of minerals together with an adequate fertilisation regime enables one to produce tubers rich in mineral components. Flis et al. (2012) claim that organic production system can increase the content of P in potato tubers, but the impact of production systems on the tuber content of K and Mg is weaker.

The ionic balance of a plant is one of the main factors shaping the quality of yields, as excessive absorption of certain cations or anions limits the concentration of other, often valuable, macro- and micronutrients. High potassium content depresses, while high concentrations of calcium and magnesium improve the nutritional value of crops grown for human consumption or for animal feed.

The content of minerals in potato tubers is a resultant of the plant’s genotype, soil fertility and climatic conditions in the area of potato production (Barczak and Nowak 2015; Wierzbowska et al. 2015; 2016; Głosek-Sobieraj et al. 2019; Zarzecka et al. 2019). Likewise, the hydrothermal conditions in the consecutive years during our study had an effect on the content of the analysed macronutrients in potato tubers (Figure 3). The least of N, P, K, S and Cl was determined in the 2013 year of the experiment (8.92, 2.07, 20.89, 0.84, and 3.57 g kg⁻¹ d.m., respectively). In turn, the lowest content of Ca and Na (0.30 and 0.33 g kg⁻¹ d.m., respectively) was found in the first year of the research, when the potato tubers were found to contain the highest levels of Mg (by 10% more than in the other years) and P and K (by 35% and 32% more than in the other years), and the content of Cl was more than double the one detected in 2013.

In a study by Gugała et al. (2015), the climatic conditions in particular seasons of plant growth had a significant effect on the content of N-total and K in tubers of table potato. Also Lombardo et al. (2014) demonstrated that temperature and precipitation significantly differentiated the content of total protein and true protein in potato tubers. Głosek-Sobieraj et al. (2019) concluded that the content of macronutrients in potato flesh and peel depended on the weather conditions and on a potato cultivar. In freshly harvested potatoes, the content of Ca and Mg in flesh and peel and K in flesh were the highest under the influence of rainfall deficit. The accumulation of protein is favoured by the dry weather from July to September. On the other hand, high precipitation (325.4 mm) and moderate air temperature (14.6°C) led to a decline in the content of total and true protein in potato tubers. The highest content of P and Mg was noted in the tubers of potato plants grown in a warm and moderately wet season (Zarzecka et al. 2019). In contrast, a higher Ca content in potato tubers was noted when the ambient temperature was high and rainfalls were abundant.

Due to the fluctuations of concentrations of mineral nutrients in tubers over the years of the research, mutual proportions between these nutrients in potato tubers changed (Table 5). According to Barczak and Nowak (2015), a typical N:S ratio value is between 11 and 12:1. Sulphur affects the uptake and accumulation of P and K in the dry mass of potato tubers; moreover, S deficit may manifest itself by the accumulation of non-protein forms of nitrogen in a plant (Klikocka et al. 2015). In the experiment presented herein, the weather conditions prevailing in the consecutive years did not affect the mass proportions of N:S. However, a significant variation in the values of this ratio was found depending on the crop production systems. Wider N:os were detected for potato tubers from the conventional and integrated crop production system (12.4:1 and 11.8:1, respectively) than from the organic ones (A – 8.9:1 and B – 8.3:1). There was also a significant variation of this ratio depending on the cultivars, with the widest N:S ratio determined for tubers of the very early cv. Miłek (11.2:1), in comparison with the ratio of 9.4:1 in the tubers of the early cv. Etola. In the opinion of Barczak et al. (2013), a narrow N:S ratio confirms that the plant is able to utilise nitrogen highly efficiently, which is reflected by higher protein content in potato tubers. However, in our experiment, the excessively narrow N:S ratio in organic potato tubers might suggest a deficit of N available to plants.

Table 5. Proportions of chemical elements in potato tubers.

Specification	N:S	K:Mg	K:Ca	K:(Ca + Mg)	(K + Na):(Ca + Mg)
Years of the experiment
2012	10.4^a*	7.7^a	47.3^c	6.6^c	6.7^c
2013	10.7^a	6.4^b	19.5^a	4.8^a	4.9^a
2014	10.1^a	7.5^a	24.6^b	5.7^b	5.9^b
Crop production systems
Conventional	12.4^b	7.6^b	30.9^a	5.9^a	6.1^a
Integrated	11.8^b	7.6^b	28.2^a	5.8^a	6.0^a
Organic (A)	8.9^a	6.9^a	32.0^a	5.6^a	5.7^a
Organic (B)	8.3^a	6.7^a	30.8^a	5.4^a	5.6^a
Cultivars
Miłek	11.2^b	7.3^a.c	31.8^a	5.8^a,c	6.0^ab
Cyprian	10.6^ab	6.4^b	27.5^a	5.1^b	5.2^c
Etola	9.4^a	6.8^bc	30.1^a	5.5^bc	5.6^a,c
Tajfun	10.2^ab	7.7^a	31.9^a	6.1^a	6.2^ab
Tetyda	10.5^ab	7.8^a	31.0^a	6.1^a	6,3^c

*- data indicated with the same letters do not differ significantly at P < 0.05.

Due to the reduction in the K content in potato tubers in the second year of the study, significantly the narrowest ionic ratios of K⁺:Mg²⁺, K⁺:Ca²⁺, K⁺:(Ca²⁺ + Mg²⁺) and (K⁺ + Na⁺):(Ca²⁺ + Mg²⁺) were obtained (6.4:1, 19.5:1, 4.8:1 and 4.9:1, respectively). However, in the first year of our experiment, when the K content of tubers was the highest, and this was accompanied by lower concentrations of Ca, Na, the above ratios were broadened, especially the K:Ca proportion. Potato tubers from organic production system were characterised by narrower K:Mg than potato tubers from conventional and integrated production. There were also some differences in ionic proportions in potato tubers caused by differences in the mineral composition of particular cultivars. Barczak and Nowak (2015) reported much narrower ionic proportions in potato tubers.

The PCA separated the tested crop production systems. Potato tubers originating from the conventional and integrated systems were determined to be richer in the analysed nutrients than potato tubers from the organic crop production variants (Figure 4). The components PC1 and PC2, which together explained 75% of the variation, were taken into consideration. PC1 received the highest contribution from K and Cl (at 0.91 each), Mg (0.87) and P (0.80). PC2, on the other hand, differentiated the compared crop production systems in terms of Na (0.89), N (0.75) and Ca (0.73). The content of S was not significantly connected with the PC1 or PC2 components. The content of the analysed macronutrients varied in the years. The weather conditions prevalent in May and June 2012 (excess rainfall) caused a lower content of the nutrients in tubers in comparison with the determinations achieved in the other two years of research (Table 3).

Figure 4. The plot for eight macronutrients in terms of principal components 1 and 2 (PC1 and PC 2) as well as depicting relationships between crop production systems (C – conventional; I – integrated; O/A – organic A; O/B – organic B); years of study (12–2012, 13–2013, 14–2014).

The research carried out proved that potatoes production systems affect the chemical composition of tubers, which is additionally modified by genetic features of cultivars and weather condition during the growing period. Potato tubers from organic farming contained about 20% less N than tubers from conventional or integrated systems. However, potato tubers from organic cultivation had a lower content of K, Ca and Na in comparison with the conventional and integrated systems. Organic cultivation was beneficial for the accumulation of P and S. The crop cultivation systems did not affect the content of Mg and Cl⁻ in tubers.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Notes on contributors

Jadwiga Wierzbowska, PhD, is a professor at Department of Agricultural Chemistry and Environmental Protection, University of Warmia and Mazury in Olsztyn, Poland. Her fields of research include plant nutrition investigations in relation to crop quality and soil fertility.

Bogumił Rychcik had completed PhD in agricultural sciences. He works at Department of Agroecosystems, University of Warmia and Mazury in Olsztyn, Poland. His research interests focus on plant production under different cropping systems, particularly in organic farming.

Milena Kaźmierczak-Pietkiewicz had acquired PhD in agricultural sciences in Department of Agroecosystems, University of Warmia and Mazury in Olsztyn, Poland.

Arkadiusz Światły is a PhD student at Department of Agricultural Chemistry and Environmental Protection, University of Warmia and Mazury in Olsztyn, Poland.

Additional information

Funding

This project work was financially supported by the Minister of Science and Higher Education in Poland in the range of the program entitled ‘Regional Initiative of Excellence’ for the years 2019–2022, Project No. 010/RID/2018/19.