Effect of different rates and methods of application of NPK-fertilizers on the quality of potato tubers

Research on the effect of different rates and methods of application of mineral NPK fertilizers on the quality of potato variety ‘Carrera’ was conducted on a luvisol of the Radočelo Mountain massif. Treatments included an unfertilized control, NPK 16:16:16 (1500 kg/ha) applied in-furrow at planting, NPK 16:16:16 (1200 kg/ha) applied in-furrow at planting, and NPK 16:16:16 applied at 700 kg/ha during seedbed preparation and at 500 kg/ha in-furrow at planting. Results on the nutritional value of potato tubers showed that the levels of tested nutrients were higher in the skin than in the flesh. The concentrations of tested nutrients in potato tubers were highest at the highest NPK fertilizer rate, whereas the lowest levels of all nutrients, except Fe, were determined in tubers at NPK rates of 700 kg/ha applied pre-plant and 500 kg/ha applied at planting.


Introduction
In order to achieve high, stable and quality yields, the potato plant requires the presence of all macro (N, P, K, Ca, Mg, S) and microelements (Fe, Mn, B, Zn, Cu, Mo), which must be in optimal amounts. The importance of nitrogen primarily lies in the fact that it is a constituent element of proteins, and it influences the development of numerous physiological and biochemical processes, plant growth, the formation of organs, and therefore it affects the structure and quality of yield. Phosphorus accelerates tuber growth, increases the quality of tubers (they contain more starch), reduces the scum of tubers. Potassium has an important role in the physiological processes of the plant, improves the quality of cooking and processing, increases the resistance of tubers to injuries and affects the concentration of dry matter. Due to potassium deficiency, it is difficult to keep the tubers, which are then more susceptible to the formation of dark spots. Microelements are included as catalysts in numerous metabolic and physiological processes in plants. Iron is an essential microelement involved in the complex oxidation reaction of ferro-chlorogenic acid during the cooking of potatoes, and the presence of which may cause darkening, which is undesirable; therefore, the optimum amount of this element is necessary in order to avoid this phenomenon. Manganese has a positive effect on the resistance to diseases of potatoes, the synthesis of organic substances, better utilization of the nitrate and ammonium forms of nitrogen. Zinc is a component of many enzymatic systems, and it plays an important role as an ingredient of the hormone auxin. Copper influences the transfer of sugar from the above-ground parts of plants to the tubers, and thus enhances the synthesis and storage of starch in the tubers (Gvozden, 2016).
Excessive amounts of fertilizers can negatively affect not only tuber yield but also tuber quality. Bugarčić (2000) concluded that high rates, above 1500 kg/ha, can negatively affect the productive and technological characteristics of potato tubers. Hajšlova et al. (2005) found that the amount of nitrogen used is negatively correlated with the content of dry matter and starch. Also, nitrogen over-fertilization can cause an increase in nitrogen content in tubers relative to potassium content, thus increasing the susceptibility of tubers to after-cooking darkening, as well as a decline in their nutritional value (Rengel and Damon, 2008).
The use of mineral fertilizers, together with other sources of soil contamination, may lead to the occurrence a high concentration of harmful and hazardous substances in the soil. These substances are adopted by plants and included in the plant-human diet chain (Zhuang et al., 2009;. In potato tubers, a high content of nitrate nitrogen usually comes from harmful substances (Bošković-Rakočević and Pavlović, 2009). Due to the presence of cadmium in crude phosphates, hazardous materials,, used as raw material for the production of phosphorus fertilizers, can cause an increase in cadmium content in potato tubers (McLaughlin et al., 1999;Pavlović i sar., 2014). The presence of other heavy metals is mainly due to their high content in the parent material, the atmosphere, irrigation water , exhaust gases, etc. (Al-Khashman, 2004;Wei and Yang, 2010).

Materials and methods
The effect of different rates of mineral fertilizers on the yield and quality of 'Carrera' potatoes was examined in 2015. The experiment was laid out in a randomized block design with three replications at the village of Bzovik, Municipality of Kraljevo (Latitude 43 0 25' 33" N, Longitude 20 0 25' 53" E; 1107 m altitude), on a luvisol of the Radočelo Mountain massif.
Before trial establishment, soil samples were collected from a depth of 30 cm for the following analyses: soil pH was measured at a 1:2.5 ratio of soil to distilled water and 1M KCl; humus content was determined by oxidation with KMnO 4 solution (according to Kotzman); total nitrogen was estimated by Kjeldahl analysis (Gerhardt Vapodest); available phosphorus and potassium were evaluated by extraction with 0.1M NH 4 -lactate and 0.4M CH 3 COOH, according to Egner-Riehm (P was analyzed spectrophotometrically by the phosphovanadate colorimetric method and K was determined by flame photometry); available Al was measured by Sokolov's method. Available Cu, Fe, Zn, Mn, Pb and Ni were determined by extraction with 0.005M DTPA + 0.01M CaCl 2 + 0.1M TEA solution, pH=7.3 (Lindsay and Norvell, 1978), and analyzed using atomic absorption spectroscopy.
After harvesting, potato tubers were sampled for determining the nutritional value (levels of major macro-and micronutrients) and the content of some harmful substances in the skin and the flesh. Nitrogen content was determined by elemental analysis using a Vario EL III Elemental Analyzer; the levels of K, Fe, Mn, Cu, Zn, Pb, Ni and Al were assessed by digestion in concentrated HNO 3 and 30% H 2 O 2 , with K readings made by a flame photometer and measurements of the other elements taken by an ICP.

Results and disscusion
Mean air temperature datafor Kraljevo for the growing season of potatoes in 2015 show that temperatures during potato germination were within the optimal range of values (Table 1). However, in July, especially during the last ten days, the average temperature (25.3 0 C) exceeded the optimal value (21-25 0 C), which reduced tuber bulking. According to the results of Bugarčić (2000), temperatures above 25 0 C significantly reduced tuber bulking. During the growing season in 2015, there was a considerable deficit of rainfall, especially in July and August (Table 1). During germination and potato tuber set, there was enough moisture in the soil, which had a positive effect on potato tuber set. In July, there was only 8.4 mm rainfall precipitation (25 days without precipitation), while in August it was 36.7 mm.
The results of agrochemical testing (Table 2) showed a strongly acid reaction of the soil, and high contents of total nitrogen, available phosphorus and potassium in the soil. The humus content was medium. Regardless of the strongly acid reaction of this soil, mobile aluminum content (3.38 mg/100 g soil) was within the limits tolerable to plants (Jakovljević et al., 1991), which is of high importance given the particularly deleterious effect of excess mobile aluminum in the arable layer as evidenced by the decrease in root penetration depth and, hence, reduction in the uptake of nutrients and water from the soil (Foy, 1974). The content of available Zn (1.52 mg/kg) was medium, the levels of of DTPA-Fe (55.6 mg/kg) and DTPA-Mn (61.5 mg/kg) were high, while the content of available Cu (0.64 mg/kg) was within the low limits (Ankerman, 1977), (Table 3). The high content of available Fe and Mn was expected, considering the stongly acid reaction of the soil. The content of available forms of Pb and Ni was within the maximum permissible concentration range. Results on the nutritional value of potato tubers showed that the levels of tested nutrients were higher in the skin than in the flesh ( Table 4). The content of nitrogen in the flesh of the tuber was in the range of 1.89% (treatment with NPK 16:16:16 700 kg/ha during seedbed preparation and 500 kg/ha in-furrow during planting) to 2.16% (NPK 16:16: 16 1500 kg/ha applied during planting). In the skin, the content of nitrogen varied from 2.42 to 2.61% in the same treatments. The content of potassium showed a similar tendency of variation in treatments (flesh 2.16-2.34%, skin 3.70-4.02%), as well as nitrogen. The nitrogen content in the flesh was slightly higher than the average values of 1.56% (Bártova et al., 2013) and 1.49-1.80% (Rostami et al., 2015), whereas the level of K was somewhat lower than the range of 2.6-3.6% (Trehan and Sharma, 2002). These findings were particularly evidenced in treatments with 1500 kg/ha NPK fertilizer, in support of the reports by Rengel and Damon (2008), who found that excessive nitrogen can increase nitrogen content in tubers relative to potassium. Iron is an essential microelement involved in the complex oxidation reaction of ferro-chlorogenic acid during the cooking of potatoes. Deficiency of Fe is the reason underlying after-cooking darkening as an adverse side-effect of heat treatment; therefore, its content in the tuber should be within the average values (21-58 mg/kg, Kabata-Pendias, 2011). In the present experiment, the content of Fe was higher in the skin of the tuber (skin 98.78-158.07 mg/kg; flesh 21.82-28.56 mg/kg), whereas the lowest content was determined in the treatment with 1200 kg/ha of NPK fertilizer applied once, in-furrow at planting. The highest content of Fe was found at the highest application rate of NPK fertilizer.
The results on the content of copper in potato tubers indicated that a higher content of this nutrient was found in the skin (6.50-9.32 mg/kg) compared to the flesh (3.26-5.48 mg/kg). The highest content was obtained in the treatment with 1500 kg/ha of NPK fertilizers applied in-furrow at planting. However, the content of Cu in the flesh was within the average values (3.0-6.6 mg/kg; Kabata-Pendias, 2011) in all treatments.
As with the previously analyzed micronutrients, the content of zinc was within the average values (10-26 mg/kg; Kabata-Pendias, 2011), which is very important in view of the fact that zinc participates in the production of many enzymatic systems (Gvozden, 2016). A higher content of Zn was determined in the skin compared to the flesh (23.38-36.88 mg/kg, 16.00-20.09 mg/kg, respectively).
The content of Mn in the skin of potato tubers ranged from 50.85 to 87.43 mg/kg, while its levels in the flesh were 10.26-17.03 mg/kg, with the highest content in treatment with 1500 kg/ha of NPK fertilizers applied in-furrow at planting.
As already stated, the contents of Cu, Fe, Zn, and Mn in the flesh of the tubers were within the range of average values indicated by Kabata-Pendias (2011), but the contents of these elements were slightly higher than the results obtained by Bártova et al. (2013).
In terms of safe food production, it is very important to determine the content of harmful substances, primarily heavy metals in agricultural products, as this is their most common way of entering the plant-human nutrition chain (Zhuang et al., 2009;. The major problem in growing potatoes on acid soils can be caused by the increased content of aluminum, which significantly reduces yields (Bošković-Rakočević and Bokan, 2003). Also, it can be accumulated in the tubers. The obtained results (Table 5) showed that Al accumulated in the skin of tuber (138.42-295.57 mg/kg), while the content of Al in the flesh was insignificant (4.58-12.89 mg/kg), and was below the average values (76 mg/kg; Kabata-Pendias, 2011). The contents of Pb and Cd in the flesh of the potato tubers were below the limit of detection (Table 5), while the content of Ni (1.38-2.68 mg/kg) was significantly below the toxic concentrations (>10 mg/kg; Kabata-Pendias, 2011). All tested heavy metals were detected in the skin of the tuber, but these values were also low, indicating that potatoes have developed mechanisms at the root that prevent the translocation of heavy metals from the root into edible parts. This mechanism for the sequestration of certain heavy metals (Cd, Cu and Zn) is made possible through sulphur-containing proteins which bind these metals to create metabolically inactive complexes that accumulate in vacuoles within the cells. Thus, these elements are excluded from the translocation process. Characteristically, Mn forms complexes with malate in the cytoplasm through which Mn is transported to the vacuole, where it dissociates from malate and complexes with oxalate as the "terminal acceptor" (Memon et al., 2001). In Pb, this mechanism is based on the formation of Pb-lignin complexes in roots (KrishnaRaj et al., 2000).
As indicated by the analysis of the effect of different rates and methods of application of NPK fertilizer on tuber quality, the highest levels of all tested nutrients were obtained at the highest rate (1500 kg/ha), as opposed to the lowest levels of all nutrients, except Fe, under NPK treatment at 700 kg/ha applied preplant and 500 kg/ha applied at planting, with all values being within the optimal range.

Conclusion
The soil was strongly acid reaction (pH/KCl 3.80), with a high content of total nitrogen (0.21%), available phosphorus (23.26 mg/100g) and potassium (47.27 mg/100g). The humus content was medium (3.10%). The content of available Zn (1.52 mg/kg) was medium, the levels of DTPA-Fe (55.6 mg/kg) and DTPA-Mn (61.5 mg/kg) were high, while the content of available Cu (0.64 mg/kg) was within the low limits. The content of available forms of Pb and Ni was within the maximum permissible concentration range. Results on the nutritional value of potato tubers showed that the levels of tested nutrients were higher in the skin than in the flesh. The highest levels of all tested nutrients were obtained at the highest rate (1500 kg/ha), as opposed to the lowest levels of all nutrients, except Fe, under NPK treatment at 700 kg/ha applied pre-plant and 500 kg/ha applied at planting, with all values being within the optimal range.