Advances in Crop Science and Technology
Open Access

Potato; Dry matter; Protein; Specific gravity; Starch; Tuber quality; Tuber number

Potato (Solanum tuberosum L.) is a near optimum nutrient balance for human consumption [1]. It is increasingly helping rural poor people to improve their health status, food security and poverty alleviation [2]. It is rich in carbohydrates, quality protein (Lysine), minerals, such as potassium and iron, nutrient salts, vitamins and an enormous amount of vitamin C. Nevertheless, it constitutes 80 percent of the caloric intake of humans worldwide and provides cheap energy to many developing countries [3]. As well as being high in phenolic compounds, it is almost free of soluble sugar, which lowers blood sugar levels [4].

In potato production, quality is an important characteristic and it depends on a number of factors. Among these factors, proper plant nutrition has a significant impact on the quantity and quality of the harvested product [5-7]. Fertilization with potassium and phosphorus is essential to achieve optimum potato yields and quality. In addition to tuber yield, potassium contributes to quality attributes such as reducing total yield, specific gravity, vitamin C content, sugars and shelf life [8,9]. It contributes to the quality formation by promoting the synthesis of photosynthesis in potato leaves, enhancing their transport to the tubers and maximizing their conversion to starch, protein and vitamins [10,11]. As a result, it influences quality attributes like tuber size, specific gravity, black spot susceptibility after cooking, sugar content reduction and fry color and storage quality [12,13]. Phosphorus also serves various functions in plant metabolism [14,15]. It influences tuber quality by regulating cellular division, starch synthesis in the tubers, and the storage of starch [16]. Furthermore, it can significantly affect the tuber setting, particularly early in the growth cycle, but it also enhanced tuber maturity later [17]. Apart from these effects, it can also affect the size category, specific gravity, and dry matter of potato tubers, as well as their texture, color and flavor once cooking has taken place [18]. It affects the starch, ascorbic acid, protein and sugar composition of tubers [19].

Apart from their importance and effect on quality, the availability and accessibility of these plant nutrients in the soils are very crucial. The pH of the soil, the type and content of clay, the contents of carbonate and the amount of iron and aluminum oxides, all have an impact on phosphorus adsorption and fixation [20]. The soils of western Ethiopia are known for their low phosphorus availability, and high capacity to fix phosphorus, while potato has a limited ability to absorb phosphorus from the soil. Due to these reasons, it is necessary to grow with high amounts of phosphate fertilizers to produce large tubers and high tuber yields. Furthermore, soil solution K in highly rainfall areas has a high possibility of leaching and subsequent losses from the soil system. Several studies claim that Ethiopian soils have a good supply of potassium and are least deficient in potassium. Acidic soils (Nitosol) may, however, contradict this claim. Due to the high precipitation in southwestern and western Ethiopia, the soils are generally acidic, poor in fertility and have problems of low soil pH, phosphorus fixation, and K leaching. Thus, realizing that the use of inorganic and organic fertilizers in potato production for tuber production with optimum quality is vital. This research, therefore, evaluated the effect of different rates of inorganic potassium and phosphorus fertilization on potato tuber quality.

Description of experimental site

The study was conducted at the Assosa Agricultural Research Center (AsARC), one of the Ethiopian Institute of Agricultural Research (EIAR) centers, which is located at 10°02' N and 34°34' E in western Ethiopia, about 665 kilometers from the capital, Addis Ababa. It is located approximately 1553 meters a.s.l. The experiment was carried out during the main cropping season, and the area receives an average annual rainfall of 1100 mm. The rainy season lasts from May to October, with the greatest amount of rain falling between June and August. It has a warm, humid climate, with mean annual maximum and minimum temperatures of 32°C and 17°C, respectively.

The soil is reddish to brown Nitosol with a pH of 5.1 in the area. It has a silty texture and is composed of 49% silt, 34% clay and 17% sand. The soil contains 4.86 percent organic matter, as well as 0.068 percent total nitrogen, 8.52 mg kg^-1 soil of available phosphorus and 0.136 cmol kg^-1 soil of exchangeable potassium. According to Landon and Mengel, phosphorus and potassium levels are extremely low.

Treatments and experimental design

The treatments consisted of four levels of potassium (0, 100, 200 and 300 kg K₂O ha^-1) and six levels of phosphorus (0, 46, 92, 138, 184 and 230 kg P₂O₅ ha^-1). The experiment was laid out as a Randomized Complete Block Design (RCBD) in a 4 × 6 factorial arrangement and replicated three times. Each plot received one of 24 treatment combinations, which were assigned at random. Each plot had a gross area of 11.25 m² with 3 m in length and 3.75 m in width. Each plot contained five rows of potato plants, with each row accommodating 10 plants per row with a total population of 50 plants per plot at the spacing of 0.75 m and 0.30 m between rows and plants, respectively. The planting material used for this experiment was Gudanie (CIP-386423-13).

Phosphorus was provided by Triple Superphosphate (TSP, 46% P₂O₅) while potassium chloride (KCl, 60% K₂O) was used as a source of potassium. The nitrogen source was Urea (CO(NH₂)₂) (46% N). The granules of potassium and phosphorus fertilizers are applied below and around the seed tubers at planting. To avoid leaching as a result of high rainfall, potash was applied in two parts (half when the plant emerged and a half at mid-stage after planting), while phosphorus was applied all at once. Each plot received 138 kg N ha^-1 of urea evenly applied three times as recommended (1/4^th at planting, 1/2 at mid stage (about 40 days after planting) and 1/4^th at tuber initiation (at the beginning of flowering)).

The land was prepared from May to June using a tractor and human labor. Medium sized (40 g-60 g) and sufficiently sprouted potato tubers (with 2 cm-3 cm long sprouts) were planted. Weeds were controlled by hoeing. Earthing up was done as required to prevent exposure of tubers to direct sunlight for promoting tuber bulking and for ease of harvesting. Mancozeb (C₈H₁₂MnN₄S₈Zn), the active ingredient of maneb and metiram, was sprayed at the rate of 50 g per 20 L of water to control late blight disease.

Data collection and measurements

Tuber quality data were collected from the three middle rows, leaving plants in the border rows alone to avoid edge effects.

Mean tuber weight (g): It was determined at harvest as a ratio of the total mass of all tubers obtained from randomly selected five plants (hills) to the total tuber numbers.

Tuber dry matter content (%): Ten fresh tubers were randomly selected and weighed from each plot. The tubers were then chopped and dried at 65°C until their weights were constant, and their dry weights were recorded. The following formula was used to calculate the dry matter percent.

Tuber-specific gravity: It was calculated using the ratio of the weight of tuber in the air to the weight of tuber in water method. Tubers of various shapes and sizes weighing between 3 kg-5 kg were randomly selected from each plot. After weighing the samples in the air, they were reweighed in the water. The following formula was then used to compute specific gravity.

Tuber size distribution by weight: It refers to the proportional weight of tubers based on their size. A Lung'aho classification was used to categorize the tubers as small (under 39 grams), medium (39 grams-75 grams) or large (over 75 grams).

Tuber protein content: Kjeldahl's method of protein digestion, distillation, and titration was used to determine tuber protein content from powdered potato samples. The following formula was used to calculate the nitrogen percentage:

Where V_s=Volume (ml) of acid required titrating sample, V_b=Volume (ml) of acid required titrating the blank and M acid- Molarity of the acid. The crude protein (%) in the sample was then calculated using the formula:

Whereas, F=The conversion factor, which is equivalent to 6.25.

Starch content: The percentage of starch was calculated from dry matter percent.

Statistical analysis: The data were subjected to Analysis of Variance (ANOVA) using SAS version 9.4's Generalized Linear Model (GLM) and interpretations were derived using the Gomez and Gomez technique. The least significance difference test was used to distinguish significant differences between treatment means at a 5% level of significance.

Potato tuber quality parameters

Average tuber weight: Table 1 depicts the analysis of the variation of average tuber weight. The main effect of potassium was an increase in average tuber weight. Similarly, phosphorus had a considerable effect on this parameter; however, the interaction of phosphorus and potassium did not affect average tuber weight. Increasing the phosphorus level from nil to 92 kg P₂O₅ ha^-1 enhanced the mean tuber weight by approximately 26% over the zero phosphorus condition. However, no further increases in average tuber weight were observed at this phosphorus application dose. In contrast, raising the potassium rate from nil to 100 kg K₂O ha^-1 increased the average tuber weight by nearly 18%. However, the average tuber weights obtained in response to 100, 200 and 300 kg K₂O ha^-1 applications were all statistically parity.

In contrast to the present finding, Panthi found that increased potassium application resulted in significant increases in average tuber weight while but not increased phosphorus application. Israel in agreement with the current study, observed significant yield increases due to potassium nutrition, attributing it to the effect on average tuber weight. The increase in average tuber weight in response to increased potassium availability may be attributed to more growth that is luxuriant, increased leaf area and foliage area and a greater supply of photosynthesis, which may have stimulated the production of larger tubers, resulting in higher yields. To put it another way, increased haulm size and duration as a result of increased nutrition availability may have favoured tuber weight. In contrast to this finding, Sahota and Singh discovered that combining phosphate and potassium increased tuber weight significantly. The fact that phosphorus application tends to increase tuber numbers rather than tuber weights could explain the lack of response in tuber weight to increased phosphorus application. This is in agreement with Rosen and Bierman's findings that phosphorus application increased small tuber production and decreased large tuber production.

Table 1: Potato tuber quality attributes mean squares as influenced by phosphorus, potassium application rate and their interaction.

Tuber dry matter percentage: The main effects of phosphorus and potassium, as well as their interaction, had no statistically significant impact on tuber dry matter content. This study's findings are consistent with Simiret and Koch, who found that potassium, had no significant influence on tuber dry matter. However, Bansal and Trehan and Simson observed that the dry matter content of tubers negatively influenced by increasing rates of potassium fertilizers. With regard to phosphorus, similar observation was reported by Sparrow who observed no significant variations in the accumulation of tuber dry matter in response to increased phosphorus use. According to Storey and Davies, the buildup of dry matter within the tubers was regulated by many factors that affected crop growth and development, most notably intercepted solar radiation, soil temperature, accessible soil moisture and cultural interventions.

Specific gravity: Tuber specific gravity was significantly (P<0.01) affected by potassium. There was, however, neither effect of phosphorus nor its interaction with potassium on this plant parameter.

Increasing the potassium rate from 0 kg to 100 kg K₂O ha^-1 does not affect tuber specific gravity. However, increasing the mineral fertilizer rate from 100 kg to 200 kg K₂O ha^-1 resulted in a decrease in tuber specific gravity. Furthermore, when potassium was supplied at a rate of 300 kg K₂O ha^-1, tuber specific gravity reverted to statistically the same value as reported at lower mineral nutrient rates.

This suggests that tuber specific gravity reacted inconsistently to higher potassium rates.

This result is consistent with that of Kumar and Niguse, who observed a declining trend in specific gravity with an increased potassium rate. Contrarily, Al-Moshileh and Khan observed that increasing the amount of potassium fertilizer resulted in increased specific gravity. However, Abdelkadir observed that specific gravity did not respond to potassium application. Alternatively, Lujan and Smith found no significant influence of phosphorus on tuber specific gravity, which is consistent with the findings of this study. In contrast to the findings of this study, Dubetz and Bole found that as the amount of phosphorus fertilizer rose, specific gravity decreased. Mulubrhan both observed that phosphorus treatment raised specific gravity considerably.

Protein and starch: Protein and starch content of potato tubers were significantly (P<0.01 influenced by potassium. These parameters were not affected by either the main effect of phosphorus or its interaction with potassium. Increasing the potassium rate from 100 kg to 300 kg K₂O ha^-1 does not affect tuber protein, however, the highest protein content of the potato tuber was observed at the control level. In the case of starch content, except for the application of 200 kg K₂O ha^-1 the other level of potassium yielded tubers with similar content of starch content. The highest starch value was obtained from 100 kg K₂O ha^-1.

In a study by Koch starch yield in potato plants with K deficiency was significantly reduced. Similarly, Niguse found that P and K interaction did not affect potato tuber starch content. Furthermore, Eleiwa reported that potassium and phosphorus levels increased starch content in potato tubers. As compared to that, Eremee report that potassium and phosphorus fertilization reduced potato starch levels significantly. In contrary, Sharma and Arora report significant increases in proteins and starch when potassium is applied, this in turn increases tuber dry matter yield.

Tuber size distribution

Large and medium sized tubers: Potassium application exhibited a significant main effect on medium and large sized tuber yields, whereas phosphorus did not affect these parameters. The two parameters did not affect the yields of medium and large tubers. Increasing potassium from 0 kg to 100 kg K₂O ha^-1 enhanced medium sized tuber production by about 34%. The yields of medium sized tubers produced at 100 kg and 200 kg K₂O ha^-1 were not significantly different. When the potash rate was increased from 200 kg to 300 kg K₂O ha^-1, the yield of medium sized tubers increased by around 22% when compared to the yield of medium sized tubers observed for plants in the control treatment. Similarly, raising the potassium rate from nil to 100 kg K₂O ha^-1 boosted the production of large sized tubers by around 91%. When potash was increased to 300 kg K₂O ha^-1, the yield increased by 150%. The yields of large sized tubers recorded at 100, 200 and 300 kg K₂O ha^-1, on the other hand, were statistically equal (Table 2). Potassium fertilization has been shown to increase tuber yields in medium and large sizes. This is because potassium stimulates photosynthetic activity, phloem load capacity and translocation and it also stimulates large molecular weight substances to be synthesized within storage organs, which may cause tubers to bulk up rapidly.

Potassium promotes phloem loading and unloading of photosynthesis (mainly amino acids and sucrose) to physiological sinks in tubers by increasing the proportion of phloem-sap solute transport. This job of potassium is related to its contribution to the osmotic potential in the sieve tubes, as well as its function in ATP synthesis, which provides the energy for photosynthate loading. As a result, potassium aids in the growth of large, heavy potato tubers. It is in line with the findings of Haddad, who found that potassium deficiency significantly decreases potato yield and Khandakhar, who found that potassium application was primarily responsible for tuber size distribution, as increased potassium application increased the number of large tubers and declined the number of small tubers.

Table 2: Tuber quality parameters of potato as influenced by phosphorus and potassium application at Assosa.

Small sized tubers: Small tuber yield was significantly affected by phosphorus main effect, as well as its interaction with potassium. The main effect of potassium, however, did not affect the small sized tuber yield. In general, raising the phosphorus rate significantly enhanced the yield of small sized tubers across all potash rates. However, increasing potash had a less pronounced effect on small sized tuber production across increased phosphorus rates than increasing phosphate across increased potash rates (Table 3).

Plants treated with the greatest phosphate content (230 kg P₂O₅ ha^-1) and 200 kg K₂O ha^-1 produced the most small sized tubers. This yield, however, was statistically equivalent to those recorded for plants grown at combined phosphate and potassium application rates of 92 kg P₂O₅ and 0 kg K₂O ha^-1, 138 kg P₂O₅ and 0 kg K₂O ha^-1, 230 kg P₂O₅ and 0 kg K₂O ha^-1, 184 kg P₂O₅ and 100 kg K₂O ha^-1, 230 kg P₂O₅ and 100 kg K₂O ha^-1. Plants grown at nil phosphate and nil potash yielded the least amount of small sized tubers. This could be due to the phenomenon that increased phosphorus availability leads to enhanced cell division and partitioning in various storage tuber organs. This finding is consistent with those of Sanderson, who discovered that phosphorus influences the tuber size distribution, tuber number and tuber set. Tuber number and tuber size typically exhibit an inverse connection; however, increase in tuber number with phosphorus fertilization has been accompanied by both increases in tuber numbers and decreases in tuber size.

The results obtained in this study accord with those of Sharma and Arora, who showed significant, increases in the production of small sized tubers and a decrease in the yield of large sized tubers in response to higher phosphorus application. Rosen and Bierman previously found that increasing the rate of phosphorus promotes the formation of small sized tubers at the price of large sized tuber production. Enhanced rates of phosphorus application increased the yield of small sized tubers, which is similar to the observations of Rosen and Bierman, who found that increasing phosphorus rates generally resulted in the formation of much more small sized tubers than large sized tubers.

Table 3: The yield of small-sized potato tubers (t ha^-1) as affected by P × K interaction at Assosa.

The most significant effect on average tuber weight and tuber specific gravity was exerted by potassium. Aside from that, potassium had a significant impact on medium and large sized tuber yields, as well as potato tuber protein and starch content. While phosphorus had no effect on tuber dry matter content, tuber specific gravity, protein and starch content of potato tubers and medium and large sized tuber yields, but it did influence average tuber weight. Except for the small sized tuber yield, the interaction of phosphorus and potassium application did not affect any of the tuber quality parameters discussed here. In this study, potassium application resulted in a higher proportion of large and medium sized tubers, whereas phosphorus application resulted in a higher proportion of small sized tubers.

I would like to acknowledge the contributions of the field workers, technical support assistants, and others. Finally, I would like to thank the Ethiopian Institute of Agricultural Research for its financial assistance.

1. Alva A, Fan M, Qing C, Rosen C, Ren H (2011) Improving nutrient use efficiency in Chinese potato production: Experiences from the United States. J Crop Improv 25: 46-85. [Crossref][Googlescholar]
2. Devaux A, Kromann P, Ortiz O (2014) Potatoes for sustainable global food security. Potato Res 57: 185-199. [Crossref][Googlescholar]
3. Leff B, Ramankutty N, Foley JA (2004) Geographical distribution of major crops across the world. Global Biogeochem Cycles 18: 1-27. [Crossref][Googlescholar]
4. Akyol H, Riciputi Y, Capanoglu E, Caboni MF, Verardo V, et al. (2016) Phenolic compounds in the potato and its byproducts: An overview. Int J Mol Sci 17: 835. [Crossref][Googlescholar][Indexed]
5. Tesfaye A, Wongchaochant S, Taychasinpitak T (2013) Evaluation of specific gravity of potato varieties in Ethiopia as a criterion for determining processing quality. Kasetsart J Nat Sci 47: 30-41. [Googlescholar]
6. Stark JC, Porter GA (2005) Potato nutrient management in sustainable cropping systems. Am J Potato Res 82: 329-338. [Crossref][Googlescholar]
7. Rosen CJ, Kelling KA, Stark JC, Porter GA (2014) Optimizing phosphorus fertilizer management in potato production. Am J Potato Res 91: 145-160. [Crossref][Googlescholar]
8. Bhattarai B, Swarnima KC (2016) Effect of potassium on quality and yield of potato tubers: A review. Int J Agric Environ Sci 3: 7-12. [Crossref][Googlescholar]
9. Khan MW, Rab A, Ali R, Sajid M, Aman F, et al. (2019) Effect of potassium and zinc on growth yield and tuber quality of potato. Sarhad J Agric 35: 330-335. [Crossref][Googlescholar]
10. Bansal SK, Trehan SP (2011) Effect of potassium on yield and processing quality attributes of potato. Karnataka J Agric Sci 24:48-54.
11. Koch M, Naumann M, Pawelzik E, Gransee A, Thiel H, et al. (2020) The importance of nutrient management for potato production part I: Plant nutrition and yield. Potato Res 63: 97-119. [Crossref][Googlescholar]
12. Perrenoud S (1993) Potato: Fertilizers for yield and quality. 2nd Edition, International Potash Institute, Switzerland.
13. Alexopoulos A, Petropoulos SA (2021) Post-harvest physiology of potato tubers. The Potato Crop Management, Production and Food Security, Nova Science Publishers, USA, pp. 230-238.
14. Raghothama KG (2000) Phosphate transport and signaling. Curr Opin Plant Biol 3: 182-187. [Crossref][Googlescholar][Indexed]
15. Marschner P (2012) Marschner’s mineral nutrition of higher plants, 3rd Edition, Academic Press, Australia.
16. Houghland GVC (1960) The influence of phosphorus on the growth and physiology of the potato plant. Am Potato J 37: 127-138. [Crossref][Googlescholar]
17. Jenkins PD, Ali H (2000) Phosphate supply and progeny tuber numbers in potato crops. Ann Appl Biol 136: 41-46. [Crossref][Googlescholar]
18. Freeman KL, Franz PR, Jong RW (1998) Effect of phosphorus on the yield, quality and petiolar phosphorus concentrations of potatoes (cv. Russet Burbank and Kennebec) grown in the krasnozem and duplex soils of Victoria. Aust J Exp Agric 38: 83-93. [Crossref][Googlescholar]
19. Sheard RW, Johnston GR (1958) Influence of nitrogen, phosphorus and potassium on the cooking quality of potatoes. Can J Plant Sci 38: 394-400. [Crossref]
20. Klein LB, Chandra S, Mondy NI (1980) The effect of phosphorus fertilization on the chemical quality of Katahdin potatoes. Am Potato J 57: 259-266. [Crossref][Googlescholar]