Journal of Agricultural Science and Practice

Volume 2. Page 97-101
Published 23rd October, 2017
ISSN: 2536-7072

Full Length Research

Effects of NPK fertilizer rates on the carbohydrate and crude protein contents of three varieties of potato (Solanum tuberosum L.) in Sudan Savanna of Nigeria

H. Yusuf1*, A. Muhammad2 and J. M. Ambursa1

1Department of Preliminary Studies, College of Science and Technology, Waziri Umaru Federal Polytechnic, Birnin-Kebbi, Kebbi State, Nigeria.
2Department of Crop Science, Kebbi State University of Science and Technology, Aliero, Kebbi State, Nigeria.

Received 10th September, 2017; Accepted 26th September, 2017

*Correspondence: Dr. H. Yusuf, Department of Preliminary Studies, College of Science and Technology, Waziri Umaru Federal Polytechnic, Birnin-Kebbi, Kebbi State, Nigeria. Email: Tel: +2348036385900.

Copyright © 2017 Yusuf et al. This article remains permanently open access under the terms of the Creative Commons Attribution License 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


Field trials were conducted during the 2015/16 and 2016/17 dry seasons. The aim was to study the effects of NPK fertilizer rates on tuber carbohydrates and crude protein (CP) of three varieties of potato (Solanum tuberosum L.). The research was conducted at Teaching and Research Farm of Kebbi State University of Science and Technology, Aliero, located at Jega in the Sudan savanna zone of Nigeria. Treatments consist of four rates of NPK [20:10:10] fertilizer (0, 300, 600 and 900kg NPK ha-1) and three varieties (Bertita, Diamant and Nicola) of potato. Factorial combinations of fertilizer rates and varieties were laid out in a randomized complete block design, replicated three times. The size of each plot was 4.5 x 3 m (13.5m2). Results revealed that CP was affected by the dosage of fertilizer applied. 600 kg NPK ha-1 gave the highest CP above which there was no significant increase. Higher tuber carbohydrate content was recorded by Nicola and Bertita than Diamant; while crude protein was highest in Bertita compared to Nicola and Diamant. In this study, Bertita variety was higher in crude protein; and was comparable to Nicola, Bertita was lower (p ≤ 0.05) than Nicola and Diamant in carbohydrates. The use of 600 kg NPK ha-1 was optimum for good tuber quality.

Key words: Crude protein, NPK fertilizer, Sudan savanna, tuber carbohydrates, variety.


Irish Potato (Solanum tuberosum L.) is one of the word's prime sources of human nutrition. The protein/ carbohydrate ratio is higher than for most cereals and other tuber and root crops (Okonkwo et al., 1995). In terms of composition, potato tuber contains 70 to 82% water, 17 to 29% dry matter, 11 to 23% carbohydrate, 0.8 to 3% protein, 0.1% fat and 1.1% minerals (Singh, 2004). Production per hectare of important vitamins, such as thiamine, riboflavin, niacin and vitamin C is higher in potato than in other major crops, such as rice, maize and wheat (Rolot, 2001; Okonkwo et al., 1995; Beukema and Vander Zaag, 1990; Harris, 1992). According to Okonkwo et al. (1995), daily consumption of 1 kg of potato tuber provides all the essential amino acids needed by the body, except cystine and methionin. Apart from more conventional ways of using potato as food stuff, it is now a prominent industrial raw material in developed countries. This has considerably widened the range of possible uses of the crop. In the food sector, potatoes are processed into deep-frogmen chips and mashed potato. By-products such as potato starch, glucose and dextrose are used in the brewing industry, confectionary products and in the distillation of alcohol. In the non-food sector, by- products such as potato starch, starch proper and dextrin are used in processes for the manufacture of cardboard, glues, textiles and paints, and in the laundry business as ironing sprays (Harris, 1992; Rolot, 2001).

High tuber dry matter content is particularly desirable in potatoes for processing. However, according to International Potato Institute, IPI (1993), N and K application reduced tuber dry matter content, with the effect being less by K than N. However, P was reported to slightly increase tuber dry matter content. Protein content usually expressed as a percentage of dry matter is favorably influenced by increase in N and K, but reduces by increasing application of P (Singh, 2004). To achieve high tuber quality, it is recommended to apply NPK fertilizer with less N than P and K to potato (Vos, 1999).

Depending on the mineral fertilizer forms, rates and nutrient ratios applied to potatoes; the contents of dry matter, starch, protein and other substances may either increase or decrease. Excessive application of fertilizer reduces starch, dry matter and sugar contents in potato tubers that deteriorate more rapidly in storage (Shock et al., 1999). This results from the fact that nitrogen promotes growth of potato vines, and when lasting drought occurs, the vines that have grown so large are most dramatically affected and the growing season is extended. Tubers harvested under such conditions, may not reach harvest maturity or amass maximum dry matter content, while starch also accumulates less rapidly, since part of it is used for respiration (Eiasu et al., 2007). Dry matter content is affected by various factors, among which the most significant are tuber maturity, growth character, plant nutrient and water uptake (Harris, 1992). Thus, this study was carried out to determine carbohydrates and crude protein contents of some selected potato varieties in response to NPK fertilizer rates.


The experiments were conducted during 2015/16 and 2016/17 dry seasons at the Teaching and Research Farm of the Kebbi State University of Science and Technology at Jega (latitude 120 11' N; longitude 40 16' E) in the Sudan savanna ecological zone of Nigeria. The climate of the area is semi-arid with average rainfall between 550 to 650 mm per annum. Relative humidity ranges from 21 to 47% and 51 to 79% during the dry and rainy seasons respectively. Temperature ranges between 14 to 30oC during dry season and 27 to 41oC during the rainy season (Anonymous, 2012). The mean monthly temperatures, relative humidity and solar radiation at the experimental site during the period of the experiments (2014, 2015, 2016 and 2017) are shown in Appendix II.

The treatments consisted of four rates (0, 300, 600 and 900kg ha-1) of NPK (20: 10: 10) fertilizer and three potato varieties (Nicola, Bertita and Diamant). The treatments were laid out in a randomized complete block design with three replications. The planting material (seed tubers) for the three varieties was sourced from the Potato Program Unit of the National Root Crop Research Institute (NRCRI) sub-station Vom, Jos, Plateau State. The seed tubers were pre-sprouted for six weeks and dressed with fungicide (Muncozeb powder) a day prior to planting. Planting was done manually with whole or cut tubers of approximately 30 g weight per hill at intra-row spacing of 30 cm and a depth of 8 to 10cm. Plots of 3.0 x 4.5m (13.5 m2) were marked out, leaving a 1 m space between main plots. Six ridges spaced 75 cm apart were constructed in each plot. The net plot area consisted of the two middle rows (3.0 x 1.5 m) (4.5 m2).

Water pump machine was used to draw water from a tube well to the field through the water channels. Irrigation was scheduled at 4 to 7 days interval depending on the water need. The whole field was watered 3 days before and after planting. Compound fertilizer (NPK 20: 10: 10) was used at the variable treatment rates of 0, 300, 600 and 900kg NPK ha-1. These rates were applied according to the treatments in two split doses. The first and second doses were applied as basal and at 4 weeks after planting (WAP) respectively. Fertilizer was incorporated about 10 cm away from each plant stand at 5 cm depth and covered with soil. Weeds were controlled manually using hoe at 4 and 7 WAP. Karate insecticide (lambdacyhalothrin) was sprayed at 4 ml active ingredient L-1 of water to control insect pests. The crops were harvested on 16th February, 2016 and 11th February, 2017 for the 2015/16 and 2016/17 trials respectively. A light irrigation was given to all plots a day to harvesting to facilitate easy lifting of tubers.

Harvested tubers were sampled from each net plot, sliced to very small pieces and oven-dried at 700C to a constant weight in separate paper bags. The oven-dried tuber samples were grounded, sieved and analyzed for total N contents in the laboratory using one gram each of the sample by Micro Kjeldhal procedure. The physical and chemical properties of the soil of the experimental site in 2015 and 2016 dry seasons are shown in Appendix I.

The crude protein content of tubers was estimated by multiplying the total nitrogen (N) content of the tuber by 6.25 as described by Yusuf et al. (1999). Tuber carbohydrate content was estimated as Nitrogen Free Extract (NFE) and was calculated as: NFE = 100 – (crude protein + moisture + crude fiber). Data generated were subjected to analysis of variance (ANOVA) in accordance with Gomez and Gomez (1984). The treatment means were separated using the Duncan's Multiple Range Test (DMRT) (Duncan, 1955) and Statistical Analysis System (SAS).


Tuber Carbohydrate Content (%)

Fertilizer rates did not have any significant effect on percent tuber carbohydrate content throughout the seasons and in the combined data as shown in Table 1. In 2015/16, Nicola had the highest tuber carbohydrate content followed by Diamant, and the lowest was in Bertita. In 2016/17 and the combined data, Nicola and Bertita were at par, with significantly higher percent carbohydrate content than Bertita. Interactions of irrigation intervals, NPK rates and varieties on percent tuber carbohydrate content were not significantly different in all trials and the combined data.

Table 1

Tuber Crude Protein Content (%)

Effects of fertilizer rate and variety on tuber crude protein (CP) content are presented in Table 1. Application of 600 and 90 0kg NPKha-1 produced similar and significantly higher percent tuber crude protein than the two other rates in 2015/16 and the combined means, except in 2016/17 where 0 and 300kg NPKha-1 was at par with 600 kg NPKha-1. Variety Bertita consistently gave higher tuber crude protein content than Diamant and Nicola in the two trials and the combined data. In the combined data, Diamant in turn gave more crude protein content than Nicola. Interactions of irrigation intervals, NPK rates and variety on tuber crude protein content were not significant in all trials and the combined data.


The CP contents of the tuber were maximized with the application of 600 kg NPKha-1 above which there was no further significant increase. The high content of the applied fertilizer particularly the 600 and 900 kg NPKha-1 rates, in addition to the inherent high soil fertility might have resulted in ample uptake and translocation of N, which is a factor in protein synthesis; to the plant system and its subsequent mobilization to other plant parts, including the tubers. According to Adhikari and Sharma (2004), N and K application reduced tuber dry matter content, with the effect being less by K than N, but P was reported to slightly increase tuber dry matter content. Protein content usually expressed as a percentage of dry matter was enhanced by increase in N and K but reduced by increasing P application (Shukla and Singh, 1976; Ezzat et al., 2011). The significantly high tuber carbohydrate content in varieties Nicola and Bertita over Diamant, and crude protein content in Bertita over Nicola and Diamant further indicated the genetic variation among the three varieties tested in this study (Babaji et al., 2008; Manrique and Barthlomew, 1991).


In this study, variety Bertita was superior in terms of tuber crude protein; and was similar to Nicola in terms of carbohydrate contents of the tuber. The application of 600 kg NPK ha-1 was optimum for crude protein and carbohydrate contents of potato tuber (Bertita, Diamant and Nicola).


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