Response of Potato to nitrogen and phosphorus fertilizer at Wag-Lasta Areas of Eastern Amhara, Ethiopia

Background: Production and productivity of potato in Ethiopia is far below the world average due to soil fertility problem, pest, disease, and agronomic factors. Nutrient depletion due to soil erosion is a serious problem in Ethiopian highlands. Annually, nitrogen 122 kg ha -1 P 13 kg ha -1 and K 82 kg ha -1 were estimated to deplete from Ethiopia (Haileslassie et al., 2005). The essential nutrients like, nitrogen and phosphorus are the most important inuential nutrient for the production of potato but they are decient in most Ethiopian soils and thus an application of these nutrients could increase signicantly the crop yields. In this context, an experiment was conducted at Sekota and Lasta Lalibela districts (Woleh and Kechin Abeba irrigation command areas) of eastern Amhara to investigate the effects of nitrogen and phosphorus fertilizers for yield and yield component of potato under irrigation condition. Methods: Four rates of nitrogen (0, 46, 92, and 138 kg N kg ha -1 ) and four rates of phosphorus (0, 23, 46, and 69 Kg P 2 O 5 kg ha -1 ) were combined in the factorial arrangement and laid out in randomized complete block design with three replications. Results: The result of the study revealed that nitrogen and phosphorus had a signicant effect on plant height, marketable, and total yield of potato at Kechin Abeba. But phosphorus did not show a signicant effect on plant height and unmarketable yield at Sekota district of Woleh irrigation command area. The highest yield 45.55 t ha -1 was obtained from in combined application of 138 N and 23 P 2 O 5 in Lalibela and 17.12 t ha -1 was obtained from in combined application of 138 N kg ha -1 and 46 P 2 O 5 kg ha -1 from Sekota districts of Woleh irrigation command area. Conclusion: The application of nitrogen and phosphorous fertilizer in the study area were more ecient in terms of tuber yield in Lalibela (Kechin Abeba) than Woleh. The application of 138 kg ha -1 N combined with 23 kg ha -1 P 2 O 5 is found to be the appropriate rates for optimum productivity of Potato at Lalibela (Kechin Abeba)

Sekota districts of Woleh irrigation command area.
Conclusion: The application of nitrogen and phosphorous fertilizer in the study area were more e cient in terms of tuber yield in Lalibela (Kechin Abeba) than Woleh. The application of 138 kg ha -1 N combined with 23 kg ha -1 P 2 O 5 is found to be the appropriate rates for optimum productivity of Potato at Lalibela (Kechin Abeba) and Sekota (Woleh) under irrigation conditions and the same agro-ecology.

Background
Potato (Solanum tuberosum L.) is one of the most important agricultural crops in the world. In the volume of production, it ranks fourth in the world after maize, rice, and wheat, with an estimated production area of 18.9 million hectares (Naz et al., 2011). Its yield in sub-Sahara Africa is below 10 t ha -1 while the attainable yields with good crop management are well above 30 t ha -1 . In Ethiopia its productivity is very low there are many reasons for this low actual yield of potato tubers (Haverkort et al., 2012). Factors like soil fertility problem, lack of good quality seed, unbalanced mineral nutrition and inadequate application of fertilizers, pests and disease, irregularity of water supply and traditional irrigation schemes and schedules are the main reason for low productivity of potato in Ethiopia (Emana and Nigussie, 2011). In addition to this continuous cropping without replacing the removal nutrient from crop biomass and another organic source is a major problem of nutrient depletion in Ethiopia. Plants require a variety of elements for growth and development of which N and P are the most important of the essential nutrients to plants because they are required in large quantities. The de ciency of these elements is manifested in the detrimental effects on the growth and development of the plants (Tisdale et al., 1995). Fertilizer usage plays a major role in the universal need to increase food production to meet the demands of the growing world population. Its' application resulted in marked crop yield increases, which for most crops was more than a hundred percent (Mengel and Kirkby, 1996). Applications of nitrogen and phosphorus fertilizer have shown that good yield response for different crops including potato in the country. Research conducted by Friew et al., (2016) shows that combined application of nitrogen and phosphorus fertilizer had increases the yield of potato by 12.26 t ha -1 as compared to control (0 N, 0 P).
Similarly, Wubengeda et al.,(2016) reported that increasing the rate of the two (N and P) nutrient increases the yield and yield components of potato. Desalegn et al., (2016) also, reported that increasing the rate of nitrogen and phosphorus increases the tuber yield by 361 and 358 % as compared with control. Generally, the above-mentioned study shows that appropriate agronomic practices including fertilizer recommendation play a signi cant role in potato production. However, in the study areas, farmers utilized inorganic and organic fertilizers with a blanket recommendation to increase the potato production. But, a site-speci c recommendation is very vital to produce much more economic production of potato for smallholder farmers. Moreover, there was no appropriate fertilizer rate recommendation for potato in the study area. Therefore, the experiment was conducted to determine the optimum rates of nitrogen and phosphorus fertilizers for potato production at Wag-Lasta areas of the Amhara Region Ethiopia.

Materials And Methods
Description of the study area

Experimental treatments, design and procedures
The experiment was conducted in the 2015 and 2017 irrigation season at Woleh and Kechin Abeba irrigation command areas. Four levels of nitrogen (0, 46, 92, 138 kg ha -1 ) and four levels of phosphorus (0, 23, 46, 69 kg ha -1 ) were arranged in a factorial combination, giving a total of 16 treatments set in a Randomized Complete Block Design (RCBD) with three replications. The entire rate of P 2 O 5 and the half rate of the N fertilizers were applied at the time of planting. The remaining half of N was applied 45 days after planting. Urea (46% N) and Triple Super Phosphate (46% P 2 O 5 ) were used as fertilizer sources for N and P, respectively. Medium size and well-sprouted potato tubers were planted at a spacing of 75 cm between rows and 30 cm between plants. The total plot size was 3mx3m (9 m 2 ), the spacing between plots and replications were 0.5 and 1 m, respectively. Cultural practices like cultivation, weeding and ridging were practiced as per recommendation. Watering was done within 5 days interval based on the recommendation. Gera potato variety was used for the study.

Data collection and analysis
Plant height (cm), marketable tuber yields (ton), unmarketable tuber yield (ton) and total tuber yield (ton) were collected from the middle rows of the experimental plots. Data were subjected to analysis of variance using proc GLM (general linear model) procedure of SAS 9.0 software (SAS 2004). Treatments means were compared with LSD at 5% signi cance level. To determine the nutrient content of the soil before planting, composite soil samples were collected from 0-20 cm depth using the Edelman auger from the experimental sites. Samples were air-dried and ground to pass through a 2-mm sieve to get the ne earth fraction (<2 mm separates). Particle size distribution (sand, silt, and clay separate) was determined by the hydrometer method as outlined by Bouyoucos (1965). Soil pH was determined from the ltered suspension of 1:2.5 soils to water ratio using a glass electrode attached to a digital pH meter. Organic carbon of the soils was determined following the wet digestion method as described by Walkley and Black (1934). Total nitrogen was determined by the micro-Kjeldahl digestion, distillation and titration method (Bremner and Mulvaney, 1982). The available phosphorus was determined by the standard Olsen method (Olsen et al., 1954).

Partial budget analysis
The partial budget analysis was carried out for every treatment based on CIMMYT (1998) to indicate the economic superiority of alternative treatments over the control treatment. The varying costs (fertilizer and labor) were estimated based on the existing rate of fertilizer purchase and daily labor cost. The average yield was adjusted downward by 10% from the exact yield to re ect the difference between the experimental yield and yield of farmers. MRR (%) was calculated as changes in net bene t divided by changes in cost.

Results And Discussion
Pre planting soil property of the study sites According to Landon, (1991) soil pH rating is classi ed as neutral and slightly alkaline. Similarly, the electrical conductivity of the study sites soil was free from salt (Landon 1991). The organic carbon and total nitrogen content were rated at low categories. According to Olson (1951), the available phosphorous was high on both sites. This might be the area had a long history on agriculture without replacing the complete removal of cover crop and burning crop residue as fuel are the main cause for nutrient loses.
A signi cant interaction (N*P) was observed for plant height and marketable yield in both years but total tuber yield was interacted in the year 2015 at Lalibela Kechin Abeba irrigation command area (Table 2 and 3). But the main effect of nitrogen was signi cantly affected all the parameters and the main effect of phosphorous was affect the only marketable and total yield of potato in 2017. In the year 2015, the main effect of phosphorous was affected marketable, total yield and plant height. Whereas at Woleh irrigation command area except for plant height signi cant interaction was observed on the other stated parameter ( Table 4). The main effect of nitrogen and phosphorous fertilization was signi cantly in uenced the plant height, marketable yield, unmarketable yield and total yield of potato but, phosphorous was not affecting the plant height.    production year. This is probably due to variation in irrigation water availability in the year between 2015 and 2017 (Table 5). That means water is essential for the germination of seeds, growth of plant roots, and nutrition, photosynthesis, transpiration, to maintain the turgidity of cell walls and multiplication of soil organisms. There was a

Marketable yield
Both the main and interaction effect of nitrogen and phosphorus fertilizer application were affect the marketable yield signi cantly at Kechin Abeba and Woleh. The increasing rate of nitrogen and phosphorus signi cantly increases the marketable yield of potato in both sites. In Kechin Abeba marketable yield was increased with the increasing rate of nitrogen and phosphorus. The highest marketable yield (45.55 t ha -1 ) was recorded from 138 kg ha -1 nitrogen is combined with phosphorus at a rate of 23 kg ha -1 in 2017 and 19.57 t ha -1 was recorded from 138 nitrogen is combined with phosphorus at a rate of 23 kg ha -1 in 2015 (Table 6)  kg ha -1 nitrogen in combined with phosphorus at a rate of 46 kg ha -1 whereas the lowest marketable yield (8.16 t ha -1 ) was recorded from the control treatment (0, 0 NP kg ha -1 ) ( Table 7). There was tuber yield reduction in Woleh and Kechin Abeba in 2015 by half as compared to tuber yield gained in 2017 Kechin Abeba. This was attributed to the fact that water is the most important limiting factor for potato production and it's possible to increase production levels by well-scheduled irrigation programs throughout the growing season (Liu et al., 2006). Similarly, (Demile, 2012) observed that 4.09 t ha -1 (64%) and 39% tuber yield reduction were recorded from the application of 25%, and 50% (de cit) of the total crop water requirement at all stages, respectively. However, the marketable yield was increased by 24.95 t ha -1 and 8.96 t ha -1 over control treatment at Kechin Abeba and Woleh respectively. This might be the fact that nitrogen is part of the chlorophyll molecule, which gives plants their green color and is involved in creating food for the plant through photosynthesis and phosphorus is involved in the metabolic processes responsible for transferring energy from one point to another in the plant. It's also critical in root development and owering. The current study is in agreement with previous study Zelalem et al.,   were obtained from the unfertilized treatment. Similarly highest unmarketable yield was obtained from the application of N 138 and P 2 O 5 46 kg ha -1 (Table 8). Partial budget analysis The application of nitrogen at a rate of 138 kg ha -1 with 23 kg ha -1 phosphorus had the highest marketable yield (15.29 t ha -1 ) and net benefit (164597 Ethiopian Birr) in Woleh irrigation command area ( Table 9). The MRR (1606.90 %) was gained from the treatment of 138 N and 23 P 2 O 5 kg ha -1 , this implies that for each Birr that invested in the new technology, the producer can receive to recover the one Birr invested plus an additional return of 16.06 Ethiopian birr.  Figure 1 Description of the study area