The Effects of Nitrogen and Phosphorus Fertilizer Rates on Yield and Quality of Tomato (Solanum lycopersicum L.) in Hawzen, Ethiopia

Four different rates of N (0, 69, 138 and 207 kg Nha − 1 ) and P (0, 46, 69 & 92 kg Pha − 1 ) were laid out in a randomized complete block design with three replications. Soil analysis were done before executing of the experiment. Data were collected on yield, and fruit quality of tomato. The revealed that of were signicantly inuenced by the of and The interaction of N and P rates only had signicant effects on marketable, total fruit yield, mean fruit weight, and fruit length. Compared to control, application Nha − Pha − 1 increased mean fruit weight, marketable fruit weight and total fruit and rates remarkable sugar, acidity, ascorbic acid, lycopene content, and protein. A signicant P detected varied P. fruit N concentration was signicantly inuenced by rates N only.


Conclusion
Application of N and P at rates of 138 and 69 kgha − 1 , respectively, showed the highest values in most yield and quality measurements and particularly increased yield by 70.5% as compared to the nil fertilizer application. Therefore, 138 kg Nha − 1 and 69 kg Pha − 1 can be used by producers for better yield and quality of tomato in the area.

Background
Tomato (Lycopersicon esculentum Mill) is important in the daily diet and economy of Ethiopian people. It is an important source of income for small-scale farmers and sources of employment for many people and regional export crops (Wiersinga and de Jager, 2009). According to Gemechis et al. (2012), it is consumed in every household in different modes, but in certain areas, it is also an important co-staple food and source of vitamins and minerals. One medium ripe tomato fruit can provide up to 40% of the recommended daily allowance of vitamin C and 20% of vitamin A (Kelly and Boyhan, 2010). It also has beta-carotene, which is an antioxidant and promotes good health (Willcox et al., 2003). Tomatoes also contain lycopene, a red pigment serving as a natural antioxidant (Shi and Manguer, 2000), calcium, water, and niacin, which are essential for metabolism (Olaniyi et al., 2010).
In Ethiopia, vegetables cover approximately 1.67% of the area under all crops and contribute 2.23% of the production of the total crop at a national level (CSA, 2019). Tomato production in Tigray is approximately 26.5% of the vegetable production area (CSA, 2017). However, the productivity is very low, approximately 5.7 ton ha − 1 (CSA, 2017), compared to Egypt (18.6 ton ha − 1 ), Spain (76.4 ton ha − 1 ), and Turkey (35.8 ton ha − 1 ). Productivity is highly constrained by several factors in Tigray and other parts of Ethiopia. One of the contributing factors to this low yield is suboptimal fertilizer use by tomato growers (Edosa et al. 2013, Balemi, 2008Lemma, 1992). In particular, N and P are major nutrients de cient in almost all soils of the cultivated area of Tigray.
In Ethiopia, the depletion rates of N, P and K are 122, 13, and 82 kg N ha − 1 year − 1 , respectively, which are more severe in highland areas (Haileslassie et al., 2005), such as Tigray. Soil fertility loss is related to cultural practices such as low fertilizer use, removal of vegetative cover (such as straw or stubble), and burning plant residues or the annual burning of vegetation on grazing land (Endale, 2011). More than 50% of the soil in Tigray is de cient in macro essential nutrients (ATA, 2014). Despite the de ciency, N and P fertilizer applications for vegetable crops such as tomato are limited and are expressed in low yields. This is more noticeable in the highland areas of Tigray as soil erosion is severe.
Tomatoes are considered "heavy feeders" because of their rapid growth and long production season, making the crop highly sensitive to soil fertility. To obtain one ton of fresh fruit, the plant needs to absorb on average 2.5-3 kg N, 0.2-0.3 kg P, and 3-3.5 kg K (Hedge, 1997). In addition, P, the second most important plant growth-limiting nutrient after N, is abundantly available in soils in both organic and inorganic forms (Khan et al., 2009). Despite a large reservoir of P, the amount of available forms to plants is generally low. Plants are unable to utilize phosphate because 95-99% phosphate is present in the insoluble, immobilized, and precipitated form (Pandey et al., 2007).
It is well documented that N and P application enhances the yield and yield-related traits of tomato (Edosa et al., 2013). It also affects the quality of fruits (Ronga et al., 2020). However, optimum N and P rate effects varied with variety, soil type, season, climatic condition, and other management practices.
Speci c agronomic protocols and extension services are required to optimally manage tomato crop systems (Ronga et al., 2020). The required amount of nutrient determination is vital for optimum plant growth and development without posing unnecessary expenditure, related negative health, and environmental effects. The soil in the Hawzen district is low in total nitrogen (0.05%) and available P (5.45 ppm) (Tekalign, 1991). However, limited literature exists on the N and P rate determination in the district. Although the producers are using the national recommendation with an amount of N and P, an optimum amount of fertilizers varies with genetic potential/responses of varieties in use, microclimate and edaphic factors, the market of the input and product. Hence, this study helps generate information on the effects of N and P on tomato fruit yield and yield-related components, quality and biochemical composition in Hawzen, Eastern Tigray.

Description of the Study Area
The trial was conducted in Hawzen district, Tigray Ethiopia, speci cally at the Hayelom site (Fig. 1).
Hawzen is located at 39° 27' 2'' E and 13° 15' 16'' N, an altitude of 2120 meters above sea level. The longterm average  annual precipitation is 536 mm, while the total rainfall of the growing season is 469 mm. The average minimum temperature was 10 °C, and the maximum temperature was 27 °C.

Experimental Treatments, Design, and Procedure
The treatments consisted of four rates of N (0, 69, 138 and 207 kg N ha − 1 ) and four rates of P (0, 46, 69 and 92 P 2 O 5 ha − 1 ) laid out in a randomized complete block design with three replications. Tomato var.
Miya (semi-determinate type) seed, N fertilizer in the form of urea (46% N) and phosphate fertilizer in the form of P 2 O 5 were used for the study.
The whole amount of P fertilizer was applied to the experimental plots three days before transplanting. N was applied three times as the ½ amount was at the time of transplanting, one fourth (¼) at the midgrowth stage (35 days after transplanting) and one fourth (¼) at early owering. The size plot was 3 m x 3 m (9 m 2 ), and the distance between the plots and blocks was 1 m and 1.5 m apart, respectively. The spacing between rows and plants was 75 cm and 30 cm, respectively. The land was plowed to a ne tilth by repeated harrowing and leveling using oxen and labor force. Recommended raised seedbeds of 5 m in length and 1 m in width were prepared near the experimental site to reduce transplanting shock during transplanting time. Seedlings were transplanted four weeks after sowing, where seedlings attained 2-3 true leaf stages.

Soil sampling procedure and physico-chemical characterization
Composite surface soil samples (0-30 cm depth) were collected randomly in a zig-zag pattern from ten sampling spots of the entire experimental site before planting for the determination of selected physicochemical properties of the soil. Soil texture was determined using the Boyoucous hydrometer method (Bouyoucos, 1965). Soil pH (McLean, 1982) was determined from a suspension of 1:2.5 of the soil:water ratio using a glass electrode attached to a digital pH meter. The organic carbon was determined by the dichromate oxidation method and subsequent titration with ferrous ammonium sulfate (Walkley and Balck, 1934), and % of organic matter (OM) was obtained by multiplying % OC by 1.74 assuming that the average C concentration of organic matter is 58%. The total soil nitrogen was estimated using the Kjeldahl procedure. The determination of available phosphorus was carried out following the Olsen extraction method (Olsen et al., 1954). Exchangeable bases (K, Ca, Mg, and Na) and cation exchange capacity were determined by the leaching method with ammonium acetate solutions (1 M NH 4 OAc). The concentrations of exchangeable Ca and Mg were measured from the extract with an atomic spectrophotometer, while exchangeable K and Na were measured with a ame photometer (Van Reeuwijk, 2002).

Crop-related data
Yield-and yield-related attributes: Plant height (cm) was determined with the use of the tape rule measured from the base of the plant above the ground to the last expanded leaf of the growing tip. The leaf area index, number of branches per plant and number of ower clusters were collected. The number of clusters per plant, number of fruits per cluster, and average number of fruits per plant were recorded.
The average fruit weight per plant (kg), marketable fruit weight (ton ha − 1 ), and fruit yield per hectare (ton ha − 1 ) were recorded.
Fruit quality analysis: Well-ripened fruits were collected, washed, dried, sliced, and homogenized. Quality parameters such as titratable acidity expressed as the percentage of citric acid, total soluble solids measured using a hand refractometer at 0-32°Brix value and sugar to acid ratio were computed (Waskar et al., 1999). The ascorbic acid content of fruits was analyzed using the method of AoAC (1995). Lycopene content was estimated from the absorbance measurement at 503 nm UV-spectrophotometer (Lime et al., 1957). The sample plant tissue was dried at 65 °C until a constant weight was obtained and ground and sieved for the determination of tissue N and P content.

Statistical Analysis
Data were subjected to analysis of variance (ANOVA) using Statistical Analysis System (SAS), version 9.2 (2009). Detection of differences among treatment means for signi cance was done using least signi cant difference (LSD) at the 0.05 probability level.

Soil chemical and physical characteristics of the experimental site
The physical and chemical properties of the soils of the experimental elds before planting are indicated in Table 1. The analytical results indicated that the particle size distribution of the surface soil (0-20 cm) of the experimental sites was dominated by clay, with a proportion of 69% sand, 14.4% silt, and 16.6% clay in the Hawzen district. Hence, the experiment had a sandy loam, textural class. The pH of the soil was 6.25, showing that the nature of the soil was slightly acidic (Bruce and Rayment, 1982), which is in the range of productive soils.
The available P was 6.15 mg kg − 1 before planting in Hawzen. According to the Holford and Cullis (1985) classi cation, soils with available P contents < 25 mg kg − 1 are rated as very high, 10-17 mg kg − 1 are moderate, 5-10 mg kg − 1 as low and < 5 mg kg − 1 as very low. Hence, the soil grouped under a medium level. Soil with OM values within 1.70 to 3.0% is rated as moderate (Charman and Roper, 2007).
Accordingly, the experimental soil in the area has a value of 1.1%, which is rated as low in OM. More importantly, total N in the experimental area was found to be 0.08% in Hawzen, which is a low rate according to Bruce and Rayment (1982 (Table 2a). Nevertheless, the interaction effects of N and P showed non-signi cant (p < 0.05) effects (Table 2a). Using N at rates of 69, 138, and 207 kg ha − 1 increased the plant height of tomato by 12.1%, 19.5%, and 31.2% compared to the control (Fig. 2). This might be due to N fertilizer ensuring   Similar to the N rate, the application of 46, 69, and 92 kg Pha − 1 produced taller plants compared to the control by 8.8%, 13.04%, and 16.5%, respectively (Fig. 2). Increasing the P level increased the plant height linearly. In line with this, Singh and Sangama (2000) suggested that P is a constituent of nucleoprotein, known to play a leading role in photosynthesis, cell division, and tissue formation, which may contribute to plant height.
Leaf area index (LAI): The results revealed that a signi cant (p < 0.05) increasing trend in LAI was recorded with increasing applied N and P, but their interaction effects were non-signi cant (Table 2a). Increasing N rates resulted in increasing LAI and widest leaf recorded at the highest rate of N (Fig. 2).
Similarly, Tie et al. (2002) reported a sharp increase in LAI in response to the application of N fertilizer. N is necessary for photosynthesis, the formation of chlorophyll and nucleic acids in which its absence or de ciency causes stunted growth (Tisdale et al., 2003). Similarly, as applied P increased, there was an increase in LAI widest recorded at 92 kg ha − 1 but similar to 69 kg ha − 1 (Fig. 2) Primary branches: The results revealed that the rates of N and P signi cantly (p < 0.05) in uenced the number of primary branches per plant. However, their interaction resulted in non-signi cant (p < 0.01) effects (Table 2a). Nil fertilizer treatment resulted in the lowest branch compared to the other treatments of N and P, whereas no more increase in a primary branch was detected when N was applied beyond the rate of 69 kg ha − 1 (Fig. 3). N supply enhanced the tomato branch, which could be due to the positive impact stimulation of meristematic growth and the new branches and leaves. Similar to this, Rao et al.
(2014) reported that increasing N assisted chloroplast function, thus increasing the growth of a plant.
An increase in the primary branch was observed with increasing P rate. Application of P at rates of 92, 69 and 46 kg ha increased lateral branches by 44.3%, 34.4%, and 19.4%, respectively, compared to the control (Fig. 3). This may support the fact that P encourages the formation of ATP and supplies energy for new cell formation, which may help to form new branches. In contrast, Etissa et al. (2013) reported that the application of P did not affect a number of lateral branches of cv. Melkashola under vertisols. The inconsistent results could be due to the variation in soil physical and chemical properties and cultivars.
Flower and fruit number per cluster: The results revealed that the number of ower clusters was signi cantly (p < 0.01) in uenced by different rates of N and P. However, no signi cant in uence was observed due to the interaction of N and P (Table 2a). The lowest number of ower and fruit clusters was recorded in the control treatment compared with any other plots that received additional N fertilizer. This could be because N helps chlorophyll formation, which helps photosynthesis. Similar to this suggestion, Lu and Zhang (2000) also reported that N de ciency decreases the quantum yield of photosystem II, the electron transport system, and the maximum photochemical e ciency of photosystem II. Similarly, the highest and lowest ower and fruit clusters were obtained from 92 kg Pha − 1 and the control, respectively (Fig. 3). An increase in ower clusters per plant was observed with an extra addition of P. The maximum cluster number could be due to the effects of P in promoting blossom bud formation.

Effects of N and P on fruit yield
Mean fruit weight and fruit length: ANOVA results demonstrated that the individual effects of N, P, and their interaction showed signi cant (p < 0.01) effects on mean fruit weight and fruit length (Table 3).
Mean fruit weight and fruit length showed the highest values due to applications of 138 kg P ha − 1 and 92 kg P ha − 1 (Table 3). Roy et al. (2011) also reported that the average fruit weight increased in capsicum. Generally, at low rates of N and P, the average fruit weight and fruit length were low. The positive response shown by yield parameters to N and P could be directly linked to the well-developed photosynthetic surfaces and increased physiological activities leading to more assimilates being produced and subsequently translocation of assimilates and utilized for fast fruit development.
Marketable and total fruit yield: Signi cant N and P (p < 0.01) interactions (p < 0.05) were noted for marketable fruit yield and total fruit ( Table 3). The lowest values of marketable and total fruit yield were obtained from the control treatment. The highest values of marketable and total fruit yield were highest with the treatments of 138 and 92 kg ha − 1 (Table 3). This could also be due to the individual mean fruit weight increment. Similar to the current ndings, Balemi (2008) also reported the highest fruit yield obtained from the highest rate and lowest from the lowest rate of NP. The nutrient requirement of the tomato is an important factor if large quantities of high-quality fruits are to be produced effectively and e ciently (Anderson et al., 1999). Higher yields at high levels of N and P are due to better fertilizer responsiveness of the tomato crop (Mishra et al., 2004). Means with different superscript letters in a column differ signi cantly

Effects of N and P on fruit chemical composition
Total soluble solids (TSS): A signi cant difference in TSS was observed among different rates of N and P but not due to their interactions (Table 2b). Except for the control treatments, no difference in TSS values was detected under different rates of N, and the lowest value of TSS was recorded in the control (Table 4). Similarly, TSS contents increased with P applications, and the lowest values were observed in the control. N is a constituent of protein, and amino acids directly affect the TSS (Kirimi et al., 2011  Total sugar content: The results demonstrated that the total sugar content of fruits was signi cantly (p < 0.05) affected by different rates of N and P. However, the interaction effects of N and P were not signi cant (Table 4) and did not affect the total sugar content (p < 0.05). The lowest total sugar content was obtained in the nil fertilizer rate (Table 4) Ascorbic acid: N and P rates did signi cantly affect vitamin C in tomato fruit. Nevertheless, the interaction effects of N and P were nonsigni cant in both traits ( Table 4). As shown in Table 4, the vitamin C content of tomato fruits increased with increasing amounts of N and P fertilizer added. However, only the control treatment showed the lowest vitamin C content in the fruits. Similar to the current ndings, Taiwo et al. (2007) reported that the control had the lowest vitamin C content. Conversely, Dumas et al. (2003) and Simonne et al. (2007) recommend a low rate of N to obtain a high ascorbic acid level. This indicated that a higher concentration of P in the soil can increase the content of vitamin C.
Titratable acidity (TA): Signi cant variation in TA was shown due to N and P rates but not their interaction (  (Sainju et al., 2003). However, different rates of P and the interaction between N and P were not signi cant (p < 0.05) for lycopene content (Table 4). Conversely, others reported that increased concentrations of P in the soil increased lycopene content in tomato fruits (Zdravković et al. 2007;Dumas et al. 2002). Thus, the results indicated that the lycopene content of tomato fruit could respond in different ways to individual fertilizer depending on variety, soil condition, environmental factors, and other management practices.
N concentration: P fertilization did not affect fruit N concentration. However, the N concentration of fruits was signi cantly (p < 0.05) in uenced by N fertilization (Table 2a). The lowest N concentration of fruits was recorded from the control treatment (Table 4). However, there was no further variation as much increase in the rate of N. High nitrogen application can increase fruit N concentration, which indicates variable N effects due to the corresponding applied amount. Hormonally, Santamaria (2006) reported that fresh fruit tomato is classi ed as a very low-nitrate accumulating vegetable. This may be the reason for the similar N concentrations recorded in the different rates of N fertilizer except the control treatment.
P concentration: The P content of fruits was signi cantly (p < 0.01) affected by the application of P fertilizer added at different rates. However, neither levels of N nor the interaction between N and P resulted in a signi cant (p < 0.05) effect on the P content of fruit (Table 2b). With increasing P rates, an increase in the P content of tomato fruit was recorded. The highest value of P content was recorded from the rate of 92 kg Pha − 1, and the lowest was recorded with the control treatment (Table 4). Similar to this nding, Gill and Verma (2018) reported that P uptake increased with increased NPK levels, and they suggested that it can be due to improved absorption and utilization of P at higher rates of application. In fact, fruit mineral composition can vary due to the response of cultivars to fertilizer application. Conversely, fruit P composition decreased with increasing rates of N (Ronga et al., 2020). This could vary due to climatic conditions, soil types, and time of application.

Conclusion
The application of N and P increased plant height, leaf area index, number of primary branches per plant, number of clusters per plant, number of ower clusters, number of fruit per clusters, and fruit set. The interaction of N and P rates had signi cant effects on marketable, total fruit yield, mean fruit weight, and fruit length. Likewise, the main effects of N and P rates were remarkable on total soluble solids, total sugar, titratable acidity, ascorbic acid, lycopene content, and protein. A signi cant in uence on the P content of tomato was detected due to varied rates of P but not N and its interaction with P. Fruit N concentration was signi cantly in uenced by N rates only. Application of N and P at rates of 138 and 69 showed the highest values in most growth, yield and quality parameters recorded, although similar to the higher rats of both treatments. Overall, producers can use 138 kg Nha − 1 and 69 kg Pha − 1 in Hawzen, Northern Ethiopia.

Declarations
Ethics approval and consent to participate No ethics approval or consent to participate was needed for any part of this study.

Consent for publication
The author has reviewed and approved the manuscript for submission.

Figure 1
Map of the study area Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.