3.1. Soil chemical and physical characteristics of the experimental site
The physical and chemical properties of the soils of the experimental fields 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.
Table 1
Physical and chemical properties of the soil before transplanting
Parameters
|
Values
|
Rate
|
Sand (%)
|
68
|
|
Silt (%)
|
15
|
|
Clay (%)
|
17
|
|
Textural class
|
Sandy loam
|
|
pH (1:2.5 H2O)
|
6.25
|
Slightly acidic
|
OM (%)
|
1.1
|
Low
|
TN (%)
|
0.083
|
Low
|
Available P (mg Pkg− 1)
|
6.15
|
Low
|
CEC (cmolc kg− 1)
|
18.53
|
Very high
|
Exchangeable Na (cmol (+)kg− 1
|
0.1
|
low
|
Exchangeable K (cmol (+)kg− 1
|
0.3
|
Moderate
|
Exchangeable Ca (cmol (+)kg− 1
|
6.5
|
Moderate
|
Exchangeable Mg (cmol (+)kg− 1
|
4.0
|
High
|
Percentage of base saturation
|
58.82
|
Moderate
|
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) classification, 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). The soil analysis result of the experimental area indicated that the CEC values of soils were 18.5 cmolc kg− 1 in the experimental site, showing high rate values (Hazelton and Murphy, 2007). The experimental soil has medium levels of exchangeable K, moderate exchangeable Ca2+, low exchangeable Na+, and high exchangeable Mg2+ (Metson, 1961) (Table 1). The base saturation of the soil is moderate (58.8%) and satisfactory for tomato production (Metson, 1961).
3.2. Effects of N and P on the growth performance of tomato
Plant height: The results showed that plant height was significantly (p < 0.01) affected by different rates of N and P (Table 2a). Nevertheless, the interaction effects of N and P showed non-significant (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 favorable conditions for the elongation of stems with optimum vegetative growth. Other researchers also reported that too little N in the soil stunts plant growth (Etissa et al., 2013; Mary, 2006; Sainju et al. 2003).
Table 2a
Results of analysis of variance of plant height, number of primary branches, number of fruit clusters per plant, number of flower clusters per plant, number of fruits per cluster, marketable fruit yield, total fruit yield, mean fruit weight
SV
|
PH
|
LAI
|
NPB
|
NFC
|
NFrC
|
NF/C
|
MFY (ton ha− 1)
|
TFY (ton ha− 1)
|
MFW (g)
|
FL (cm)
|
N
|
**
|
**
|
**
|
**
|
**
|
**
|
**
|
**
|
**
|
**
|
P
|
*
|
*
|
**
|
**
|
**
|
**
|
**
|
**
|
**
|
**
|
N*P
|
ns
|
ns
|
ns
|
ns
|
ns
|
ns
|
*
|
*
|
**
|
*
|
CV
|
15
|
5.8
|
12.6
|
15.3
|
4.75
|
24.32
|
17.33
|
16.55
|
5.49
|
10.31
|
SV = source of variation, PH = plant height, LAI = leaf area index, NPB = number of primary branches, NFrC = fruit cluster number, NFC = flower cluster number, MFY = marketable fruit yield, TFY = total fruit yield, MFW = mean fruit weight, FL = fruit length |
Table 2a
Results of analysis of variance of total soluble solid (TSS), total sugar, titratable acidity (TA), ascorbic acid content, lycopene content N and P
Source of variation
|
TSS (oBrix)
|
Total sugar
|
TA
|
Ascorbic acid
|
Lycopene
content (mg g− 1)
|
N(mg g− 1)
|
P(mg g− 1)
|
N
|
**
|
*
|
*
|
**
|
ns
|
**
|
ns
|
P
|
**
|
**
|
**
|
**
|
ns
|
**
|
**
|
N*P
|
ns
|
ns
|
ns
|
ns
|
ns
|
ns
|
ns
|
CV
|
15.09
|
13.89
|
19.98
|
17.86
|
25
|
3.0
|
21.17
|
TSS = total soluble solids, TA = titratable acidity, *, **, ns F-test significant at p < 0.05, p < 0.01, and not significant, respectively |
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 significant (p < 0.05) increasing trend in LAI was recorded with increasing applied N and P, but their interaction effects were non-significant (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 deficiency 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). Similarly, Khavarinejad et al. (2009) reported that a low phosphate supply reduced leaf area in P-deficient plants. Phosphate deficiency causes a negative effect on leaf cell expansion, which is the result of carbohydrate deficiency (Louw-Gaume et al., 2010).
Primary branches: The results revealed that the rates of N and P significantly (p < 0.05) influenced the number of primary branches per plant. However, their interaction resulted in non-significant (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 flower clusters was significantly (p < 0.01) influenced by different rates of N and P. However, no significant influence was observed due to the interaction of N and P (Table 2a). The lowest number of flower 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 deficiency decreases the quantum yield of photosystem II, the electron transport system, and the maximum photochemical efficiency of photosystem II. Similarly, the highest and lowest flower and fruit clusters were obtained from 92 kg Pha− 1 and the control, respectively (Fig. 3). An increase in flower 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.
3.3. 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 significant (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: Significant 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 findings, 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 efficiently (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).
Table 3
Effects of N and P on marketable, total fruit yield, mean fruit weight and fruit length of tomato under Hawzen conditions
N:P (kgha− 1)
|
Mean fruit weight (g)
|
Fruit length (cm)
|
Marketable fruit yield (ton ha− 1)
|
Total fruit yield (ton ha− 1)
|
0:0
|
47.00h
|
3.67h
|
16.33h
|
17.87g
|
0:46
|
49.67gh
|
4.67efg
|
27.00fg
|
28.87ef
|
0:69
|
54.00gh
|
4.67efg
|
29.33def
|
31.00de
|
0:92
|
53.67fg
|
5.33cde
|
28.67defg
|
30.50e
|
69:0
|
49.67gh
|
4.00gh
|
19.00gh
|
20.37fg
|
69:46
|
54.33gh
|
5.00def
|
31.00def
|
32.50cde
|
69:69
|
58.33gh
|
5.33cde
|
32.33def
|
34.00cde
|
69:92
|
63.33bcd
|
6.00abc
|
38.33bcd
|
40.33bcd
|
138:0
|
51.00gh
|
4.33fgh
|
30.33def
|
31.83cde
|
138:46
|
60.00cde
|
5.00def
|
31.33def
|
32.83cde
|
138:69
|
65.00bc
|
5.33cde
|
42.67bc
|
45.17b
|
138:92
|
73.33a
|
6.67a
|
55.33a
|
57.00a
|
207:0
|
51.33gh
|
4.67efg
|
28.00efg
|
29.83ef
|
207:46
|
56.67b
|
5.67bcd
|
33.00cdef
|
34.33cde
|
207:69
|
67.67ab
|
6.33ab
|
46.00ab
|
48.00ab
|
207:92
|
65.00bc
|
5.33cde
|
38.00bcde
|
41.00bc
|
LSD
|
5.18
|
0.93
|
9.70
|
9.5
|
Means with different superscript letters in a column differ significantly |
3.4. Effects of N and P on fruit chemical composition
Total soluble solids (TSS): A significant 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). Other researchers also reported that soluble solids decreased with decreased N (Kuscu et al. 2014; Simonne et al. 2007). Conversely, Ronga et al. (2020) and Warner et al. (2004) reported that N had no effects on the TSS of tomato.
Table 4
Fruit total soluble solid (TSS), total sugar, titratable acidity (TA), protein, ascorbic acid, lycopene content, and P content of tomato fruits as influenced by different rates of P and N at Hawzen
Factors
|
Treatments
|
TSS (oBrix)
|
Total sugar
|
TA
|
Ascorbic acid
|
Lycopene content (mgg− 1)
|
N(mgg− 1)
|
P(mg/g− 1)
|
N (kg/ha)
|
|
|
|
|
|
|
|
|
0
|
5.26b
|
4.00b
|
0.37b
|
8.25c
|
0.05b
|
0.31b
|
7.5
|
69
|
6.40a
|
4.38ab
|
0.36b
|
9.33bc
|
0.06ab
|
0.33a
|
7.6
|
138
|
7.02a
|
4.63a
|
0.41ab
|
10.25ab
|
0.07a
|
0.34a
|
8.33
|
207
|
7.08a
|
4.85a
|
0.46a
|
11.58a
|
0.07a
|
0.34a
|
7.33
|
LSD
|
0.8
|
0.52
|
0.07
|
1.47
|
0.01
|
0.008
|
1.36
|
P (kg/ha)
|
|
|
|
|
|
|
|
|
0
|
4.91c
|
3.60b
|
0.34b
|
8.25c
|
0.005
|
0.3
|
5.75c
|
46
|
6.20b
|
4.00b
|
0.38b
|
9.33bc
|
0.061
|
0.33
|
7.41b
|
69
|
7.15a
|
5.00a
|
0.40b
|
10.25ab
|
0.072
|
0.33
|
8.17ab
|
92
|
7.50a
|
5.25a
|
0.48a
|
11.58a
|
0.070
|
0.34
|
9.42a
|
LSD
|
0.81
|
0.51
|
0.06
|
1.47
|
0.01
|
0.008
|
1.36
|
Total sugar content: The results demonstrated that the total sugar content of fruits was significantly (p < 0.05) affected by different rates of N and P. However, the interaction effects of N and P were not significant (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). Likewise, the application of 69 and 92 kg Pha− 1 showed higher total sugar compared to the control and 46 kg P ha− 1. Total soluble contents are an indicator of mineral nutrient concentration in fruit, and these values generally increase with fertilization (Kirimi et al., 2011). The sugar to TSS ratio reached the highest value at 50 kg ha− 1 N supply compared to below and above these levels (Ronga et al., 2020). Sainju et al. (2003) also reported that N deficiency reduced tomato taste. Wang et al. (2007) reported that sugar increased due to increasing N supply.
Ascorbic acid: N and P rates did significantly affect vitamin C in tomato fruit. Nevertheless, the interaction effects of N and P were nonsignificant 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 findings, 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): Significant variation in TA was shown due to N and P rates but not their interaction (Table 4). The control treatment had the lowest TA value compared to 138 kg ha− 1, but further, an increase in N supply beyond 138 kg ha− 1 did not increase the TA of the fruit. Other studies also reported that TA increased with increasing N supply (Wang et al., 2007; Kuscu et al., 2014) also reported increasing TA with increasing N rate, and Taiwo et al. (2007) reported that the control had the lowest TA. In the case of the P rate, an increasing trend was shown with the increasing rate but did
Lycopene Content: The application of N significantly (p < 0.05) increased the lycopene content of tomato fruits. An increasing trend occurred when N was added at the rate of 69 kg ha and became significant at 138 kg ha− 1 compared to the control (Table 4). Lycopene is an important trait in tomato, accounting for 90% of carotenoids (Dumas et al., 2003). Correspondingly, Simonne et al. (2007) also reported increased β-carotene with increasing N rate. Adequate nutrient availability is vital for the production and nutrient content of tomatoes (Sainju et al., 2003). However, different rates of P and the interaction between N and P were not significant (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 significantly (p < 0.05) influenced 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 classified 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 significantly (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 significant (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 finding, 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.