Fertilizer management for improving yield and quality of off-season tomatoes in high tunnel

Tomato is well known for its quality and nutritional value all over the world but untimely and imbalanced fertilizer application has a severe effect on yield & quality of tomato. To investigate the effect of specific timing of fertilizer application for off-season tomato (Sahel hybrid), a study was conducted at Nuclear Institute for Food and Agriculture (NIFA), Peshawar. Nitrogen, phosphorus and potassium (NPK) fertilizers were applied at the rate of 10-10-15 kg ha at 7, 14 and 21 days intervals, starting after establishment of crop (30 days after transplanting) till mid of June. The current study was carried out under randomized complete block (RCB) design having three replicates of four treatments. The maximum fruit yield (2.94 t/ 10 Marla tunnel) was recorded in the treatment receiving NPK @ 10-10-15 kg ha at 7 days interval that was significantly (P≤0.05) higher as compared to those fertilized at 14 days (1.75 t/ 10 Marla tunnel) and 21 days (0.83 t/ 10 Marla tunnel) intervals, while minimum (0.5 t/ 10 Marlas) was recorded in control. Maximum grade-A tomatoes having above 100 g weight per fruit (62%), density (1.30 g/cm), chlorophyll content / SPAD value (57.63), total acidity (0.44%), protein content (15.4%), total soluble solids (5.4 °Brix), N (1.25%), P (0.20%), K (1.45%) in fruit and improved shelf-life (9 days) were recorded with the NPK application at 7 days intervals. The concentration of vitamin C showed increasing trend along with decrease in N dose at all application times. This study suggested that application of timely and balanced fertilizer may be a good strategy to get higher and good quality yield of off-seasonally grown tomatoes in the tunnels.

tomato producer province was Baluchistan followed by Sindh, KP and Punjab [4]. Contrary to the recent past, nutrition is being considered as an integral part of food security [5]. Accordingly, all four dimensions of food security, viz. the accessibility, availability, stability and utilization of macro and micronutrients are deemed components of food security [6]. Subsequent to this; a focus on the role of small producers in the agriculture sector is an important element. Production of off-season vegetables under high tunnels is an expanding opportunity to enhance food production of the country. High tunnel farming techniques are used to produce off-season vegetables such as tomatoes, cucumbers, chilies, sweet peppers, egg plants and gourds. Similarly, high tunnels have a wide range of advantages like yield and quality enhancement, avoiding the adverse environmental conditions, early crop production aiming at achieving high market value, etc. [7]. Tomato, one of the vital and widely cultivated high tunnel vegetables, possesses a wide range of vital compounds including solid & insoluble solids (5-7%), water (about 90%), carotenoids, citric acid, phenols, flavonoids, vitamins and minerals [8]. It also contains remarkable amounts of lycopene, antioxidants, and carotene that play an important role in preventing cancer of many types [9]. However, the ultimate goal is to enhance quality production of tomatoes in high tunnels while improving fertilizer use efficiency and enhancing the socioeconomic status of subsistence-level growers and small land holders. Nutrient management plays a significant role in enhancing production whether it is applied in open fields or while growing off-season vegetables. Nitrogen, phosphorus and potassium application in the form of organic and inorganic sources at critical growth stages (flowering, fruit setting and fruit development) of the tomato has been reported to enhance their growth and development. For example, N requirement of tomatoes is moderate during foliage growth, until fruit development. P is very important for vigorous growth and fruit production. Likewise, K is needed for fruit development and enlargement [10]. However, USDA (United States Department of Agriculture) together with the Fertilizer Institute promotes a specific framework called 4R nutrient stewardship (4Rs) that aims at increasing productivity and profitability for growers in both highly intensive agricultural systems and subsistence-level growers. The first 2Rs (right source and right rate) are commonly followed during crop production in Pakistan. However, the other 2Rs (right time and right place) are reported to be rarely practiced by farming communities, which results in low nutrient use efficiency and low economic returns [3, 5]. The core objectives of the current study were to assess the suitable time interval (right time) for NPK application on yield and quality parameters of F1 hybrid "Sahel" tomatoes grown in high tunnel.

Materials and Methods
The study was planned in a high tunnel measuring 28 ft. × 120 ft. with 11 ft. high bend fitted with a polyethylene plastic and 6 ft. roll-up sides at Nuclear Institute for Food and Agriculture (NIFA), Peshawar. The experiment was laid out in Randomized Complete Block (RCB) design having three replicates per treatment. The treatments consisted of different time intervals for the application of NPK to the tomato crop. A nursery of the F1 hybrid tomato cultivar "Sahel" was raised in plastic tubes (6 cm x 4 cm). After thirty days of germination, the nursery was transferred to tunnel. The soil of experimental tunnel was silt loam, alkaline in nature and non-saline. The soil was marginal in terms of the contents of organic matter (OM), P, K and adequate with respect to Potassium (K2O) ( Table 1). The spacing between plants and rows were maintained at 1.5 ft. and 3.0 ft. respectively. After thirty days of germination, the nursery was transferred into the tunnel. Upon establishment of crop about one month after transplanting, NPK @ 10-10-15 kg/ha was applied at 7 days intervals (T1: high dose), 14 days intervals (T2: medium dose) and 21 days intervals (T3: low dose) during the whole season. All the three doses were compared with control (T0: no fertilizer). The experiment was properly maintained following standard cultural practices as commonly observed in high tunnel farming. Tomato fruit yield in each treatment was collected at each harvest and weighed in kilograms (kg). The total yield per treatment was calculated by adding the yields at all harvests for the same treatment. Fruit and straw samples were washed initially with tap water and then with distilled water. After washing, these samples were firstly airdried and then oven-dried at 70 o C till constant weight. All these samples were then subjected to grinding in a stainless steel grinder. Soil samples collected at depth of 0-30 cm from different places in the tunnel were mixed to make a composite sample. Stones and plant residues were removed from these samples and then the samples were dried in the laboratory. The soil samples were then grinded and sieved through 2 mm sieve. After labeling, the soil samples were stored in plastic containers. These soil samples were analyzed in the laboratory for determination of N, P, K and soil organic carbon. Particle size analysis was done using hydrometer method [11]. Soil nitrogen was determined by using Kjeldahl apparatus [12]. Soil phosphorus was assessed using spectrophotometer at 880 nm [13]. Soil potassium was determined by using flame photometer. Organic carbon content in the soil was determined through dichromate approach, using combustion at 160°C for 30 minutes [14]. Nitrogen in plant samples was determined using Kjeldahl apparatus, according to Gunning and Hibbard's method [12]. Total phosphorus in plant was determined by analyzing the samples through spectrometer at 400 nm [13] while potassium in plant samples was assessed by using flame photometer [13]. The protein and moisture contents of the tomato fruits were determined by standard protocols [15]. The K and Zn concentrations of were assessed by Atomic Absorption Spectrophotometer, P by UVvisible spectrophotometer and N by Kjeldahl apparatus [16]. The total phenolics contents and flavonoids were assessed by the Folin Ciocalteu method by [17]. Vitamin C contents were determined according to 2,6dichlorophenolindophenol method by [18]. The data collected were arranged and subjected to analysis using Statistix 8.1. Data were recorded as mean of three replications and the means of various treatments were compared using LSD (Least Significance Difference) test.

Results and Discussion Soil physico-chemical properties
The soil samples were collected before crop transplanting from two depths i.e. 0-15 cm & 15-30 cm. samples of same depth were mixed to form their composite samples and then were analyzed for soil physical & chemical parameters (Table 1). Results showed that the soil in experimental tunnel was silt loam and deficient in nitrogen, phosphorus and organic carbon. The organic matter (0.88%) in the upper depth (0-15 cm) was higher than the lower depth i.e. 15-30 cm (0.41%). The amount of organic C in upper depth (0-15 cm) was 0.52% which was found 0.24% in the lower depth (15-30 cm). Total nitrogen content in upper depth was found 0.05% as compared to 0.03% in the lower depth. Available phosphorus in the 0-15 cm depth was 7.5 µg P/g soil and decreased to 5.9 µg P/g soil in 15-30 cm depth. Maximum K (80 µg/g) was recorded in upper portion (0-15 cm) while minimum K (60 µg/g) was found in the lower portion (15-30 cm). Soil was alkaline in reaction (pH 7.9-8.0), non-saline (EC 0.24-0.25 dS/m), moderately calcareous in nature 15.0 to 14.5% lime.  Tomato fruit size/grading The data regarding the effect of NPK applied after each 7, 14 and 21 days intervals on fruit size (Grading) is presented in (Table 3). The maximum % age of A-Grade (100-140 g/fruit) was recorded in T1 (62%) by application of NPK after each 7 days interval followed by T2 (48%) and T3 (36%) after each 14 and 21 days intervals, respectively. This increase in weight may be attributed to more nutrients availability and increased water and nutrients uptake by the plant that ultimately may result in higher photosynthesis rate and in return more food accumulation in fruits. The lowest (25%) A-Grade was found in control (T0). It was examined the performance of eight tomato hybrids under plastic tunnel for early growth and various agronomic parameters [22]. He reported that the average single fruit weight of various tomato cultivars ranged between 103-202 g which is in line with our findings. Similar finding were obtained by [23,24] who reported that under plastic tunnel the no. of fruits/ plant, single fruit weight and total weight of fruits/ plant were in the ranges of 20-57, 53-167 g and 2.0-4.78 kg, respectively. Likewise, [25] evaluated tuff and sand soil-less substrates in comparison with conventional growing in soil for the growth of tomato in a non-circulating open culture. Tomato plant grown in soil or tuff gave higher weight of fruit which ranged between 120-163 g whereas substrate has no effect on weight of fruit, which was quite similar to our findings. Moreover, foliar fertilization significantly increased the tomato growth, yield as well as quality [26]. The average fruit weight (81.60 g) was high with foliar spray along with NPK application.  life [29]. The results reveal that both treatments T1 and T2 have maximum shelf life (9 days) followed by T3 (8 days) and T0 (7 days). This may be attributed to the application of potassium, which was not applied to the control treatment (T0). The findings were also supported by

NPK contents in tomato fruit
Effect of NPK fertilizer applied with different days' intervals on the NPK content in fruit of tomato is described in (

NPK contents in tomato straw
Effect of NPK fertilizer applied with different days' intervals on the NPK contents in straw of tomatoes is described in (Table 6). Maximum N was recorded in T1 (1.25%) followed by T2 (1.19%), T3 (1.05%) and T0 (0.80%). Similarly, the highest P content was found in T1 (0.20%) followed by T2 (0.18 %), T3 (0.17%) and T0 (0.10%). In case of K, the highest K content was observed in T1 (1.45%) as compared to T2 (1.35%), T3 (1.23%) and T0 (1.08%). The results of the finding revealed overall uptake of NPK by straw was higher when NPK fertilizer was applied at 7 days intervals as compared to other treatments. It was reported that nitrogen, phosphorus and potassium are required for better plant growth and fruit development, so their lower amounts decreased the yield of tomato plant [21]. Nutritional quality of tomato fruit TSS (total soluble solids), pH, total acidity, protein & vitamin C content are key quality parameters contributing towards the stability of tomatoes while processing and storage. The effect of NPK fertilizer applied with different days' intervals on the nutritional quality of tomatoes is described in (