Impact of Fertilizer Sources and Water Supplies on the Growth and Yield of Watermelon Varieties in Oyo State, Nigeria

Background and Objective: The use of organic fertilizers is an agricultural practice that enhances soil fertility and plant quality. This study reported the influences of fertilizers on the yield of watermelons. Materials and Methods: Field experiments were conducted to assess the impacts of mineral fertilizers, organic fertilizer (Sawdust Compost, SDC) and their combinations on two varieties of watermelon during early and late raining season and under irrigation in Ogun-Osun River Basin Project Site Sepeteri, Oyo-State. Plants without mineral or organic fertilizers served as control treatments. All experiments were arranged in a Randomized Complete Block Design with three replications. Data collected, which included Crop Growth Rate (CGR), Relative Growth Rate (RGR), Net Assimilate Ratio (NAR), dry matter weight, fruit yield and plant nutrient uptake, were subjected to analysis of variance at a 5% level. Results: The results showed significant differences (p<0.05) for all the treatments tested when compared to the control. The highest value of CGR (39.86/plant) was obtained between 6-8 weeks with Kaolack variety fertilized with 13.2 t ha G 1 SDC during the late raining season. Kaolack fertilized with 13.2 t ha G 1 SDC during the early raining season produced the highest dry matter weight with the value of 88.81 g/plant. The highest mean fruit weight and fruit yield of 5.05 kg and 98.96 t ha G 1 respectively were recorded with Kaolack fertilized with 13.2 t ha G 1 SDC under irrigation. Conclusion: It is concluded that Kaolack watermelon variety fertilized at 13.2 t ha G 1 SDC fertilizer under irrigation is recommended for high fruit yields.


INTRODUCTION
Watermelon (Citrillus lanatus L.) is a member of the family Cucurbitaceae, kingdom, Plantae, order: cucurbitales, Genus: Citrullus and species: Lanatus 1,2 . It is the vine-like plant (Climber or Trailer) herb 3 . Watermelons are consumed as deserts, fresh-cut fruit and as juice 4 . It is an excellent source of vitamin A, C and vitamin E. It also provides a significant amount of vitamin B as well as, minerals such as Potassium (K), Magnesium (Mg), Iron (Fe), Manganese (Mn), Phosphorus (P), Sodium (Na) and Zinc (Zn) 5 . Cultural practices have been reported to affect fruit yield, quality and phytochemical contents of watermelon The sawdust compost used was preparedfrom sawdust biomass and well-cured poultry manure. The poultry manure and sawdust biomass were sorted of non-biodegradable materials. The heap method of composting was adopted. The materials were laid out in 3:1 sawdust to poultry manure on dry weight and composted in twelve weeks. The compost heap was watered and turned once a week for the first three weeks then fortnightly until maturity. At maturity, the compost was evacuated and air-dried and samples were taken for determiningtotal nitrogen (1.1 g kgG 1 ), available phosphorus (5.22 g kgG 1 ), exchangeable K, Na, Ca, Mg and Zn with the values of 28.1, 1.8, 1.95 and 0.12 g kgG 1 respectively.

Methodology:
The treatments consisted of two varieties of watermelon (Sugar baby and Kaolack) and twelve nutrient sources and rates were: T 1 = 0 kg N haG 1 (Control), T 2 = 333.3 kg haG 1 NPK 15-15-15, T 3 = 666.6 kg haG 1  , T 4 = 999.9 kg haG 1 NPK 15-15-15, T 5 = 1333.2 kg haG 1 NPK 15-15-15, T 6 = 4.4 t haG 1 sawdust compost (SDC), T 7 = 8.8 t haG 1 SDC, T 8 = 13.2 t haG 1 , SDC, T 9 = 17.6 t haG 1 SDC, T 10 = 333.3 kg haG 1 NPK+4.4 t haG 1 SDC, 500 kg haG 1 NPK+6.6 t haG 1 SDC, 666.6 kg haG 1 NPK+8.8 t haG 1 SDC. The factorial combination of these treatments gave 24 treatments combinations arranged in a split-plot with three replicates. Plots that received compost only or its combination were uniformly applied to the soil a week before planting while plots that received mineral fertilizer were applied two weeks after sowing. Plot size of 3×3 m (9 m 2 ) and contained 24 plants/plot and spaced out at 1×0.75 m. Weeding was done manually as required and insect pests were controlled using Cypermethrin and Lambda-cyhalothrin at the rate of 20 mL to 18 L of water with an application at 2 weeks' interval starting from 2 weeks after sowing.
Irrigation management was done using a Centre Pivot Irrigation System (CPIS) to supply water. The overhead sprinkler gross rate of water application was approximately 10 mm at 3 days intervals for the first 6 weeks after sowing and reduced to twice a week after fruiting at 15 psi. The overhead CPIS was operated at night when the wind was at a minimum.
Data were collected on leaf area, leaf area index, crop growth rate, relative growth rate, net assimilate ratio, dry matter weight, fruit yield and plant nutrient uptake. For the determination of plant nutrient uptake three fully expanded leaves (trust leaves) were selected per plant. Sampled leaves were oven-dried at 80°C for 72 hrs to the constant weight and grounded in a Willey mill to reduce the material to a fineness suitable size. The grounded samples were stored in airtight plastic containers for chemical analysis. Total nitrogen was determined by digesting 0.5 g dry leaf samples with 68% H 2 SO 4 in the Kjeldahl digestion unit until the sample was colourless and titrated with 0.1 N of H 2 SO 4 using selenium and sodium as a catalyst. Total N was determined from the digest by steam distillation with excess NaOH. The P, K, Fe, Cu and Zn plant tissue contents were determined by ashing 0.2 g of the plant samples in a muffle furnace at 600°C for 3 hrs. The ash was cooled and dissolved in 1 N hydrochloric acid and the solution passed through filter paper into a 50 mL volumetric flask and was made up to the mark with distilled water.
From the digest, P concentration was determined by the vanadomolybdate yellow colourimetry method using a spectrophotometer. The K was determined by using a flame photometer (Cornin Model 410 manufactured by Sherwood Scientific Ltd., Cambridge, United Kingdom) while micronutrients (Fe, Cu, Zn) were estimated with atomic absorption spectrophotometer (Perkin Elmer AAS-300 product of Agilent, Stevens Creek Blvd Santa Clara, CA 9505, United State).
Nutrient accumulation in the plant was evaluated using the method described by Ombo (1974) as: Nutrient uptake = Tissue nutrient content×sample dry weight (%) Statistical analysis: Data collected were subjected to analysis of variance for split-plot and mean were compared using least significant different at 5% probability levels.

RESULTS
The soil texture during both seasons and irrigation were loamy-sand The organic carbon was very low (<10). The total nitrogen is high (>0.2). Available phosphorus (P) was low (<10.0) and potassium was high (>0.3), sodium of early and late was of medium (0.1-0.3) while that of irrigation was high (>0.3). The calcium during the rain-fed and irrigation season was of medium (2)(3)(4)(5). The Mg of both seasons was of medium (0.3-1) while, that of irrigation is high (>1). The soil pH of the early and late season was slightly acidic (6.49 and 6.58), respectively while that of irrigationis neutral (7.01). The result of the physicochemical soil properties showed that the experimental area is high infertility ( Table 1).
The average temperature during the Irrigation cropping season (January-March) was 30.09°C which was higher than the early season (March-May) with a value of 27.33°C and lower than the late season (August-September) with the value of 31.44°C ( Table 2). The compost used for the experiment had a pH of 6.7 with nitrogen contents of 1.1 g kgG 1 . Phosphorus and Potassium contents were 5.22 and 28.1 g kgG 1 respectively (Table 3). Calcium, Magnesium and Zinc contents were 1.8, 1.95 and 0.12 g kgG 1 , respectively (Table 3).
Crop growth rate: The growth and yield parameters assessed were depended on the fertilizer sources and rate of application. There was a significant increment in the crop growth rate of the two varieties of watermelon assessed from 4-6 and 4-8 weeks after sowing during the three growing seasons (   1.1 P (g kgG 1 ) 5.22 K (g kgG 1 ) 28.1 Ca (g kgG 1 ) 1.8 Mg (g kgG 1 ) 1.95 Zn (mg kgG 1 ) 0.12 All measurement was on dry weight Table 4: Effect of fertilizers sources and rates on crop growth rate (CGR) (g/week) of watermelon varieties under rain-fed and irrigation at Sepeteri compared with Sugar baby without fertilizer with the least of 11.76 g/week (Table 4). During irrigation, Kaolack fertilized at 13.2 t haG 1 SDC produced the highest CGR of 35.86 g/week when compared to 16.52 g/week produced from Kaolack without fertilizer (Table 4).
Relative growth rate (RGR): Fertilizer sources and rates significantly affected the relative growth rate (RGR) of the two watermelon varieties during the three growing seasons (  Table 5: Effect of fertilizers sources and rates on relative growth rate (RGR) (g/g/week) of watermelon varieties under rain-fed and irrigation at Sepeteri

Dry matter weight and nitrogen uptake:
The data collected on dry matter weight and nitrogen plant uptake were significantly influenced (p<0.05) by fertilizer sources and rates in both seasons and under irrigation (  Table 8: Effect of fertilizers sources and rates on total dry matter weight (TDMW) and nitrogen uptake of watermelon varieties under rain-fed and irrigation at Sepeteri
Phosphorus and potassium uptake: For the P uptake, during the early season, Sugar babies fertilized with 13.2 t haG 1 SDC had the highest P uptake of 28.20 g kgG 1 while, Kaolack without fertilized gave the least of 8.22 g kgG 1 . During the late season, Kaolack fertilized with 17.6 t haG 1 SDC had the highest P uptake of 15.30 g kgG 1 while sugar baby without fertilizer gave the least of 6.00 g kgG 1 (Table 9). When irrigation was applied, Kaolack with the application of 13.2 t haG 1 SDC produced the highest P uptake of 7.68 g kgG 1 (Table 9).
For the K uptake, during the early season, Kaolack fertilized with 13.2 t haG 1 SDC had the highest K uptake of 21.90 g kgG 1 meanwhile during the late season, Kaolack fertilized with 1333.2 kg haG 1 NPK 15-15-15 had the highest K uptake of 26.90 g kgG 1 while during the irrigation, Kaolack with the application of 13.2 t haG 1 Sawdust Compost (SDC) produced the highest K uptake of 13.97 g kgG 1 . Sugar babies without fertilization had the least K uptake during the three growing seasons (Table 9).
During the early season, Kaolack with the application of 13.2 t haG 1 SDC significant (p<0.05) increase the fruit yield with the value of 53.74 t haG 1 when compared to the least of 15.96 t haG 1 which was obtained from sugar baby without fertilizer. In the late season, Kaolack fertilized with 13.2 t haG 1 SDC gave the highest fruit yield of 38.15 t haG 1 when compared to Sugar baby with the control that gave the least of 12.22 t haG 1 . When irrigation was applied, the yield of 98.96 t haG 1 was obtained from Kaolack with the application of 13.2 t haG 1 SDC which was % higher than (Table 10).

DISCUSSION
The result of the soil analysis of the three growing seasons showed they were sandy loam and acidic. The available nutrients in the soil were below the nutrient requirements of the watermelon. The relatively low amount of the major nutrients required by the crop alerts the need for augmentation to enhance optimal performance. Inadequate availability of essential nutrients usually limits optimum crop performance. The results of the experiments show the superiority of fertilized plants over non-fertilized plants. These results show that NPK 15-15-15, compost and their combinations significantly influenced the growth, dry matter production and fruit yield in the three growing seasons. During the growing seasons, crop growth rate,   net assimilate ratio and dry matter production nitrogen uptake and fruit yield was considerably enhanced by increasing fertilizers application. These parameters were obtained with the application of 13.2 t haG 1 Sawdust Compost (SDC). This indicated that high dry matter production at the high level of nutrientsfavoured the development of plant parameters which culminated in better production of dry matter. When nutrient is available in the right proportion, the photosynthetic activity of the plants will be considerably favoured and conversion of photosynthetic products to yield. This result is similar to the finds of Akanbi et al. 29 . They reported an increase in dry matter production of Okra at a higher nitrogen rate. Liu et al. 30 and Ouda and Mahadeen 31 stated that the combined application of organic and inorganic fertilizers results in the vigorous vegetative growth of plants 32 . Fertilized plants showed a significant response on fruit yield when compared with the unfertilized plant. This result shows that watermelon yield could be enhanced by the application of fertilizer. This result conforms with the result of previous studies 26,33,34 , who reported an increase in growth and yield component of watermelon in response to an increased level of fertilizer application.
The highest fruit yield at 13.2 t haG 1 SDC in the three growing seasons could be due to the availability of compost due to its slow release of nutrients and favourable nutrient mineralization of compost as a result of the influence of the mineral component on the organic content of the compost. A similar finding was reported by many scientists 29,[35][36][37] . This result could also be attributed to the enhancement of decomposition of the organic mineral and mineralization of nutrients especially N and P by the presence  23 whereby the yield of Amaranthus cruentus was affected with the application of organic and organomineral fertilizer 39 on watermelon.
This result confirms the result of the previous authors 16,40 , who reported the increase in melon and watermelon growth and yield with an increase in nitrogen application. A similar report was observed with Agba and Enga 40 , they reported an increase in growth and yield component of cucumber in response to an increased level of fertilizer application. The result of other study by Aguyoh et al. 28 , also shows that as fertilization application increases, the yield of watermelon also increases.
Varietal differences were obtained to growth development in the two varieties of watermelon used in this study. From the three growing seasons and most cases, the Kaolack variety had the highest values over Sugar baby varieties. This could be attributed to differences in its genetic constitution to suitability to the agro-ecological conditions. This result is similar to the findings of Enujeke 41 , who reported that the genetic constitution of crop varieties influence the growth characters. It is also in harmony with the findings of Iken and Amusa 42 that attributed the growth and yield differences among crop varieties to the right choice of the suitable agro-ecological zone. A similar observation was recorded by the previous studies 43,29,44 . They reported variability in plant genetic potential leading to differences in the observed performance. Planting Kaolack with the application of 13.2 t haG 1 sawdust compost under irrigation is considered as best agronomic practice towards achieving a high fruit yield.

CONCLUSION
Fertilizer sources and rates significantly influenced the growth and fruit yield of two watermelon varieties assessed during the three growing seasons. Kaolack variety produced better growth and fruit yield when compared to Sugar baby.It was observed that an increase in fertilizer resulted in better growth and fruit yield. Watermelon planted with irrigation produced better and highest fruit yield when compared to rainfed.

SIGNIFICANCE STATEMENT
This study established the appropriate combination of organic and inorganic fertilizers and time of planting on the growth and yield of watermelon cultivated in Oyo State, Southwestern Nigeria. In addition, the study laid to rest the controversy surrounding the optimum plaining condition for an enhanced yield of watermelon in the chosen area. This information is required to guide farmers who are involved in the production of watermelon in Southwestern Nigeria.