Watermelon (Citrullus lanatus L.) has stood out among the main horticultural products in Brazil. In the past years, there has been an increase in the area planted with the crop, especially in the brazilian states from the north  and northeast. From 2001 to 2014, the area planted with watermelon in Rio Grande do Norte had a 310% increase, going from 1,655 to 5,133 hectares, while in the same period there was 13% reduction in  the  productivity (PAM, 2014). Along with the increase in the producers’ interests for growing watermelon, there is also growth in the use of watermelon hybrids, mainly due to its productive potential. However, productivity has declined in recent years (PAM, 2014), demonstrating that this is a characteristic of low heritability (Ferreira et al., 2003) and strong environmental influence, it becomes then necessary to review the management currently adopted for this crop. Among the several production factors, fertilization is one of the factors that most lack information in the watermelon crop. Many producers, despite using hybrids, use old information for crop fertilization (Trani et al., 1997; CFSEMG, 1999; Cavalcanti et al., 2008), based on cultivars of open pollination and smaller productive potentials and, consequently, demand of nutrients. Among the nutrients, we can highlight phosphorus (P), which, despite being absorbed in small amounts by the watermelon plant (Grangeiro and Cecílio Filho, 2004; Nogueira et al., 2014), is key to obtain high productivity and quality of watermelon fruits. P is a component of nucleic acids (e.g. DNA), phospholipids, enzymes and molecules such as ATP (where the plant stores metabolic energy); it is present in the structures of the cellular membrane and participates in the synthesis of proteins and vitamins. Phosphorus also has an important function in the energy transfer system inside the plant, taking part in processes such as photosynthesis and respiration (Hawkesford et al., 2012). The use of this nutrient in the appropriate dose and time, favors growth of the roots, flowering and fruit set, besides accelerating their ripening and improving  the  sugar  content  (Molina,  2006).  Most Brazilian soils (91%) have a low content (<9 mg dm^-3) or middle (9 to 12 mg dm^-3) P (Lopes, 1998; Anghinoni, 2004) then being considered phosphorus poor, owing to their source material and high determination of this element, causing low P content available, especially in soils where there is a predominance of minerals sesquioxides (Novais and Smyth, 1999; Raij, 2011), for which there is often shortage of element in plants resulting in low productivity (Leon et al., 2008; Maluki et al., 2015.). However, excess phosphorus fertilization can also be harmful to the plant, especially the antagonistic effect on zinc absorption (Souza et al., 2011a), as well as to the environment due to eutrophication of water sources (Bennett et al., 2001). The present paper aimed at assessing the effect of doses of phosphorus upon productivity, quality and profitability of the watermelon crop.

Site location and characterization

The experiment was done from August to October 2012, in Mossoró, RN, with geographic coordinates 5° 03’ 37” S and 37° 23’ 50” W and altitude of 72 meters. During the experiment period, the average temperature and the average relative humidity were 26.7°C and 56.8%, respectively, without rain. The soil of the experimental area was classified as Red Yellow Argisol (EMBRAPA, 2006), whose physico-chemical characteristics of the layer 0-20 cm are presented as in Table 1.

Treatments and experimental design

The experimental design was randomized complete blocks, with four replications. The treatments were arranged in a 6 × 2 factorial design, composed by six doses of phosphorus (0, 45, 90, 135, 180 and 225 kg ha^-1 of P₂O₅) and two watermelon (Citrullus lanatus) cultivars (‘Top Gun’ and ‘Olímpia’). Each experimental unit was composed by three rows of eight plants, with a space of 2.5 m between the rows and 0.8 m among the plants, the total area was 48 m², and the plants used were the eight plants from the central row from each parcel as useful area.

Field establishment

Soil preparation consisted of plowing and harrowing, followed by thrown distribution of limestone (Relative Neutralization Total Power of 95%), in total area, and incorporation with grid, in order to elevate base saturation to 70%. Irrigations by sprinklers were done for 20 days, using a 5 mm daily slide. After this period, the next step was furrowing, with approximately 10 cm deep, and fertilization of planting with triple superphosphate (43% P₂O₅), in the dose corresponding to each treatment. Right after that, the sites were raised and drip irrigation was employed, with one hose per site, with issuers with 0.40 m in space between each other and 1.5 L h^-1 of flow. The sites were covered with black plastic, and holes of approximately 4 cm in diameter were made in it, with a 0.80 m space between each other. The seedlings were produced in polyethylene trays with 200 cells, and transplanted when they presented one permanent leaf. After that, the plants from each site were covered with white polypropylene  tissue,  weighting  15 g m^-2, forming a tunnel in order to avoid the attack of leafminer (Liriomyza trifolli) and the whitefly (Bemisia tabaci), so the cover remains for 15 days. Topdressing was done daily through irrigation water, beginning right after transplanting, and the total following amounts were applied 98.63; 198.53; 32.88; 6.26; 2.04 and 7.8 kg ha^-1 of N, K₂O, Ca, Mg, B and Zn, respectively. The fertilizers used were urea, potassium nitrate, potassium chloride, calcium nitrate, magnesium sulfate, boric acid and zinc sulfate and 1.16 L ha^-1 of Root^® (amino acid based product used to improve rooting of seedlings). Irrigation was done daily according to the need of the crop, with a total slide in the cycle of 286.4 mm.

Harvesting and parameters evaluated

The harvests of fruits were done in the useful area of the experimental unit, at 56 and 63 days after transplantation, and an evaluation was made of:

 a) Total and commercial productivity (t ha^-1): total productivity corresponded to the sum of the masses of healthy fruits (from all classes), while for the commercial productivity (t ha^-1) fruits with mass ≥ 5.0 kg were considered;

 b) Number of fruits per plant, total and commercial (fruits plant^-1): obtained by dividing the number of total or commercial fruits by the number of plants;

 c) Mass of the fruits and commercial fruits (kg): corresponded to the result of the division of total or commercial productivity by the number of total or commercial fruits, in 1 hectare;

 d) Content of solid soluble (ºBrix): portions of the pulp were removed from close to the flower scar, central and close to the fruit peduncle, which were homogenized by extracting juice, and the readings were determined in a digital refractometer;

 e) Titratable acidity (g citric acid per 100 mL of juice): at 1 mL of the pulp juice, used to evaluate the content of soluble solids, 49 mL of distilled water and 2 drops of phenolphthalein 1% were added. Titration was done until the turning point with NaOH solution (0.1 N), previously standardized;

 f) Peel thickness (mm): determined with the employment of caliper rule in the median region of the fruit; Pulp firmness (N): obtained by the average of three readings in the pulp, one in the distal end, one in the center, and another close to the peduncle, with the help of a manual penetrometer with an 8 mm diameter door;

 g) Content of phosphorus on the leaf: was removed from the fifth leaf from the main branch in the early fruiting (35 days) (Trani and Raij, 1997) and P contents were determined by the method of phospho-molybdic complex in reducing environment, according to EMBRAPA (2006).

 h) Content of phosphorus in the soil after harvest: phosphorus content in the soil after harvest: after harvesting the fruit and eliminated the shoots of each plot were taken at random two samples of soil in the row and phosphorus extracted by Mehlich-1 (HCl 0.05 mol L^-1 H₂SO₄ + 0.0125 mol L^-1) in a soil extractor ratio 1:10 (5 cm³ of TFSA and 50 mL of extraction solution) and analyzed by flame photometry. Based on the adjusted equation for commercial productivity, an economic analysis of the experiment was carried out. Costs with fertilizers, increase in productivity, cost-effective factors and economic return were calculated. Increase in productivity was calculated by the difference between the productivity obtained by the dose used and the productivity of the treatment without phosphorus.  The cost-effective relation resulted in the division between the increase in production and the cost of the phosphatic fertilizer. The economic return was obtained by multiplying the value of the cost-effective relation by the price of 1 t of commercial watermelon (R$ 180,00) or R$ 0,18 kg^-1, which corresponded to the average price of sales of the fruits intended for the domestic market. To calculate the costs with fertilizers, the cost of a unit of P₂O₅ (R$ kg^-1) was used, and  a  bag  of  25 kg  of  triple superphosphate was acquired at the cost of R$ 50,95. The fixed cost, which corresponds to the sum of production and management costs, was not considered, because it is the same for all treatments.  The data was submitted to a variance analysis (F test), using the statistics software SAS (SAS System, version 9.0). The regression equations were obtained by the table curve (Jandel Scientific, 1991), except for the total and commercial productivity, where the SAEG 9.0 (SAEG, 2005) software was used, due to the specificity of the equations.

Content of phosphorus in the soil after harvest

The content of phosphorus (P) in the soil, assessed after the harvest of fruits was only influenced by the factor dose of phosphorus (p < 0.01) (Table 2), and as a higher dose was applied, the greater availability of P for watermelon was observed (Figure 1). Sandy textured soils have a better balance between content and P availability to plants, not being affected by the mounting process due to its lower content of clay, iron and aluminum oxides and calcium (Corrêa et al., 2011). 

The response of cultivars of Olimpia and Top Gun watermelons to the increase in provision of P to the plants was similar, with maximum commercial productivity reached with 49.37 kg ha^-1 of P₂O₅. The increase in the dose of phosphorus from 0 to 225 kg ha^-1 of P₂O₅, in Red Yellow Argisol with 6.4 mg dm^-3 of P, does not influence the quality of watermelon fruits. The maximum economic return for the crop of watermelon in Red Yellow Argisol with 6.4 mg dm^-3 of P is reached with the dose of 49.37 kg ha^-1 of P₂O₅. 

The authors have not declared any conflict of interests.