Effect of drip irrigation regimes and mineral fertilizer rates on watermelon yield in mulched fields

. Crop growth in arid and semiarid regions usually depends on irrigation, but inappropriate irrigation practice can result in water stress. Water needs of watermelon are the highest during the period of strong formation of vegetative and generative organs. Thus, 85-90% moisture is required from seeds for seed germination. Before the formation of fruit, soil moisture should be maintained at 70% of the total field moisture capacity of the soil, and at the time of fruit ripening at 60%. In this article, the results of experiments based on the development of optimal irrigation regime and fertilization rate for the cultivation of watermelon plants in mulched fields by drip irrigation method, differentiated soil moisture (60...70...60%, 65...75...65%, 70...80...70%, 75...85...75% SAM (SAM - Soil available moisture)), and fertilization rates (without fertilizer, N 100 P 80 K 40 , N 120 P 100 K 50 and N 140 P 120 K 60 kg/ha) have been written. Also, the effect of fertilization rates (N 100 P 80 K 40 , N 120 P 100 K 50 and N 140 P 120 K 60 kg/ha) on the quality indicators of watermelon fruit, the amount of dry matter, sugar, nitrates and "C" in the fruit has been noted.


Introduction
Watermelon (Citrullus lanatus [Thunb.] Matsum and Nakai; family: Cucurbitaceae) is the most commonly grown cucurbit in the world. Watermelon is now grown in 122 countries and around 1,200 cultivars are grown on all cultivated continents [1,2].
Crop growth in arid and semiarid regions usually depends on irrigation, but inappropriate irrigation practice can result in water stress. The reduction in plant growth and yield caused by water stress has been well documented [3,4].
Watermelon consumes water unevenly. Water needs of watermelon are the highest during the period of strong formation of vegetative and generative organs. Thus, 85 -90% moisture is required from seeds for seed germination [5]. Before the formation of fruit, soil moisture should be maintained at 70% of the total field moisture capacity of the soil, and at the time of fruit ripening at 60% [6].
Watermelon plants, provided with good moisture, grow with large leaves -each plant is 3.6 m 2 long and is able to absorb moisture not only from the soil surface, but also from deeper soil horizons through a well-developed root system. In connection with the moisture in the soil, the interval from the flowering of the watermelon to the formation of mass fruit is important. Irrigation at the beginning of blooming accelerates fruit ripening and increases fruit size after blooming [7]. When the crop formation period ends, water consumption decreases.
All field crops absorb nutrients from the soil at a relatively high level. Wat ermelon absorbs 15.3-34.0 kg of nitrogen, 5.3-11.0 kg of phosphorus, 23.6-41.8 kg of potassium to produce 100 s/ha of fruit, depending on the applied fertilizers [8]. The demand for phosphorus is the highest in the early stages of watermelon development. F ailure to meet the demand for phosphorus for a long time weakens the process of protein synthesis and organic mass accumulation in plants, which slows down the growth process. Potassium affects the metabolism, growth and development of plants, the formation of generative organs in it, and increases resistance to environmental stresses [9]. It regulates the ratio of male to female flowers and affects the location of fruit on the stem [10]. In addition, the use of potash fertilizers leads to early ripening of watermelon.
At a time when the climate is changing sharply and the water shortage is increasing, drip irrigation and the amount of mineral fertilizers needed for the plant, dissolved in water, will make it possible to get a high yield and ecologically cle an product from the watermelon plant. However, the norms of optimal watering and fertilizing of the watermelon plant in the method of drip irrigation have not been developed in the soil and climate conditions of the Republic of Uzbekistan.

Materials and methods
Experiments were carried out in 2021-2022 at the "Center for Innovative Developments and Consultancy in Agriculture" DUK pilot farm at the Tashkent State Agrarian University.
The fertilizing experiment was carried out by using mineral fertilizers according to the options, the fertilizing rates were as follows in the phenophases of the Experiments were conducted in four replications in a randomized block (reversible) method. Watermelon seeds were sown in pairs (2.50+0.70): 2x0.50 cm. We used white and black polyethylene films with a width of 140 cm and a thickness of 0.02 cm for mulching the watermelon fields. Polyethylene films of this size are the optimal size for mulching (2.50+0.70): 2x0.50 cm beds. This experiment was conducted on the "Shirin" variety and "Dolby F1" hybrid of the watermelon plant.
Soil temperature (soil thermometer), seed germination, number of seedlings, phenological observations and biometric measurements, irrigation method, duration and number of irrigations (by V.YE. Kabayev's zuldar method) [5], yield amount, leaves surface area, number of leaves, stem and root weight, chemical composition of watermelon fruit, economic efficiency were studied from the day after planting watermelon seeds.
Irrigation norms were determined based on the lack of moisture in the soil according to I.A. Kostyakova's formula [4]. The watering time based on soil moisture was determined once every seven days by the thermostat-gravity method. The calculation of irrigation water was carried out through the Chipoletti spillway (maximum limit 50 cm). Soil moisture was determined by the weight method. Soil samples were taken every 10 cm to a depth of 100 cm in triplicate [9].

Results and discussion
The results of field experiments and correlation analysis showed that the yield of watermelon fruit depends on the irrigation regimes and fertilization rates used during the growing season, as well as various agrotechnical (for example, mulching) measures.
When we analyzed the yield of the watermelon plant on average over the years of research on the experimental options, it became clear that the yield of the "Shirin" variety in 2021 under white polyethylene mulch within all options was from 16.3 to 31.9 t/ha and in the options grown under black mulch was 17.2 -33.2 t/ha, it was observed that these indicators were slightly higher in "Dolby F1" hybrid compared to "Shirin" variety. In 2021, the yield of the hybrid "Dolby F1" was 17.1 to 36.1 t/ha in all options under white polyethylene mulch and 18.3 to 37.7 t/ha in options under black mulch (Table 1). In 2021, the highest yield was achieved by keeping the pre-irrigation soil moisture at the level of 75...85...75% SAM together with the application of mineral fertilizers in the amount of N140P120K60 kg/ha. Considering the variety and hybrid, the highest yield was obtained from "Dolby F1" hybrid in the option N140P120K60 kg/ha + 75...85...75% SAM, the yield was equal to 37.7 t/ha. The highest yield of the "Shirin" variety (33.2 t/ha) was also obtained in this option (N140P120K60 kg/ha + 75...85...75% SAM).
The lowest yield was obtained by keeping the soil moisture limit before irrigation at 60...70...60% SAM without fertilizer. With such a background of mineral nutrition and 70...80...70% SAM regime, the yield of the "Shirin" variety under white mulch was 17.3 t/ha and under black mulch was 18.9 t/ha, and in the hybrid "Dolby F1" was respectively 18.1 -20.2 t/ha. In the option with soil moisture before irrigation of 75...85...75% SAM, the yield of the "Shirin" variety under white and black mulch was 17.9 -20.2 t/ha, and in the hybrid "Dolby F1" it was 18.8 21.7 t/ha.
We found out that the conditions of moisture supply and the rate of applied mineral fertilizers have a significant effect on the evaluation of the effect of mineral fertilizers on the quality indicators of watermelon fruits. Thus, in the fields mulched wit h white and black polyethylene films, the amount of dry matter in fruits in the N140P120K60 kg/ha option was 11.82±% in the "Shirin" variety, which is 0.64% higher compared to the N120P100K50 kg/ga option, 2.07% more compared to the control option. It was observed that the content of dry matter in the fruits of the hybrid "Dolby F1" grown in the option N140P120K60 kg/ha was higher than other fertilization rates, it was 1.14% higher than the control (Figure 1). One of the important indicators of fruit quality is the content of nitrates, the amount of which depends on the doses of fertilizers. According to the research carried out ov er the years, the average amount of nitrates in the control option was 10.9 mg/kg in the "Shirin" variety and 11.4 mg/kg in the "Dolby F1" hybrid. In the highest fertilizing options (N120P100K50, and N140P120K60 kg/ga), the amount of nitrate in the fruits of the "Shirin" variety was 39.8 and 49.5 mg/kg, and in the fruits of the hybrid "Dolby F1" it was respectively 40.5 and 50.8 mg/kg. It was observed that the amount of sugar and vitamin "C" was also different in the section of varietiy and hybrid according to options (Graph 1).
The results of the 2022 study showed that the distribution of indicators in this year was the same as in the previous year, but small changes were observed in productivity indicators. Also, when we analyzed the yield results, it was clear that the yield of watermelon variety and hybrid increased under white mulch and decreased under black mulch compared to the crop in 2021. The main reason for these indicators was the high amount of precipitation and relatively low air-soil temperature in 2022. Even so, it was observed that the yield of watermelon on black mulch was higher than the yield of watermelon varieties and hybrids grown on white mulch.

Conclusions
Analyzing the results of the research, we can conclude that in order to obtai n a high and ecologically clean product from watermelon varieties and hybrids, first of all, taking into account the amount of precipitation and the applied agrotechnical measures (for example, mulching), drip irrigation regimes and fertilization rates sho uld be set. Taking into account these factors, when we grew the hybrid of "Shirin" variety and "Dolby F1" watermelon plant, we managed to cultivate an abundant harvest suitable for consumption. Among our options, the optimal drip irrigation regime was 75...85...75% SAM and the best fertilization rate was N140P120K60 kg/ha. The most important aspect is that the amount of nitrites in the fruit of the watermelon plant grown under this fertilization rate was in the amount recommended for consumption.