Organic soilless cultivation techniques of small fruit watermelon

In recent years, some producers have successfully applied watermelon cultivation techniques to hydroponic systems. However, challenges such as poor root development and infections in the nutrient solution have often led to suboptimal plant growth. Since 1998, the author has experimented with organic soilless cultivation methods. After more than three years of trials, this approach has proven effective for growing premium, small-fruited watermelons. I. Cultivation Trough and Substrate (1) Cultivation Trough A simple brick-made trough is commonly used. Its length depends on the greenhouse size, typically no longer than 20 meters. The width is 90 cm, and the height is 18 cm, with a 90 cm walkway between them. The bottom of the trough is lined with a 0.1 mm thick polyethylene film to prevent contact with the ground. (2) Hydroponic Tank Utilization While hydroponic systems require precise management, especially in manure handling, organic cultivation can be directly implemented using hydroponic tanks. The substrate is filled into the tank, and a drip irrigation system is installed for efficient watering. (B) Substrate Composition The substrate consists of coconut coir and river sand mixed in a 1:1 ratio by volume. The sand should be 0.2–0.4 cm in size. If sea sand is used, it must be thoroughly rinsed with fresh water before use. For each cubic meter of substrate, 12 kg of commercial chicken manure organic fertilizer (with 20% organic matter, 8.8% nitrogen, 3.5% phosphorus, and 7.2% potassium) is added. Trace elements are also included, and the mixture is then placed into the cultivation tank. (3) Drip Irrigation System Two parallel drip irrigation belts are installed per trough. Each belt has a diameter of 15 mm, with outlets spaced 50 cm apart. A separate water supply system and pressurized pump are installed in each greenhouse to ensure even distribution. A filter is also recommended to prevent clogging. II. Watermelon Cultivation Techniques (a) Variety Selection Choose high-quality, small-fruited varieties with good taste, appearance, and high sugar content, such as "Golden Beauty" or "Black Beauty." These varieties have a short growth cycle and are ideal for premium production. (2) Seedling Sowing Arrangement Sowing times vary depending on local conditions. In the Pearl River Delta region of Guangdong, seeds are typically sown in January and September. (III) Cultivation and Management 1. Planting When seedlings have 3–4 true leaves, they can be transplanted. The substrate should be pre-moistened before planting. Two rows are planted per trough, with 70 cm between rows and 50 cm between plants. Each planting position should be close to the drip irrigation outlet to avoid root damage during the rooting process. 2. Fertilizer and Water Management Seven days after planting, apply 0.1 kg of urea and compound fertilizer per cubic meter. Twenty days later, add 4 kg of peanut bran per cubic meter (containing 6.32% nitrogen, 1.17% phosphorus, and 1.34% potassium). This organic material provides sufficient nutrients throughout the growth period. Additional quick-acting fertilizers can be applied if needed. Since commercial organic fertilizer and peanut bran contain essential micronutrients like calcium, magnesium, iron, manganese, copper, zinc, molybdenum, and boron, additional supplementation is unnecessary. If fruits grow large, apply 0.3 kg of potassium sulfate compound fertilizer per cubic meter. During disease prevention, spray 0.1% urea and 0.2% potassium dihydrogen phosphate every five days for three times. Water management should be adjusted based on plant growth and weather. When plants are young and temperatures are low, water every 2–3 days. During vigorous growth and high temperatures, especially when fruits are expanding, daily watering is necessary. However, water should be reduced before harvest to maintain fruit quality and sugar content. 3. Vine Training and Pruning When vines reach about 10 cm, use nylon or plastic ropes to support the main vine and guide its growth. For small watermelons, pollinate female flowers on the main vine between 16–25 nodes. Pruning is best done using a double-vine method. When the plant reaches 30 cm, leave one side vine besides the main vine and remove the rest. The main vine should bear one fruit, while the side vine serves as a vegetative branch. At the node stage, leave 4–5 leaves before pinching.

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