The general principle of watermelon fertilization is to apply ample organic fertilizer, appropriate amounts of nitrogen fertilizer, stable application of phosphorus fertilizer, and increased application of potassium fertilizer, using a balanced application of nitrogen, phosphorus, and potassium, and supplementing with appropriate amounts of micronutrient fertilizers. Specific fertilization methods are as follows:

* **Base fertilizer:** Base fertilizer is mainly organic fertilizer. Generally, apply 1-2 cubic meters/acre of fully decomposed chicken or cow manure, or 200-400 kg/acre of commercial organic fertilizer, 80-100 kg/acre of decomposed cake fertilizer, plus 25-35 kg/acre of balanced compound fertilizer. Make a top dressing trench about 10 cm deep, 10 cm wide, and 40 cm long between two watermelon seedlings, and tamp down the soil after application.

* **Seedling fertilizer:** From the cotyledon expansion stage to the five-leaf "clumping" stage, apply 5-10 kg/acre of urea, placing it in a trench 10 cm from the root ball and watering promptly.

* **Seedling fertilizer:** From the cotyledon expansion stage to the five-leaf "clumping" stage, apply 5-10 kg/acre of urea, placing it in a trench 10 cm from the root ball and watering immediately. Vine-extending fertilizer: From the initial vine formation to the opening of the first fruit-bearing female flower, apply 5-10 kg of superphosphate and 5-10 kg of potassium sulfate per mu (approximately 0.067 hectares).

Fruiting fertilizer: From the fruit's initial fuzz removal to its final size, apply 5-10 kg of urea and 5-10 kg of potassium sulfate per mu (approximately 0.067 hectares). Additionally, during the mid-to-late growth stages, foliar spraying with potassium dihydrogen phosphate can significantly increase the sugar content of watermelons. Generally, after fruit set, spray the leaves with a 0.3%-0.4% diluted potassium dihydrogen phosphate solution every 6-7 days, for 2-3 consecutive applications.

These are the basic methods for watermelon fertilization. It is important to note that watermelon fertilization should be rationally planned and adjusted based on the utilization rate of different fertilizers, the soil's nutrient status, and the watermelon's nutrient requirements. Different fertilizer amounts should be applied according to the specific conditions.