Why shouldn't multi-basal fertilizer be applied in greenhouse watermelon cultivation?

Watermelons have specific nutrient requirements that are influenced by both the manure law and their growth characteristics. Throughout the entire growth cycle, watermelons absorb the most potassium, followed by nitrogen, with phosphorus being the least absorbed. According to research conducted by the Institute of Soil and Fertilizers at the Henan Academy of Agricultural Sciences between 1987 and 1988, for every 1,000 kilograms of watermelon produced, the plant absorbs approximately 2.46 kg of nitrogen, 0.9 kg of phosphorus pentoxide, and 3.02 kg of potassium oxide. The absorption ratio of these three nutrients is roughly 2.73:1:3.36. The uptake of these nutrients varies significantly across different growth stages. During the germination and seedling phase, the absorption of fertilizers is minimal, accounting for only about 0.51% of the total nitrogen, phosphorus, and potassium. As the plant grows, nutrient uptake gradually increases, reaching 18% of the total during the vine extension and assimilation period. Once the fruit begins to set, the demand for nutrients rises sharply. During this critical stage, the plant absorbs the most nutrients, with potassium making up the largest share, followed by nitrogen and phosphorus, which together account for about 81.49% of the total potassium absorption. The pattern of nutrient absorption also changes depending on the growth stage. Before the vines start to spread, nitrogen is the most absorbed, followed by potassium. However, after the fruit sets, potassium becomes the primary nutrient required, followed by nitrogen. This shift in nutrient preference reflects the changing priorities of the plant as it transitions from vegetative to reproductive growth. Watermelon roots are well-developed and highly efficient at absorbing both water and nutrients. However, if too much basal fertilizer is applied, it can lead to excessive early-stage nutrient availability. This often results in overly vigorous vegetative growth, thick foliage, and poor light penetration inside the canopy. These conditions can hinder the proper transition of the plant’s nutrient focus from leaves to fruit, making it difficult for the melon buds to develop properly. If this issue isn't addressed, the plant may continue to prioritize leaf growth, further delaying fruit formation. To avoid this, it's important to manage nutrient application carefully during the early stages, ensuring that vegetative growth doesn't become too intense. The amount of basal fertilizer should be moderate. Once the fruits begin to set, the plant's nutrient focus shifts toward the developing melons. At this point, additional top-dressing should be applied to boost photosynthetic efficiency, increase dry matter accumulation, and support the growth of large, high-quality watermelons. A recommended practice is to apply foliar sprays using a combination of 0.4% potassium dihydrogen phosphate and Tianda 2116 (a specialized fruit enhancer) every 5–7 days for 3–4 applications. This not only helps increase yield but also improves fruit sugar content and overall quality. By following this approach, growers can ensure a balanced nutrient supply throughout the growing season, leading to better yields and superior watermelon quality.

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