Determination of Substrate Mixing Ratios for the Hydroponic Cultivation of Watermelon

Watermelon (Citrullus lanatus), one of widely consumed fruit vegetables in the world, is primarily cultivated in soil, and the worsening issues of soil replant failure increasingly emphasize the necessity for hydroponic cultivation technology. However, the hydroponic application to watermelon cultivation has several challenges such as low fruit weight, cracking, blood-fresh watermelon, and low sweetness. This study aimed to determine the mixing ratio of the substrate (perlite and coir) for small and middle-sized watermelon hydroponic cultivation. Watermelon seedlings of two cultivars with three to four normal leaves were cultivated under an arch-shaped vertical hydroponic system in a glass greenhouse during the semi-forcing and retarding culture period. A total of six mixing ratios of substrates were designed using perlite and coir; perlite:coir = 10:0 (perlite), 8:2, 6:4, 4:6, 2:8, and 0:10 (coir). Fruit weight and sweetness of in the small and medium-sized watermelons were highest in the 4:6 mixing ratio during semi-forcing culture. However, the fruit weight and sweetness increase as the mixing ratio of perlite increased in retarding culture. The 10:0 mixing ratio exhibited the highest fruit weight among treatments, 51% higher than 0:10. In contrast, the fruit weight of small-sized watermelons tended to increase as the coir mixing ratio increased, and 0:10 exhibited the highest fruit weight during semi-forcing culture. However, 0:10 showed a significantly higher variation in the weight of two fruits fertilized per plant. Compared with the 10:0 substrate, fruit variation increased six times, intensifying the nutrient bias. Therefore, the yield of commodity fruits in the 10:0 treatment increased by 97% compared to 0:10. We concluded that perlite substrate effectively reduces root zone temperature during high-temperature cultivation of small and medium-sized watermelons, leading to increased yield by enhancing fruit growth and reducing nutrient imbalance. Further research is needed on various types of substrates and mixing ratios for year-round watermelon production in hydroponics.