Abstract Liquid fertilizer is an essential material in agriculture, and its use tends to increase annually. Liquid fertilizer use also contributes to agricultural development; however, a white precipitate frequently forms with mixing the fertilizer and groundwater, and the effect of precipitate formation is unknown. Therefore, the purpose of this study was to clarify the characteristics of the precipitate and investigate whether it has any effect on Japanese spinach mustard ( Brassica rapa var. perviridis ). The liquid fertilizer used in this study is commercially available on Okinawa Island, and groundwater was collected from three springs. The collected samples were Ca‐rich groundwater (72.3–113 mg/L) due to geological characteristics. When the liquid fertilizer was diluted with groundwater, the largest amount of precipitate formed at a dilution of 1:10 (4.0–6.0 mg/10 mL). We identified the precipitate as brushite (CaHPO 4 ・2H 2 O) by X‐ray diffraction analysis. Based on this composition, it was suggested that brushite was formed by a combination of calcium from groundwater and phosphate from the liquid fertilizer. Comparing the measured values and the estimated values in the diluted liquid fertilizer, the calcium content decreased significantly (approximately 50%–90% reduction), whereas the phosphate content decreased slightly (approximately 5% reduction). The results showed that the formation of a precipitate reduces the calcium and phosphate content in the liquid fertilizer diluted with groundwater. However, this did not affect the growth of Japanese mustard spinach, likely because brushite is soluble.
Kinjo et al. (Sun,) studied this question.