Optimizing Nitrogen Application Rates for Organic Bok Choy Production in Subtropical Sandy Soils

Organic systems rely on nitrogen (N) supplied through organic sources that must be mineralized before uptake. The timing and rate of N release are often difficult to synchronize with crop demand, especially for short-duration crops like bok choy. We evaluated the response of bok choy to N rates in Florida sandy soils using five N rates (0, 56, 112, 168, and 224 kg N ha−1) in a randomized complete block design. Yield, N uptake, apparent N recovery, N balances, and in-season crop indicators (SPAD, sap nitrate) were measured across three years. N applications significantly increased yield in 2023 and 2025 (with no significant differences in yield above 112 kg N ha−1), but not in 2024, highlighting the variability in crop response, likely due to environmental conditions. Higher N rates increased plant N accumulation but reduced N recovery and increased N surpluses, indicating a greater potential for N losses. SPAD and sap nitrate measurements were positively related to yield and could be used as rapid indicators of in-season crop N status, with SPAD having the advantage of being non-destructive. Overall, these findings help to optimize N management to better match crop demand in organic leafy green production systems, improving productivity while minimizing environmental costs.