Phosphorus fertilizer rate and timing of application for cabbage production in sandy soils

Cabbage (Brassica oleracea var. capitata) is an important vegetable crop grown in the sandy soils of Florida. The availability of phosphorus (P), an essential nutrient for cabbage growth and yield, can be limited by soil pH, aluminum (Al), iron (Fe), and calcium (Ca). This study assessed the response of cabbage plant P uptake and yield to different rates of P fertilizer applied at pre-planting (P_pp) and supplemented with liquid P fertilizer applied at pre-cupping (Sd₁) and cupping (Sd₂) stages. The field experiment was conducted during the fall of 2018 and 2019 with four P fertilizer rates (0, 25, 49, and 74 kg P ha⁻¹) applied at P_pp using triple phosphate (0-46-0). Two subsequent side-dress applications (Sd₁ and Sd₂) of liquid P (0 or 12 kg P ha⁻¹) were applied as phosphoric acid. Soil samples were collected before treatment application and after the Sd₂ to assess soil extractable P using Mehlich-1 and Mehlich-3 extractants. Whole plant samples were collected at harvest to measure dry biomass accumulation and estimate P uptake. During the growth stages leading up to head formation, cabbage leaf tissue did not show signs of P deficiency in either season, indicating that P was not limiting during the early stages. However, after head formation, leaves from plots that did not receive P fertilizer showed P values below the sufficiency range. Increasing P fertilizer rates at P_pp significantly enhanced cabbage dry biomass accumulation, yield, and P uptake. Furthermore, an in-season application of liquid P fertilizer increased overall cabbage total yield and P uptake. The results of the field experiments suggest that the current soil P interpretations based on Mehlich-3 extractant for Florida did not reflect the plant-available soil P, as they did not predict the observed response of total and marketable yield to added P fertilizer.