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Phosphorus (P) is essential for plant growth, but excessive and continuous application of P fertilizers and animal manure has led to the accumulation of legacy phosphorus (legacy-P) in soils. While legacy-P presents a potential resource to support future agricultural demands, limited understanding of its chemical forms across different soil types hinders its sustainable management. This study aimed to characterize the chemical nature and storage potential of legacy-P in three contrasting soil types: acidic, organic, and calcareous. Key soil properties—including pH, organic matter, total P, Mehlich-3 P, and concentrations of aluminum (Al), calcium (Ca), iron (Fe), and magnesium (Mg)—were evaluated alongside P fractionation using a modified Hedley method. Furthermore, the soil P saturation ratio (PSR) was calculated to evaluate the relative saturation of soil sorption sites with P. Results showed considerable variability in Mehlich-3 P across soil types, ranging from 7–62% of total P in acidic soils, 1–8% in organic soils, and 3–47% in calcareous soils. Acidic soils were dominated by humic/fulvic-bound P (> 42%), organic soils by recalcitrant residual P (> 62%), and calcareous soils by Ca/Mg-bound P (> 69%), highlighting the influence of soil chemistry on P stability. Despite considerable variation among soil types in P fractionations, PSR results suggested that Ca/Mg-associated minerals play a greater role in retaining legacy P in organic and calcareous soils than Al- and Fe-associated minerals. In acidic soils, both mineral groups contributed similarly to P retention. 

To trace the phosphorus (P) and potassium (K) content in flooded rice (Oryza sativa L), 14 rice cultivars commonly grown in the Southern United States were evaluated for their P and K concentration in tissue and grain. Field experiments were conducted at two locations in Everglades Agriculture Area (EAA), where flooded rice was cultivated on organic Histosols. Soil pH and Mehlich-3 phosphorus (M3P) were significantly different between locations. At Site I, soil pH, M3P, and Mehlich-3 potassium (M3K) varied in the range of 6.8–7.1, 21.4–36.4 mg kg−1, and 53.9–151.0 mg kg−1, respectively. At Site II, soil pH, M3P and M3K varied in the range of 6.9–7.3, 11.2–20.5 mg kg−1, and 64.8–104.1 mg kg−1, respectively. Stem potassium was the only measured parameter that was significantly different among rice cultivars at both sites. At Site I and Site II, stem K ranged from 14.2–26.6 mg kg−1 and 10.4–19.4 mg kg−1, respectively. No significant difference in yield among cultivars was observed at Site I, whereas Site II had a significant difference in yield among cultivars. At Site I and Site II, yields ranged from 3745–7587 kg ha−1 and 2627–6406 kg ha−1, respectively. None of the cultivars ranked consistently in the same top and bottom position for each measured parameter. Total phosphorus (TP) concentration was highest in grain, whereas total potassium (TK) concentration was highest in the stem. Results suggest incorporation of rice stem into the soil could potentially add fertilizer back to the soil which helps in fertility management.