Abstract
Decades of excessive phosphorus (P) fertilizer application in intensive agriculture have left significant residues of plant-unavailable soil P, commonly referred to as “legacy-P.” There is an urgent need to utilize legacy-P in agriculture, both because fossil P reserves are dwindling and because P leakage from agricultural systems into the environment already exceeds planetary boundaries. In this opinion paper, we advocate against acidification as a “one-size-fits-all” solution for releasing legacy-P, because legacy-P can be bound in various chemical forms that respond differently to this strategy. To guide future plant physiologists and soil microbiologists in developing site-specific solutions, we collate practical methodologies to distinguish between soil systems dominated by adsorption–desorption compared to precipitation-dissolution processes, and propose potential strategies to promote P release in each system. We also aim to rekindle interest in mechanistic studies of P in soils, paying tribute to Jim N. Barrow and Uzi Kafkafi—two key figures in advancing soil P chemistry who passed away in 2024. We argue that the phenomenon of P adsorption–desorption hysteresis, which they both explored, remains enigmatic, with practical implications for how we study and model legacy-P behavior today. Furthermore, we highlight that knowledge gaps remain even regarding the role of soil pH in P dynamics, and outline a path forward, including practical directions to address the open questions. Above all, we wish to emphasize that advancing the sustainable management of legacy P will benefit from interdisciplinary collaboration between soil chemists, microbiologists, and plant physiologists, drawing on insights gained through decades of rigorous research.