Soil Phosphorus Forms from Organic and Conventional Forage Fields

Recent studies exploring the soil P status of organic dairy farms in Canada report low concentrations of Olsen soil test P (STP), yet the impact on crop productivity is unclear. It has been suggested that soil biological activity, including organic P mineralization, may supply crops with P not measured by conventional soil P tests. Organic P (P-o) mineralization depends on the forms of soil Po present; thus, the objective of this study was to compare soil P forms in perennial forage fields of three paired organic and conventional dairy farms in southwestern Ontario using solution P-31 nuclear magnetic resonance (NMR) spectroscopy. Several methods to account for degradation of orthophosphate diesters during NMR analysis were also tested. Soils from conventional systems had greater proportions and concentrations of orthophosphate in NMR extracts than those from organic systems, while total Po was more abundant in organic systems. Proportions of diesters, thought to be readily mineralizable forms of P-o, were high in all soils; this was especially true after accounting for diester degradation. The relative percentage of diesters was greater in soil extracts from organically managed systems than those from conventional fields when a moderate amount of diester degradation was assumed (26 vs. 19%). These results may in part explain why some organically managed perennial forage crops can maintain acceptable yields at low STP concentrations.