Solution 31P NMR Spectroscopy of Soil P Forms from Organic and Conventional Dairy Farms.

Soil biological activity, including a greater reliance on organic P mineralisation, in organically-managed systems has an increased role in crop P nutrition and it is not measured by the conventional soil P test (STP).  Organic P mineralisation depends on forms of soil organic P present.  The objective of this study was to compare forms of soil P using solution ³¹P Nuclear Magnetic Resonance Spectroscopy (NMR) to determine if there were any differences among organic and conventional forage fields. Forage fields on three long-term (>20 yr) organically-managed dairy farms with low STP and relatively high yields were selected and compared with adjacent conventional dairy farm fields with a long-term history of water-soluble P fertilizer application. Conventional systems were found to have greater relative and absolute amounts of orthophosphate in the NMR extracts, while organic systems had greater proportions of organic P (though differences in organic P were not significant, p= 0.0534).  After performing corrections for orthophosphate diester degradation during NMR sample preparation, proportions of orthophosphate diesters were found to be quite high for all samples (mean = 30%) and were correlated with legume content and declining soil test P levels.  The relative proportions of diesters were greater in soils from organically-managed systems (34% vs 26%), and  they made up a greater proportion of the soil P than what has been typically assumed. This new finding may in part explain why organically-managed legume-based forages have the potential to maintain sufficient plant tissue P contents and to yield well at notably low STP (< 10 mg kg^-1).