Assessing the depletion of soil P following sequential extractions with Mehlich-3 and Olsen solutions

We analyzed in soils with contrasting cultivation histories the depletion of P following sequential extractions with soil testing solutions. Soil samples were collected in three experiments in eastern Canada (L'Acadie, Levis, and Normandin) and P was sequentially extracted 16 times, once daily, using Mehlich-3 (M3) or Olsen (Ol) solution. The cumulative amount of P extracted was 252mg P-M3 kg(-1) and 77mgkg(-1) P-Ol for L'Acadie, 212mg P-M3 kg(-1) and 66mg P-Ol kg(-1) for Levis, and 424mg P-M3 kg(-1) and 83mg P-Ol kg(-1) for Normandin. The depletion of P was described by a logarithmic function (Y=a ln (N)+b) for P-M3, and a power function (Y=N) for P-Ol. The inorganic P pool decreased in the three soils. The organic P pool did not decrease possibly because soil testing solutions did not directly extract P from this pool. This study demonstrated that laboratory soil testing analysis using M3 or Ol solution principally target P from the inorganic pool, suggesting that P fertilizer recommendations to mineral soils relying on these methods do not account for the potential of the organic P pool to contribute to soil P availability.