Soil phosphorus saturation degree: Review of some indices and their suitability for P management in Quebec, Canada

Many agricultural fields contain excessive labile soil P in regard to crop needs. Its environmental fate must be assessed. The concept of P saturation degree is meaningful as it describes the portion of the soil binding sites already covered with P, and indicates the potential desorbability of soil P. The first objective of this study was to review different indices that have been proposed to estimate the degree of soil P saturation and the relationships between soil P saturation degree and P solubility. The second objective is to discuss their suitability as environmental indicators for P management in the province of Quebec, Canada. In the Netherlands, the P saturation index is defined as the ratio of P to Al + Fe contents extracted by ammonium oxalate [P-ox/(Al-ox + Fe-ox) or (P-ox/0.5(Al-ox + Fe-ox)]. This approach has been mainly used with non-calcareous soils. In Quebec, the ratio of Mehlich-III extractable P to Al (M3P/AlM3) is proposed as an alternative, which relies on routine laboratory test. However, the suitability of the M3P/AlM3 ratio has yet to be determined for some specific soil groups (e.g. gleyed soils, soils with Al-ox content >6 g kg(-1)) and for Subsoil horizons. Regardless of the chosen index, it is suggested that the best way to manage the risk of water contamination by P in Quebec (namely, defining critical levels of soil P saturation) may be to form homogeneous soil groups to account for their distinctive behaviour and characteristics.