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 h ave been proposed to estimate the degree of soil P saturation and the relat ionships between soil P saturation degree and P solubility. The second obje ctive is to discuss their suitability as environmental indicators for P man agement in the province of Quebec, Canada. In the Netherlands, the P satura tion index is defined as the ratio of P to Al + Fe contents extracted by am monium oxalate [P-ox/(Al-ox + Fe-ox) or (P-ox/0.5(Al-ox + Fe-ox)]. This app roach 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 horizo ns. Regardless of the chosen index, it is suggested that the best way to ma nage the risk of water contamination by P in Quebec (namely, defining criti cal levels of soil P saturation) may be to form homogeneous soil groups to account for their distinctive behaviour and characteristics.