Phosphorus is an essential element for life, and its availability to plant or algae may constrain the ecosystem productivity. The geochemical cycle of P limits its bioavailability in soil and only a small fraction of applied P as fertilizers may be uptaken by plants. On the other hand, excessive P rates promote P enrichment of soil, increasing the risk of losses both by leaching and runoff with transfer from soil to surface and ground waters. This means that P may be seen as an "environmental problem" in rich countries where soils have received excessive P applications and as "agronomic problem" in poor countries where the agricultural productivity is usually limited by P availability.