The need for a sustainable use of the limited resource phosphorus in agriculture is generally acknowledged though scarcity is no pressing topic. There is no doubt that phosphorus losses from agricultural soils to the environment are a major contributor to eutrophication of water bodies. There is also no doubt that the excessive application of farmyard manure in the vicinity of big livestock enterprises yields an undesired accumulation of P in soils, while on numerous arable farms phosphorus mining can be observed. The premise for a sustainable P use in agriculture is a balanced P fertilization where inputs equal outputs. This chapter summarizes the chemical behavior of fertilizer-derived phosphorus in soils, and it presents a novel approach to assess the fertilizer phosphorus utilization completely and reliably as a tool to quantify the genuine phosphorus demand of agricultural crops.