Determining the longer term decline in plant-available soil phosphorus from short-term measured values

Once phosphorus (P) applications stop, it is important to know how long it will take for plant-available P in soil to decline to a particular level for soils with a history of intensive management. The rate of decline in the absence of applied P can be expressed as a half-life, that is the time in years for it to halve relative to the lowest level measured in the soil. This knowledge is essential when planning long-term policies for managing the P status of soils. It is important to farmers who wish to optimize applications of P fertilizers and make better use of accumulated P residues to ensure that plant-available P does not fall below the critical level for the soil and crop because of the risk of losing yield and the inefficient use of other inputs, especially N fertilizer. Lower levels of plant-available P are also desirable for environmental and ecological reasons. Allowing plant-available P to decline to the critical level from unnecessarily high concentrations will reduce the risk from P on eroded soil causing adverse effects of eutrophication in surface water. Low levels of plant-available P are usually considered necessary for establishing species-rich grassland. A method for determining the longer term decline in plant-available soil P from short-term measured values is presented.