Migration of P from soils to water resources poses a risk of surface water eutrophication, and increase in P concentration in soils through manure or fertilizer addition would exace rbate this problem. Investigating the rate of increase in P concentration of surface soil receiving livestock manure is crucial to the development of best manure management strategies and prevention of eutrophication of aquatic systems. In this study, the changes in P concentrations of surface soils (0- to 15-cm depth) receiving 25 annual manure applications at rates of 0, 30, 60 and 90 Mg ha-1 yr-1 under non-irrigated conditions and at rates of 0, 60, 120 and 180 Mg ha-1 yr-1 under irrigated conditions were examined. The soil test P (STP) and total P (TP) of the surface soil increased with the TP through manure application over a 25-yr period. The STP pool was about 38% of the soil TP pool, similar to ratios of STP to TP in feedlot cattle manure. While the high proportion of STP to TP could be beneficial for crop production, it could also increase the potential for P losses from these soils through runoff and leaching. The changes in TP and STP concentrations of the surface soil were modelled with an expone ntial rise to maximum function: TP = 0.69 + 5.06 (1 − e(−0.087x)) and STP = 0.029 + 2.21 (1 − e(−0.082x)) where x is the cumulative TP applied. Although the model was developed for a specific soil and type of manure, it could be adapted to other soils or manure sources by adjusting the model coefficients for the particular soil and/or manure type. These adjustments would not require as extensive a data set as was required to develop the original model. This model could be used to determine the amount of TP that could be applied for a given critical STP. Producers, regulatory agencies, planners, and extension specialists could also use this model to make decisions on manure P management. Key words: Long-term cattle manure application, total phosphorus, available phosphorus, rate of accumulation, non-irrigated and irrigated cropping
