Influence of soil test phosphorus level and leaching volume on phosphorus leaching

Phosphorus (P) is an essential nutrient in agricultural production, yet its losses contribute to eutrophication in freshwater systems. To mitigate this, there is increasing interest in quantifying soluble P leaching losses and including them in P indices. The objectives of this research were (a) to evaluate water soluble P leaching loss from topsoils (0–15 cm) across a range of initial soil test P (STP) levels; (b) to determine the effects of soil type, physiochemical soil properties, and leaching volume on P leaching loss; and (c) to determine the predictability of P leaching losses from soil tests. Intact soil cores (3.81 cm in diameter, 0–15 cm deep) were collected from six agricultural fields in Minnesota and leached with deionized water offsite. Additional columns were collected to identify the influence of three leaching volumes on P leaching load. Phosphorus leaching loss was impacted by soil type, initial STP level, and the volume of the leaching event. Leachate P concentration increased with increasing STP level and remained consistent among leaching volumes. Concentrations were not diluted with increased leaching volumes. Consequently, leaching volume was the primary driver of total P load. Several linear and machine learning models were used to predict P leachate concentration. Phosphorus concentration was best predicted using a ridge regression model using two soil tests: the degree of phosphorus sorption and water extractable P (R² =.58, RMSE = 0.06). The model prediction of P leaching loss is promising, although further research on subsoil interactions is needed.