Phosphorus and nitrogen leaching from an organic and a mineral soil receiving single and split dairy slurry applications: a laboratory column experiment

Purpose Intensification of farming means that organic soils, of low phosphorus (P) holding capacity, are being brought into production. Consequently, farmers may have to adjust their fertiliser application regimes to reduce environmental risk. The aim of this paper is to test the hypothesis that overall loads of P and nitrogen are smaller when applications are split in two compared to a single application. Materials and methods A laboratory column experiment was conducted in which two soils, one organic and one mineral, were exposed to dairy slurry applications of 15, 30 and 55 kg P ha(-1) applied in one single application or split into two applications. The columns were uniformly irrigated weekly with 160 mL of distilled water (equivalent to average precipitation in Ireland) and the leached water was analysed for nutrients. Results and discussion There were no significant cumulative P loads in the leached water for any soil type or treatment (minimum and maximum values ranging from 0.04 to 0.12 kg dissolved reactive phosphorus (DRP) ha(-1) and 0.09 to 0.14 kg DRP ha(-1) for the organic and mineral soil, respectively, and 1.5 to 1.8 kg total phosphorus (TP) ha(-1) and 1.8 to 2.9 kg TP ha(-1) for the organic and mineral soil, respectively). There was high ammonium-N retention within the organic soil (0.2-0.4 kg ha(-1), compared to 15.0-36.8 ha(-1) for mineral soils in the leached water). Nitrate-N loads were higher from the organic soil (6.5-105.3 ha(-1)) than the mineral soil (9.7-17.4 ha(-1)), although for both soils, loads from the amended columns were lower than the controls (110.7 and 20.1 NO3-N ha(-1) for the organic and mineral soil, respectively). Conclusions The overall finding of this study was that split slurry applications had little effect on nutrient exports when compared to single applications, making the amounts of slurry applied, and not the application regime, the predominant factor in nutrient loss.