Nutrient Leaching in Undisturbed Cores of an Acidic Sandy Podosol Following Simultaneous Potassium Chloride and Di-Ammonium Phosphate Application

In south-east Queensland, Australia, extensive areas of sandy soils (Podosols) with shallow (<1 m) watertables are used for exotic pine tree production. Despite concerns that surface-applied fertilisers (di-ammonium phosphate (DAP) and potassium chloride (KCl)) may be contributing to a decline in local groundwater quality, published information on nutrient leaching in these Podosols is scarce. Large (0.3 m i.d. ×0.85 m long) undisturbed soil cores were intermittently leached with deionised water following a single surface application of KCl in combination with DAP. Potassium was applied at rates (equivalent on a surface area basis) of 0 (K0), 50 (K50), 100 (K100) and 300 (K300) kg K+/ha, and DAP was applied at a rate equivalent to 50 kg P ha−1. Applied ions appeared in the leachate very quickly after surface application, and reactive ions leached at the same rate as non-reactive ions. This behaviour was attributed to preferential soil–water flow, and limited ion sorption. About 30, 35, 100 and 25 percent of the applied K+, phosphorus (P), chloride (Cl−) and ammonium (NH4+) was leached from the soil cores. Cation exchange was the major mechanism responsible for K+ and NH4+ retention, although nitrification may have also contributed to NH4+ losses. Findings indicate that significant amounts of surface-applied fertiliser ions can potentially be rapidly leached below the tree root-zone, and into the underlying groundwater. Immobile water regions were estimated to comprise nearly 50% of the soil–water. Intermittent leaching resulted in secondary concentration peaks along the trailing edge of the ion breakthrough curves (BTCs). This was attributed to diffusion of solute from immobile water regions into mobile water regions during periods of no-flow.