Phosphorus fertilizer form affects phosphorus loss to waterways: a paired catchment study

Two catchments were used to test the hypothesis that using reactive phosphate rock (RPR), a low water soluble fertilizer, instead of superphosphate, could decrease phosphorus (P) export in streams. The sheep-grazed catchments, each ca. 12 ha in size, received 20-25 kg P/ha either as RPR or superphosphate for 3 yr. Filterable (< 0.45 mu m) reactive P (FRP) and total P (TP) in stream samples were then compared against two subsequent years when both catchments received 20 kg P/ha as superphosphate. On average, during the 5-yr study, loads of FRP and TP in stream flow were low (110 g FRP/ha and 358 g TP/ha), typical of dry sheep-grazed hill country. However, FRP (58%) and TP (38%) in stream flow were significantly less when RPR was applied instead of superphosphate. Where appropriate climatic and soil conditions exist to maintain agronomic targets, RPR could be used to decrease P export in streams.