Distribution of Soil Fertility in Sheep Grazed Pastures Irrigated with Border Dyke System

Phosphorus (P) fertilisation and irrigation are important management practices to maintain the productivity of grazed pastures in New Zealand. The Winchmore research site was used to assess the three‐dimensional distribution of soil fertility as affected by long‐term P fertilisation and flood irrigation. Results showed that long‐term P fertilisation decreased soil pH, increased total N, Olsen P, sulphate sulphur, and organic sulphur concentrations compared to the control, but had no effect on total C in the 0–75 mm soil depth. In the 75–150 mm soil depth, total C was significantly lower while total N was significantly higher under P fertilised treatments compared to the control. Phosphorus fertilisation significantly increased Olsen P concentrations in the 75–150 mm soil depth compared to the control and accumulation below the root zone was observed, especially under high P rates. Sulphur, both in inorganic and organic form, was also accumulating to depth regardless of P rate and form. A clear lateral effect was found where soil pH, Olsen P, total C, total N, and extractable organic sulphur were significantly higher in the border edges compared to the strip, potentially making them critical source areas. Regardless of P treatment and soil depth, flood irrigation had no effect on longitudinal soil fertility distribution in this grazed pasture system.