Effect of irrigation on soil physical properties on temperate pastoral farms: A regional New Zealand study

Abstract

Context: Many regions in the world have undergone rapid land use change and intensification of agricultural land, such as through irrigation expansion, upgrading irrigation systems, and changing grassland, stock, and nutrient management practices. With more intensive land use, changes to soil properties can occur, such as soil compaction and changes in soil water storage. The effects of modern sprinkler-irrigated pastoral farming on soil physical properties are not well quantified internationally, particularly for temperate climates. Aims: This regional study evaluates the effect of irrigation on soil physical properties in topsoil and subsoil, under modern pastoral grazing and sprinkler irrigation, across Canterbury, New Zealand. Methods: Paired sites were sampled, consisting of a spray-irrigated paddock (field) and an adjoined part of the same paddock that was dryland (unirrigated), with other management the same for each pair. Key results: Under irrigation there was a shift towards a greater abundance of smaller pores. This was reflected in macroporosity and readily available water capacity being significantly lower under irrigation, while semi-available water capacity and unavailable water held below permanent wilting point both increased. Conclusions: These differences reflect increased compaction under irrigated grazed pasture, particularly under dairy grazing, consistent with findings in similar studies. This study quantified changes in both the topsoil and subsoil but showed that most differences were confined to the topsoil (30 cm depth). Implications: For irrigation management, our study indicates the lower readily available water capacity on irrigated pasture is significant, with farmers potentially having to irrigate more frequently. Adopting deficit irrigation could minimise impacts of compaction.