Effect of treading by dairy cattle on topsoil physical conditions for six contrasting soil types in Waikato and Northland, New Zealand, with implications for monitoring

A study of the physical condition of soils under dairying in the Waikato and Northland regions was undertaken to determine the physical condition of the soil, possible changes from pugging damage, and the most appropriate measurements and sampling regimes for monitoring change. Sites were selected on widespread soil types (Allophanic and Gley Soils in the Waikato; Allophanic, Ultic, and Podzol Soils in Northland) and corresponded to never trodden, usual usage or conditions, and previously pugged (>18 months ago) pasture. Soil cores were collected at 50-mm depth increments for determination of bulk density, total porosity, saturated and unsaturated hydraulic conductivity, proportion of pores greater than 30 and 60 mum, and aggregate size class. The 0-100-mm depth was best for showing differences between treading regimes. Within this depth, hydraulic conductivity and aggregate size showed the greatest differences between regimes. All measurements were useful for showing differences in the Waikato data. However, for Northland, bulk density, total porosity, and proportion of pores were not always indicators of change. Approximately 20 cores were needed per regime to show differences. Soil properties on most soil types were still affected 18 months after a pugging event. Measurements selected for showing change varied depending on whether data were for geographic regions, a single region, or a particular soil type.