Changes in the pore‐space of a pasture topsoil under animal treading

Summary Changes in the Pore‐space of the top 1½ in. of Manawatu silt loam under pasture were followed in three successive winters. On each sampling date twelve hand‐carved cores were taken from randomly disposed sites in a paddock grazed by sheep. The content of pores drained at a suction of 50 cm of water was determined, and also the volume of water drawn from the saturated cores by this suction. Soil structure, as measured by these coefficients, fluctuated considerably within the space of a few months, the least drainage being found after periods of heavy grazing in wet weather. In one case the content of pores drained at 50 cm of suction fell from 10% to 3% of the soil volume as a result of grazing during the winter period. The volume of pores drained by this suction rose again following the onset of drier weather, but only after a time‐lag of 1–2 months in the cases studied. No relationship was found between these changes in pores drained by 50 ern of suction and the fluctuations of total pore‐volume which took place in the same paddocks. Soil cores containing the greatest weight of roots had the highest measured volumes of pores; on some occasions they also showed a tendency to drain more than cores containing fewer roots.