Effects of carrot growing on volcanic ash soils in the Ohakune area, New Zealand

Abstract

Effects of intensive carrot production on soils derived from young volcanic ash were determined at Ohakune, New Zealand. Erosion rates (derived from caesium-137) and key soil physical and chemical properties were determined in 3 fields with differing management history (6 and 16 years cropping) or topography (sloping and flat).Caesium-137 areal activity in cropped fields ranged from 90 to 2034 Bq/m2, compared with a reference value under long-term pasture of 602 Bq/m2. Mean areal activity was lower than the reference value in 2 sloping fields, but not in a flat field. Net erosion rates were low in both sloping fields (–16 and –5 t/ha.year), but within each field there was a wide range of erosion and deposition rates (–109 to +293 t/ha.year in Field 1 and –145 to +514 t/ha.year in Field 2). These very high rates imply total soil losses up to 238 mm and deposition up to 670 mm, consistent with observed elevation differences between the cropped fields and adjacent fields in long-term pasture and with topsoil depth variation from 145 to 1165 mm. Tillage erosion and deposition rates are high (up to c. 40 t/ha.year) but water erosion is the dominant mechanism of soil redistribution.Cropping has reduced organic matter and aggregate stability, increased bulk density in the lower part of the topsoil and subsoil, and increased aggregate size. Hydraulic conductivity was higher in the topsoil and lower in the subsoil under cropping than it was under pasture. However, it would not limit soil water movement as it was higher than typical rainfall intensities. Compacted wheel tracks were the primary control on runoff and erosion as they have low infiltration rates (4 mm/h) compared with carrot beds (853 mm/h).