Effect of development and cultivation on physical properties of peat soils in New Zealand

Abstract

The effect of development and cultivation on the physical properties of two peat soils formed from restiad/sedge and sphagnum peat and developed for agriculture in the Hauraki Plains, New Zealand is reported by comparing physical properties of the plough layer (0–15 cm) and the relatively unaffected underlying peat (15–40 cm). Drainage and cultivation reduced soil moisture content, increased bulk density and reduced organic matter content. Bulk density was correlated with organic matter content only for the plough layer and indicates that bulk density alone is not a good indicator of decomposition in weakly decomposed peat soils in this region. Moisture retention curves indicated that a large amount of the water in the soil could not be removed at high suctions (up to 65% of the water was retained at suction oof 1500 kPa) and may cause moisture stress for plants, particulary in the plough layer of sphagnum peat, despite the high water content of the soil. This has important implications for maintaining a water table which maintains an adequate available water content for plants in the unsaturated zone immediately above the water table. Hydraulic conductivity differed between peat types in both the plough layer and the underlying peat. The presence of a root layer at 35–40 cm in the restiad/sedge peat was considered to substantially influence the readings and has important implications for drainage design in the soil. A simple method for measuring shrinkage in peat soils which involves coating natural clods in a saran resin is presented. A shrinkage curve indicated that shrinkage in peat is directly related to the gravimetric water content and does not contain zones of residual or structural shrinkage which are often associated with mineral soils. It is suggested, therefore, that a simple linear model based upon normal shrinkage will successfully predict volume changes associated with drainage of peat soils for agriculture.