SOIL COMPLEXES ON TERRACES IN THE SOUTHWEST OF THE NORTH ISLAND OF NEW ZEALAND

Soil complexes are a common feature of the soil pattern on loess and loamy alluvium covered terraces in the southwest of the North Island of New Zealand. The complexes occur where rainfall is between 900 and 1250mm per annum, and where mean annual temperature is 11-13 degrees. Soil moisture regimes are ustic and udic.

Several soil complexes occur on interglacial marine terraces with multiple unit loess cover, separated by paleosols, and several metres thick, overlying beach, coastal or dune sands. Soils range from well drained Hapludands to poorly drained Fragi- and Endoaqualfs. Similar complexes occur on river aggradation terraces older than the last glacial with one to several units of loess cover.

Other soil complexes occur on last glacial river aggradation terraces where loamy alluvium 0.5 – several metres thick overlies gravels. Soils range from well drained Dystrudepts to poorly drained Endoaquepts.

The complexes occur on flat to gently undulating land without topographic control, and all members in the complex from well to poorly drained can be present within a radius of 50m. There appears to be no relationship between loess or alluvium thickness and soil class, nor a relationship with the topography of the underlying loess or gravel. However, the water content of the underlying unit may be important.

The well drained members of the soil complexes commonly contain allophane, while poorly drained members are often dominated by kandite clays. It appears that a template is established early in the weathering of the parent material that promotes certain pathways of clay mineral formation, and in turn diverse soil properties.

The presence of the soil complexes has important implications for intensive land uses, and pose a challenge for precision agriculture.