16. Ultic Soils

Ultic Soils are strongly weathered, clay-rich, soils that form on stable land surfaces, mainly on quartz- and feldspar-rich parent materials, predominantly in the northern half of the North Island. Scattered occurrences also occur on extremely old surfaces in other regions of New Zealand including Wellington, Marlborough/Nelson, Otago, and Southland. Some Ultic Soils developed on iron-rich volcanogenic, calcite-bearing sandstones of Tertiary age in the Auckland-Northland region have distinct relict reddening in lower subsoils. One subgroup of the Yellow Ultic Soils in the northern Waikato region is formed on halloysitic tephra deposits over a buried clay-rich paleosol. Ultic Soils generally have slow permeability and are prone to wetness in winter with attendant risks of compaction or pugging, and are droughty in summer. They are strongly leached and naturally acidic (pHs <5.5) with illuvial clay-rich subsoils and pale-coloured upper subsoils. They have low soil fertility. Thus substantial additions of lime and fertiliser are needed for productive use. Ultic Soils are best suited to pastoral agriculture and forestry. They are an option for urban development that avoids the more productive soils, provided the engineering design takes into account the potential for mud-sliding on the Tertiary sedimentary rocks (Waitemata Group) in the Auckland-Northland regions.