Extensive areas of land from the Coromandel Peninsula to Northland have escaped erosion or volcanic ash deposition for more than 50,000 years. Smaller areas occur in Wellington, Marlborough and Nelson. Although old for New Zealand, these soils are not old compared to the ancient soils of Australia, and are young compared to the rocks on which they are formed. Because these soils have been weathering for a long time they have a high clay content. They are acidic and of low fertility.
Ultic soils, covering 3% of New Zealand, are weathered from quartz-rich sedimentary rocks. These yellow clay soils, hard in summer and wet in winter, are well known to Auckland gardeners. Lime can correct acidity, and compost can balance the heavy clay content.
Granular soils and Oxidic soils
Granular soils, covering 1% of the country, and Oxidic soils, covering less than 1%, are weathered from ancient, dark volcanic rocks or ash. Iron oxide weathered from the volcanic parent materials causes a brown colouration. The term ‘granular’ refers to the soil structure. Despite the high clay content, stickiness when wet, and low natural fertility, large areas are being used for horticulture at Pukekohe, demonstrating the productivity of these soils when they are well managed.
Oxidic soils are the most strongly weathered of New Zealand’s soils. They contain iron and aluminium oxides. They retain nutrients poorly, but can be productive when well managed, as, for example, in the orchards of the Bay of Islands.