Environmental conditions responsible for soil formation in New Zealand are varied. The main islands extend south over a distance of a thousand miles, from latitude 34° 24' S to 47° 17' S. Their climate ranges from oceanic to subcontinental, and, from subtropic to cool temperate on the lowlands to alpine on the mountains (maximum elevation of 12,389 ft). They are mostly humid (rainfall, 40–80 in. per annum); however, since the rain comes mostly from the west, the western side of the South Island is superhumid (80–300 in.). Parts of the east and west coasts of the North Island and the east coast of the South Island in the shadow of the mountains are subhumid (20–40 in.), and interior basins of the South Island are semi-arid (12–20 in.). Most of the humid areas were originally in forest or scrub and the drier areas were in tussock grassland, but high on the mountains subalpine scrub, herbfield, or fellfield prevailed.
The topography and geology of New Zealand are similarly varied. Half the country is steep, 20 per cent is moderately steep, and only 30 per cent is rolling or flat land . The largest area of flat land is the Canterbury Plains on the east coast of the South Island. Geologically, New Zealand has a continental structure and is formed predominantly from rocks of sedimentary origin, but in the northern half of the North Island both basaltic and andesitic rocks are important soil formers, and in the middle part of the North Island various ash showers of rhyolitic or andesitic composition blanket the terrain . South of the ash-shower region the impress of the Ice Age is reflected by glacial erosion and deposits of loess and boulders in the South Island, and by corresponding loess-like and stony sedimentary beds and possibly periglacial solifluction in the southern part of the North Island. The Ice Age is also reflected on the coasts and in the valleys by terraces associated with the changes of sea level that accompanied the accumulation of ice in polar regions.