4. Brown Soils

Brown Soils are New Zealand’s most extensive soil order covering about 43% of the country. Brown Soils occur on the moister hill and mountain lands throughout New Zealand except where tephra deposits predominate. Brown Soils also occur on flat to rolling lands in wetter regions such as the Southland lowlands. Most Brown Soils occur in areas with mean annual rainfall greater than about 800 mm in the South Island, and 1000 mm in the North Island. Sandy Brown Soils and stony Brown Soils may form in areas with lower rainfalls in materials with low (<75 mm) water holding capacity. Brown Soils have grey-brown topsoils, brown to yellowish brown subsoils, and profiles with A–B–C horizonation. They form from a wide range of parent materials, predominantly siliceous quartzo-feldspathic rocks (e.g. greywacke, mudstone, schist, rhyolite) and their derivatives (e.g. sand dunes, colluvium, alluvium, and loess). Brown Soils are moderately weathered with moderate to strong leaching. The distinctive brown colours are due to thin coatings of iron and aluminium hydrous oxides, or aluminosilicates, potentially including allophane and ferrihydrite, on particle surfaces. The Brown Soils underlie much of New Zealand’s sheep and beef production along with dairying, cropping, plantation forestry, and extensive areas of public conservation land.