Collectively, Brown, Pallic, Podzol and Semi-arid soils cover more than 69% of New Zealand. They form in different climates, although rainfall has the most effect on which type develops.
Brown, Pallic and Semi-arid soils may form beneath a range of vegetation types, but Podzols are restricted mainly to native forest. All four types occur mainly on quartz-rich parent rock, commonly greywacke, schist and granite.
Brown soils cover 43% of New Zealand. They extend from the country’s mountainous and hilly backbone down to the moist lowlands, where summer droughts are uncommon and soils remain damp throughout the year (except in some sandy or very stony soils, which may experience drought). Annual rainfall is usually more than 800 millimetres (south) or 1,000 millimetres (north). The relatively wet climate causes leaching of nutrients, which are washed into drainage waters and eventually into streams and rivers. This makes the soils acid, with limited fertility. Fertilised Brown soils, such as those on the Southland plains, make good land for sheep, beef and dairy farming.
The wrong soil
When early European explorers saw New Zealand’s extensive conifer–broadleaf, beech and kauri forests, they concluded that the land was richly fertile. They gave glowing reports of the potential for agriculture. But in some areas, this was not to be. New Zealand’s native forests, unlike the northern hemisphere’s deciduous forests, can grow well on acidic and low-fertility soils.
Podzolised soils cover 13% of New Zealand. The name podzol comes from a Russian word for wood ash, describing the white subsoil – digging through the thin black topsoil exposes an almost white horizon (layer). Digging deeper reveals a bright ginger brown. These colours have been caused by strong acid leaching from native trees (especially conifers, beech and kauri) and high rainfall (more than 1,300 millimetres per year). The rain water carries acids from the trees, stripping aluminium and iron from the upper part of the soil and leaving it bleached white. It deposits the elements along with coloured organic matter, creating the ginger-brown layer. Sometimes this takes the form of an iron ‘pan’ or impervious layer in the subsoil. Podzols have low fertility and extreme acidity.
Pallic soils cover 12% of New Zealand. These soils are dry for part of the growing season, especially summer to mid-autumn. They form where annual rainfall is between 500 and 1,000 millimetres. They are called pallic because of the subsoils’ pale straw colour. New Zealand Pallic soils have hard, brittle fragipans (compact, deep subsoil layers), and are probably unique (one Russian soil scientist described them as ‘extremely peculiar’). These soils are formed in loess – wind-blown dust. In other countries, loess contains lime, but in New Zealand it does not. This is because most of the loess is formed from the lime-poor greywacke that makes up the main mountain range.
Most Pallic soils have limited uses (mostly sheep grazing) because the subsoil is dense. Roots cannot penetrate to moisture deep down, so the soil becomes even drier. Although dry in summer, the soil can be wet in winter or spring. Some younger Pallic soils are not so dense and have a range of agricultural uses.
Semi-arid soils, which cover 1% of New Zealand, are dry for most of the annual growing season. They are found in the inland basins of Otago and Canterbury, where the annual rainfall is less than 500 millimetres. There is too little rain to wash out the sodium, calcium and other elements released from the slow weathering of the parent material. Consequently, lime-rich and saline deposits accumulate in the subsoil. Irrigation gives the soil a good wash, flushing out the salts and transporting them into lower-lying areas. Nutrient levels are relatively high, but the soil must be irrigated to produce a crop.