The zonal soils are formed on normal sites from ordinary siliceous rocks and show clearly the impress of climate and vegetation. If the soils from unusual parent materials (such as volcanic ash) and those occupying special sites (such as steep slopes and hollows) are set aside, a simple pattern of zonal soils is revealed. It consists of the brown-grey earths of the semi-arid areas where the rainfall is less than about 20 in. a year; the yellow-grey earths of sub-humid areas where the rainfall is approximately 20–40 in. a year; the high country, central and southern, and northern yellow-brown earths of the humid regions where the rainfall is well distributed and is greater than approximately 40 in. a year; and the corresponding podzolised yellow-brown earths and podzols resulting from excessive leaching beneath an acid litter of decomposing vegetation.
|Table 2. Broad Groupings of Land Use in New Zealand|
|Area (Millions of Acres)|
|Occupied farm land|
|Tussock and other native grassland||13.0|
|Land in field crops, gardens, and orchards||1.4|
|Plantations of exotic trees||0.9|
|Land in fern, scrub, and second growth||5.7|
|Barren and unproductive land||1.9|
|Total other occupied farm land||12.6|
|total occupied farm land||44.0|
|Land in cities and boroughs||0.4|
|National parks, reserves, and domains||5.1|
|Other land, including waste land, such as mountains, bare rock, water surfaces, roads, etc.||7.1|
The brown-grey earths occupy the dry intermontane basins of Central Otago and the Mackenzie Plains, where rainfall is insufficient for plant requirements for most of the year (diag. 7A). They are generally rich in plant nutrients and are weakly acid to alkaline, with salty patches in places. Many of them are stony. Their chief need is water, but irrigation must be practised with care to avoid waterlogging or the spreading of salts. They produce fine wools and store sheep and, where irrigated, fat lambs; lucerne is grown and, in favoured spots, brassica and other seed crops and stone fruits.
The southern and central yellow-grey earths are the seasonally dry soils of Southern Otago, Canterbury, Wairarapa, Hawke's Bay, and Manawatu, where rainfall is inadequate for plant growth for a third to a half of the year (diag. 7B). They are moderately to weakly acid and the older soils have a thick, hard pan in the subsoil. The drier soils, formed under tussock, are used for mixed arable farming and fat lambs and the moister soils, mostly formed under forest, are used also for dairying. Pastures on these soils respond to phosphate fertilisers. Shallow and stony soils related to the yellow-grey earths cover much of Canterbury Plains: they are used for sheepfarming, including fat lambs; with irrigation they carry over four ewes per acre and grow cereal and pasture-seed crops.
The high country, southern and central, and northern yellow-brown earths are commonly referred to collectively as the “yellow-brown earths”, but, owing to their very different properties, they really comprise three groups, which are given distinctive technical names. With their podzolised counterparts, they occur in humid regions where the rainfall is sufficient on the average for plant growth for the greater part of the year (diag. 7C), and for iron compounds to decompose and stain the soils yellow.
The high country yellow-brown earths on the cold uplands east of the Southern Alps are developed under tussock at high elevations. They have yellow friable subsoils and are moderately acid and strongly leached. Where oversown with clovers they respond to molybdenised superphosphate and sulphur. They are used mostly for wool production and some store sheep. The southern and central yellow-brown earths of the cool and the mild districts, like Southland and Wellington, were formed under forest and have nutty subsoils. When sown to pasture with lime and superphosphate (in places molybdenised superphosphate) they are used for dairying and fat lambs on the rolling land and for wool production, store stock, and some fat lambs on the more hilly slopes. Cattle are also run to help control pasture growth and prevent its reversion to scrub and fern. The related podzolised yellow-brown earths and podzols are best developed in Southland. They are formed under rimu-kamahi forest and are very leached, with acid grey structureless topsoils and with thin iron pans in the subsoils in places. They are being brought under pasture for sheep and cattle grazing. Waterlogged counterparts of these soils, the gley podzols or “pakihi soils”, occur in Westland where the rainfall is 100 in. or so per year (diag. 7D). They are structureless and their chief problem is one of drainage to remove the excess water. They are not readily utilised after the native forest has been removed. The northern yellow-brown earths formed under mixed forest in the warm, moist climate of North Auckland are mostly heavy clays containing kaolin, which tends to absorb alumina and consequently to retain phosphate in a relatively unavailable form. The more fertile soils are moderately acid, but support good pastures when topdressed with lime and phosphate, in places molybdenised: the rolling land is used for dairying and fat lambs, the hills for sheep and cattle grazing. The less fertile soils support fair pastures when topdressed with lime and superphosphate: on the easier land many dairy and fat-lamb farms are situated, but pastures on the hill country are less easy to maintain. The northern podzolised yellow-brown earths and podzols have thin, grey, structureless topsoils overlying a grey, siliceous horizon and are strongly acid and very low in plant nutrients. Where formed under kauri forest they are known as “gum-lands”. In places, especially where the soils are sandy, farming should be approached with caution, because subsoil pans of humus and iron impede drainage; otherwise the soils can be brought to support good dairy pastures.