Story: Soils and regional land use
Page 1 – Overview
How land is used commercially depends on its topography, soil properties and the climate (mainly temperature and rainfall).
Almost half of New Zealand’s land is farmed commercially – over 40% of the total land area is pasture and arable cropping land, about 7% is exotic forest, and less than 2% is orchard or market garden. The remainder is indigenous forest and shrubland (33%), tussock grassland (14%), urban area (1%), alpine zone, low-lying wetland or coastal sand.
Topography and use
Flat land is used for growing crops, feeding young stock for sale, and most dairy farming. Most flat land in New Zealand is in the South Island, especially on the Canterbury and Southland plains, and in Central Otago.
Rolling and moderately steep slopes are used for sheep, beef and deer farming, and forestry. Most forestry is in the North Island. Hill country is farmed in all regions of the North Island, and in all except the West Coast of the South Island. Farmed high-country tussock grasslands extend mainly from Marlborough to Southland.
The mean annual temperature decreases from north to south – for example, in Auckland it is 15.3°C and in Dunedin it is 10.8°C. Temperature also decreases by about 0.5°C with every 100-metre increase in altitude.
Minimum winter temperatures affect the flowering and seeding patterns of some plants. If the temperature is less than 5°C, plant growth will stop. However, it is the number of frost-free days in a year that dictates where frost-sensitive plants can be grown.
Rainfall varies even more markedly across New Zealand – from more than 5,000 millimetres annually in Fiordland to less than 400 millimetres in Central Otago. Plant growth depends on available moisture in the soil, which is the difference between rainfall and evaporation in a given period. Moisture deficits occur when more water evaporates from the soil than is replaced by rain, usually in late spring and early autumn. The sum of these deficits annually across the country ranges from slight (1–20 millimetres) to high (200–300 millimetres). When soil moisture gets too low, plants stop growing, and in drought conditions they die.
Droughts are common on the east coasts of New Zealand, but not on the west. In all regions, the soil on steep slopes, especially north-facing aspects, is drier than soil on nearby flat land.
Soil fertility and growth
The natural fertility of soils varies around New Zealand, but fertilisers are spread on all soils where plants and animals are raised commercially. Fertilisers balance nutrients in the soil for plant and animal growth. Plant and animal tissue is made up of carbon, hydrogen and oxygen, plus other mineral elements. The main elements are nitrogen, phosphorus, potassium, sulfur, calcium, magnesium and sodium. Others needed in tiny amounts are the trace elements boron, iron, manganese, copper, zinc, molybdenum, chlorine, cobalt, iodine and selenium.
All New Zealand soils, particularly those in the North Island, are deficient in phosphorus, so phosphate fertiliser is applied to all pasture and crop land.
White clover is the most important and widespread legume in New Zealand pastures, and is the basis of the country’s relatively low-cost pastoral agricultural systems. Soil bacteria attached to legume plant roots convert or ‘fix’ nitrogen in the atmosphere to a form that can be used by other plants – including crops.
Nitrogen is added to supplement the natural process of nitrogen fixation – where pasture legumes convert nitrogen in the atmosphere to a form in the soil that can be used by other plants. The use of nitrogen fertiliser has increased because of the rapid expansion of dairy farming, especially in the South Island. Dairying is more intensive than sheep and beef farming, and the pasture needs to grow faster. Higher rates of other nutrients are also needed.
Plant growth is affected by soil acidity, which is measured on the pH scale. Soil with a pH of 4.8–5.2 is strongly acidic and likely to restrict plant growth, whereas a pH of 6.6–7.5 is neutral. The optimum pH for pasture growth is 5.8–6. If soil pH needs to be raised (made more alkaline), lime is applied. Lime is quarried and finely ground at several sites in New Zealand.