East Coast and Manawatū
Minimum winter temperatures are mild (3–6°C).The annual water deficit is generally low, but moderate on Manawatū’s sandy soils, the flats of Gisborne, and the flats and hill country of Hawke’s Bay and the Wairarapa.
Whereas most soils in the top half of the North Island are dominated by volcanic ash, large areas of the Hawke’s Bay, Wairarapa, Manawatū and Wellington regions are formed from mudstone, sandstone, greywacke or limestone. These soils are mainly silt loams, and in the east where there is less rainfall, and on flat or undulating land, there may be compacted subsoils. Erosion is noticeable on many steep hill-country slopes.
The lower North Island is split by the Tararua and Ruahine ranges. In the west, the land is dominated by moderate to steep hill country, which is used for sheep and beef farming. There are also dissected loess-covered ancient terraces and alluvial flood plains, used for dairying and cropping. In the east, forestry and sheep and beef farming occupy the highly erodible mudstones of Gisborne–East Cape as well as the drier siltstone hills and steeplands of Hawke’s Bay and the Wairarapa.
The Gimblett Gravels
The Gimblett Gravels is a large area of stony, shallow soil in central Hawke’s Bay. It has little organic matter and is free-draining, holding very little water. It produces little pasture because every summer it suffers drought. But the area has proven excellent for viticulture, and a range of deeper-rooting grape varieties, especially Merlot, are grown very successfully.
Orchards on the Heretaunga plains are on some of the better Hawke’s Bay alluvial soils. Grape vines have been planted on the stony soils of Hawke’s Bay and the Wairarapa, which previously supported only small numbers of sheep. The growth of viticulture and horticulture has increased the demand for irrigation water. Irrigation has made dairying possible away from the wetter fringes of the mountain ranges.
Much of the west coast of the Manawatū has sandy soils, deposited as the shoreline advanced and retreated. A large part of this area has been planted in exotic forest. On the more recent sands, nitrogen and phosphorus fertilisers may be needed for tree growth. The oldest, firmer sands have been cultivated, fertilised and used for dairying, and nearer Wellington, for vegetable growing.
Most erosion on North Island hill country is in the form of slips – the mass movement of topsoil down slopes. A slip may disturb fences or cover tracks, and so disrupt farm management and pasture production. Even when over-sown with grass seed and fertilised, slip areas virtually never recover their original productive capacity – lost topsoil takes decades to even partially redevelop.
All soils respond to phosphorus and sulfur fertiliser to varying extents, with the drier hills in Hawke’s Bay and the Wairarapa needing less than in higher rainfall zones or on flat land. Where soil pH levels are low, lime can be beneficial.