The soils of the Gisborne Plain
Gisborne winegrowers are privileged with the best soil maps of any horticultural locality in New Zealand. Alan Pullar was a highly respected scientist working for the Soil Bureau of the DSIR in the 1950s and 1960s, and his maps at a scale of 1:15,800 were sufficiently detailed from his meticulous field surveys that they guide almost everyone buying or cultivating land on the Gisborne Plain. Grape growers, drivers of harvesters, real estate agents and growers of all crops swear by them. Dig a soil profile on one side of a line dividing two soil types on the map and then move 10 metres to the other side, dig another, and the profile will be different.
Alan Pullar’s soil maps of the Gisborne Plain identified subtle variations in the fertile soils of the flood plain. Winegrowers have preferred to establish vineyards on the Matawhero silt-loams and avoided heavier, less free-draining clay soils and areas with high water tables. Nevertheless, there are fine Gisborne vineyards on the Kaiti clay-loam soils around Patutahi.
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The flood plain of the Waipaoa River is made up of soils derived mainly from finer sediments than those of either Hawke’s Bay or Marlborough. Its headwaters are in the Ruakumara Range and include the notorious Tarndale slip near Mangatu. The Waipaoa’s suspended sediment load is much higher than that of most New Zealand rivers because it is eroding mainly fine-grained and soft sedimentary rocks, especially mudstones and siltstones. Until stopbanking and other flood-control measures reduced the frequency of floods in the mid-twentieth century, these fine sediments were the parent materials for many of the soils of the Gisborne Plain. But as the flood plain developed, the physical properties of the soils were highly variable spatially. In addition, before European settlement, much of the plain comprised wetlands, and
Gisborne and the Poverty Bay flats. The Manutuke, Matawhero, and Patutahi triangle.
In parts of these lowlands the water table is close to the surface. Pullar’s maps and soil descriptions provide an accurate guide to this detailed variability, and when combined with the experience of local landowners in growing many crops – especially different varieties of grapes – the qualities of different soils, including what varieties and clones of grapes performed well on them, gradually became a form of codified knowledge.
The soils of the Poverty Bay flats have two distinctive characteristics compared with many recent alluvial soils of New Zealand. They are slightly alkaline and some are low in organic matter. The alkalinity (low to medium pH – 5.8 to 6.2) originates from the calcareous bedrocks of the Waipaoa’s catchment; the low organic matter, from the youth of most of the soils. The Waipaoa group of soils, in particular, has both
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