10 Sep 2015

New Zealand's rich diversity of soils

From Our Changing World, 9:34 pm on 10 September 2015

We have a very diverse topography, we have a very diverse climate, we have a very diverse geology and that all reflects in the soil.
Craig Ross, Landcare Research

For a small country, New Zealand has a rich diversity of soils, including some that are unique and rare, but only about 5 per cent are fertile and versatile enough to produce food without the need for any significant manipulation.

In this first of a series of soil science features to mark the International Year of Soils, I asked Landcare Research soil scientist Craig Ross to take me on a virtual tour of New Zealand soils, from north to south. In this video, he explores a pallic soil (named after the pale colours of the subsoil) in a pit dug on a Manawatu paddock that is being developed for housing. 

He says New Zealand’s soils reflect the diversity in the factors that produce them in the first place: rocks, climate and vegetation. The north of the North Island is dominated by old, oxidic soils that have formed as a result of weathering over long periods of time in volcanic ash or dark volcanic rock. The first grape vines in New Zealand were probably planted on these soils by an associate of Samuel Marsden at Kerikeri in 1817.

Granular soils cover much of the Waikato and south of Auckland. Built from volcanic parent materials such as strongly weathered tephras, often older than 50,000 years, these soils are among the most productive in New Zealand and have supported decades of horticultural production around Pukekohe.

At the other end of the productivity scale are podzols, that form on the west coasts of both islands in forested areas with high rainfall. Craig Ross says podzols leach nutrients to lower layers and their low fertility means that they need to manipulated if they are to be used in agriculture or forestry.

About 15 per cent of the North Island is covered in silica-rich pumice soils, which were long thought of as “undesirable” because of their low fertility and susceptibility to drought and erosion. But for Waikato University geoscientist David Lowe, who led a fieldtrip across the Taupo Volcanic Zone, pumice soils have gone from despised to highly desirable.

The classification of soils was first developed by categorising soils by the parent rocks that formed them, for example greywacke or volcanic rocks, the climate zones that weathered them, and the vegetation that provided the organic material.

More recently, soil taxonomy has incorporated other diagnostic criteria such as chemical properties, measurements of grain size, structure and the depth of the top soil. One of the last publications to be produced by the then Department of Scientific and Industrial Research (DSIR) was Alan Hewitt’s New Zealand Soil Classification in 1992, which is still used today.

Craig Ross says good, fertile soils are a fragile, precious and limited resource and soil scientists have long been concerned about urban encroachment on some of the New Zealand’s best soils.