The Vegetation of New Zealand
4. The Soils of the New Zealand Botanical Region
4. The Soils of the New Zealand Botanical Region.
In what follows merely general and guarded statements are made, based for the most part on the experience of agriculture and horticulture and on rapid field observations. Also the distribution of the plant communities supplies some information — not always reliable — as to the relative fertility of the soil on which they grow.page 51
The soils which occupy the widest areas are pumice, clays of various kinds, loess (in a wide sense), loams, sand and stony debris. Peat, including-raw humus, mica-schist soils, calcareous soils, soils derived from basic volcanic rocks and rocks of various kinds are common enough but of more local distribution. Certain soils are purely local, e. g., scoria, sulphur &c, soils near hotsprings &c, salt soils, magnesian soils and soil heavily manured by sea-birds.
Loess soils occur over wide areas in South Island. They have arisen from silt blown from the glacial river-beds; such accumulation and transport still goes on. Loess is frequently mixed with clay derived from the underlying rock or of glacial origin. Loess is an important ingredient of soils to the east of the Divide in South Island.
Pumice soils play a large part in the centre of North Island and the land adjacent. When pure, unweathered and perhaps mixed with scoria, they provide, even in a wet climate merely steppe or desert conditions. When weathered and mixed with humus, pumice soil is "fertile" enough as the farms of the Waikato, and, in part, those of Hawkes Bay bear witness.
Clay soils of various kinds are common in North, South and Stewart. Islands. The extremely abundant greywacke readily weathers into clay. The low hills and undulating ground of lowland Auckland, known as the "gumlands", are covered with a great depth of specially impervious clay deficient in humus which though variable in quality is generally extremely poor. This is especially so with the white clays locally termed "pipeclay". The Stewart Island clays are formed from granite. Glacier clay occurs on mountain slopes and river-valleys. The "fertility" of these clay soils is governed by the drainage-conditions and the percentage of humus. Frequently clay becomes hard and dry during a period of drought and it then offers a most unfavourable station for plant-life.
Mica-schist soils occupy a wide area in the North and South Otago Botanical Districts. They are particularly fertile and contain, when apparently dry, a considerable amount of available moisture.
Calcareous soils are a feature of the Wanganui "coastal" plain. They also occur locally throughout the Region and frequently extend over considerable areas, but the fact that a soil overlies limestone by no means proves that it is truly calcareous.
Sandy soils are frequent on and near the coast, and arise either as blown sand from the shore or from the disintegration of soft sandstone. They are also frequent on the gravel plains.
Alluvial loams form the bulk of the soil of lowland valleys in both Islands.
Humus soils are of widespread distribution. They occur at all altitudes and vary from a thin surface-layer to peat-deposits, 12 m. deep, as in the page 52Chatham and Lord Auckland Islands. The rain-forest climate is eminently favourable for the production of humus. The subantarctic and wet high mountain climates favour the formation of raw humus and peat.
Volcanic soils, pumice excepted, though of wide occurrence in many parts of the New Zealand region, are generally local and limited in extent. They are specially fertile and the distinction between the vegetation of volcanic and "gumland" soils in Auckland is striking. The other soils mentioned above need no special comment here, but some come into consideration — as do soils of all kinds — when dealing with the plant-communities.
The terms "fertile" and "fertility" have been used in this chapter, and are to be found here and there in other parts of this book. All the same, it is not a really valid ecological expression, for the idea of fertility is derived entirely from the soil in relation to crops, and to the plant in wild nature no soil is fertile or unfertile. In fact, the whole idea of epharmony is opposed to such a term.