Transcript SOILS - HBHS12Geo

SOILS AND LAND USE IN NZ
Nature of Soil
The soil is a natural body embracing not only the topsoil, but also the subsoil
and other layers above its parent rock. It is a product of its environment – of the
rock waste which is its parent material, of the climate under which it weathers,
of the kind of topography upon which it is situated, of the vegetation and other
organisms with which it develops, and of the length of time during which it has
been developing.
From the standpoint of land use the soil is regarded as a medium of plant
growth, but it is not a passive medium. It is an active body comprising not only
mineral and organic constituents, but also the soil processes that are
continually going on, as well as soil temperature and moisture regimes that
keep these processes operating. The dynamic or active soil, as defined in this
way, correlates well with the kinds and distribution of soil organisms, with plant
growth, with land capability, and with land use.
Soil-forming Factors
Environmental conditions responsible for soil formation in New Zealand are
varied. The main islands extend south over a distance of a thousand miles, from
latitude 34° 24' S to 47° 17' S. Their climate ranges from oceanic to
subcontinental, and, from subtropic to cool temperate on the lowlands to alpine
on the mountains (maximum elevation of 4000m). They are mostly humid
(rainfall, 1000 – 2000mm per annum); however, since the rain comes mostly
from the west, the western side of the South Island is superhumid (2000 –
8000mm). Parts of the east and west coasts of the North Island and the east
coast of the South Island in the shadow of the mountains are subhumid (500 –
1000mm), and interior basins of the South Island are semi-arid (200 – 400mm).
Most of the humid areas were originally in forest or scrub and the drier areas
were in tussock grassland, but high on the mountains subalpine scrub,
herbfield, or fellfield prevailed.
The topography and geology of New Zealand are similarly varied. Half the country
is steep, 20 per cent is moderately steep, and only 30 per cent is rolling or flat
land . The largest area of flat land is the Canterbury Plains on the east coast of the
South Island. Geologically, New Zealand has a continental structure and is formed
predominantly from rocks of sedimentary origin, but in the northern half of the
North Island both basaltic and andesitic rocks are important soil formers, and in
the middle part of the North Island various ash showers of rhyolitic or andesitic
composition blanket the terrain. South of the ash-shower region the impress of
the Ice Age is reflected by glacial erosion and deposits of loess and boulders in
the South Island, and by corresponding loess-like and stony sedimentary beds
and possibly periglacial solifluction in the southern part of the North Island. The
Ice Age is also reflected on the coasts and in the valleys by terraces associated
with the changes of sea level that accompanied the accumulation of ice in polar
regions.
Zonal Soils
The zonal soils are formed on normal sites from ordinary silica-based rocks and
show clearly the influence of climate and vegetation. If the soils from unusual
parent materials (such as volcanic ash) and those occupying special sites (such
as steep slopes and hollows) are set aside, a simple pattern of zonal soils is
revealed. It consists of the brown-grey earths of the semi-arid areas where the
rainfall is less than about 500mm a year; the yellow-grey earths of sub-humid
areas where the rainfall is approximately 500 – 1000mm a year; the high country,
central and southern, and northern yellow-brown earths of the humid regions
where the rainfall is well distributed and is greater than approximately 1000mm a
year; and the corresponding podzolised yellow-brown earths and podzols
resulting from excessive leaching beneath an acid litter of decomposing
vegetation. The southern and central yellow-grey earths are the seasonally dry
soils of Southern Otago, Canterbury, Wairarapa, Hawke's Bay, and Manawatu,
where rainfall is inadequate for plant growth for a third to a half of the year. They
are moderately to weakly acid and the older soils have a thick, hard pan in the
subsoil. The drier soils, formed under tussock, are used for mixed arable
farming and fat lambs and the moister soils, mostly formed under forest, are
used also for dairying. Pastures on these soils respond to phosphate fertilisers.
Intrazonal and Azonal Soils
The intrazonal soils include the soils from less common parent materials and
those influenced by high ground water; and the azonal soils are youthful, owing
to recent renewal by sedimentation or erosion.
The rendzina and other calcareous (or lime-rich) soils, mostly from limestones,
occur both in the subhumid areas (such as near Oamaru, in North Canterbury,
and in Hawke's Bay), where they are associated with yellow-grey earths. They
are fertile, with deep, dark, granular topsoils. In the drier areas they are used for
intensive arable farming and sheep farming.
The yellow-brown pumice soils of central North Island are formed mostly on two
volcanic-ash showers that fell 800 and 1,700 years ago. Their topsoils are mostly
sands or sandy loams and the subsoils pumice sands and gravels. Because the
cattle and sheep on them became bush sick, they were difficult to farm for many
years and large areas were planted in exotic forests. The ailment, however, is now
overcome by topdressing with cobaltised superphosphate and, except where
droughty, the soils can be converted into good farm land. In addition to
phosphate, they need consolidation and, after continued use, potash topdressing.
The yellow-brown loams are derived mostly from fine-textured ashes erupted by
volcanoes in central North Island and by Mount Egmont and occur around the
margin of the pumice soils, which are younger. They are very friable, loamy soils,
respond well to superphosphate topdressing, and give slight to good responses
to lime; potash responses are expected to increase with continued use. The soils
are used mainly for dairying and fat-lamb production.
The recent soils from alluvium are formed from flood sediments on river flats.
For the most part they are fertile, deep loams, but some are gravelly with
excessive drainage. They are used mainly for fat lambs and dairying; also, in
drier areas, for cereals, pasture and vegetable seed crops, pulse crops for
canning, pip and stone fruits, small fruits, and tobacco.
The steepland soils occupy about half the area of New Zealand, being most
extensive on the axial ranges. Although showing characters related to the zone
where they occur, they are, for the most part, shallow and their subsoils vary
widely in fertility according to the underlying rock. Being relatively unstable they
are periodically renewed by erosion which, when the plant cover is disturbed by
man, becomes accelerated, with obvious effects, not only on the hillsides, but
also on the rivers and river