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Bush sickness & the
discovery of trace element deficiences
Material for the sections on Soils and Radiata pines was provided by David J.
Lowe. Chris
Hendy wrote the article on bush sickness.
David
J. Lowe explaining a Horotiu soil profile, near Cambridge in the
Waikato.
Image courtesy of David J. Lowe.
Why
are soils important?
Soils are important because they:
- have a significant role in nutrient cycles,
releasing and storing nutrients;
- are the basis for agricultural production of
food and fibres;
- store water and regulate water supplies;
- regulate emissions of trace gases;
- degrade pollutants (purifying or detoxifying
them);
- produce most of our clays;
- act as a 'museum', storing information about
ancient
environments (e.g. types of pollen), including information covering
much of human history;
- provide a foundation for buildings and other
structures;
- can be used in environmental and criminal soil
forensics;
- lock away carbon (potentially lessening the
effects of climate change).
Globally,
soils (up to 1m depth) contain more than twice the amount of carbon
found in the atmosphere.
Soils contain the equivalent of about 300 times the amount of carbon
now released annually through burning of fossil fuels.
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What
is soil?
The word "soil" comes
from a Latin word (solum)
meaning ground,
although today it can have a number of different meanings e.g. the
natural medium for land plants to grow in. In that sense the thickness
or depth of soil is determined by the rooting depth of the plants
growing in it. Some definitions require soil to contain living things.
University of Waikato professor David Lowe
says that soil can be defined as:
- the natural, three-dimensional body (a soil
profile or pedon), about one metre thick, covering the land surface and
that can support rooted plants, and
which
- has
one or more soil
horizons (layers) that have evolved over time through additions,
losses, transfers and transformations of energy and matter due to
climate, living things, topography, and the original rocks, and which
- is
made up
of solids (inorganic and organic materials), liquids and gases, and which
- is
the most
complex ecosystem on Earth, and
which
- is
essential
to life through recycling of nutrients, carbon, and oxygen, and which
- is
non-renewable.
(Soil is often called 'dirt', a word which is frequently used in a
derogatory way. 'Dirt' comes from an Old Norse word (drit) meaning
faeces.)
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How
deep is soil?
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The
answer to this depends on the history of the soil and how 'evolved' it
is. ('Evolved' describes how well-developed the soil is, how thick it
is, and how many distinct layers - or horizons - there are.)
In other words, you can
find both very
shallow and very deep soils - up to several metres deep. What's more,
it's relatively common - especially in the Waikato - to find a whole
series of soils stacked upon top of one another, in what are called
'layered landscapes' e.g. a layer of soil originally formed in tephra
and was subsequently covered by another layer of volcanic ash, after
which yet another layer of soil formed from that second ash fall.
A series of buried soils and
volcanic ash layers, in a roadside cutting
near Mount Tarawera. Image courtesy of David Lowe.
When
soil scientists are comparing soil types, they usually look at the
topmost metre. But it's still hard to define where the soil stops. This
is because soils (and the layers underneath them) are altered over time
by the inteacting effects of climate, topography (e.g. whether the land
is steep or flat), and living organisms. However, the bottom of any
soil profile grades into hard rock or other unconsolidated material,
lacking animals, roots, or other signs of life. (This excludes bacteria, which
have been found growing in rocks more than a kilometre
below the surface.) And so this lack of biological activity (although
it's often hard to see) is often used to define the point at which a
soil stops and its substrate begins.
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What do you want to use it for? "Good" soils
are regarded as versatile or high-class soils.
High class soils can be
used
intensively, supporting a wide range of crops and other plants.
Typically they are deep, loamy, have good drainage, and are relatively
flat. These "good" soils support high crop yields. They can also
withstand intensive cropping, and have the capacity to absorb high
pollution loads.
Such versatile soils are
a limited and
finite resource - they are not in endless supply. Any form of
development reduces the stock of versatile soils, and they are
irreversibly lost if given over to urban development.
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New Zealand has a lot of
fertile soils - true or false?
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False - only
about 5% of
New Zealand's land area comprises "high class' or versatile
soils. The idea that our soils are particularly fertile can be traced
back to early European visitors like Joseph Banks, who wrongly assumed
that dense forest cover could be equated with rich soils (in
fact, it's the opposite). Because New Zealand has relatively high
rainfall, our soils lose a lot of nutrients through leaching. The
forest
cover reduces this effect, but once the trees are gone nutrients are
quickly lost from the soil.
It's true that some soils are very productive - e.g. soils on
well-drained volcanic ash - because of their physical properties.
However, this has to be supported by regular fertiliser applications to
supply nutrients, phosphates, and potassium, plus some trace elements.
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But what about the Waikato - we
have rich, fertile soils, don't we?
In
general this is false, although a good proportion (about 13%) of soils
in the
region are high-class soils, mainly the Horotiu soils and the
equivalent Waihou soils in and around Matamata. And the area between
Hamilton & Cambridge has soils that have a high potential
food-producing value - because of this, most subdivision in the area
was prohibited until 1991, when the Resource Management Act came into
being.
Horotiu soil profile. Image courtesy of David J. Lowe. |
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The Horotiu soil shown here is made up of a top part formed from a
series of thin tephra (volcanic ash) layers over a lower layer of
alluvial material laid down about 18,000 years ago by the young Waikato
River. The ash layers have weathered to form a special clay
mineral called allophane, which forms very fine aggregates that allow
free drainage but also retains moisture and is easy for roots to
exploit.
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How
many different soils are there in New Zealand?
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The answer
depends on how
soil types are defined, but using the soil series definition given
below, soil scientists agree that there are about 1800 soil series in
New Zealand (each soil series can include up to 6 soil types). Compare
this to around 300,000 named soil series around the world.
A soil series
is:
- a grouping of soils with similar profile
features, horizons, climatic regime, parent material, etc.;
- identifed by a geographic name (same as
types);
- comprises soils that act in the same way under
a particular land management regime.
- e.g. Horotiu series, Bruntwood series.
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Hamilton Basin: landscape & geology.
Image courtesy of David Lowe. You can also access a downloadable
version of this image
(753KB).
Hamilton
Basin: soil series & landscape. Image courtesy of David Lowe.
You can also open a downloadable
version (773KB).
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How
old are soils?
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Soils develop
through a
series of processes acting on parent material (the original rocks) over
time. These processes are influenced by climate (e.g. temperature,
rainfall, wind), organisms (i.e. plants, animals, fungi,
bacteria), and topography (i.e. whether a site is sloping or flat,
shaded or in the sun.
Soils can be up to several million years old. Many of the soils found
in New Zealand date to the end of the last glacial period, around
15,000 years
ago, although many volcanic-ash soils have taken 25,000 years
or more to
form.
Open the following links for a series of downloadable images
('snapshots')
showing how landscapes and soils in the Hamilton Basin have changed
generally over time: approximately 120,000
years ago, 20,000
years ago, 10,000
years ago, & last ~2000
years. All images courtesy of David J. Lowe.
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Radiata pines & New
Zealand Soils
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Can radiata
pine trees be grown indefinitely in New Zealand's soils?
Radiata pines are important because they make up 90% of all the trees
in our plantation forests, and their export earns billions of dollars
each year. In some places, there have now been three or four crops of
radiata pine and so the question of sustainability arises - can pine
trees be grown indefinitely? Scientists at Waikato University and at
Scion (formerly Forest Research), Rotorua, have undertaken a new study
in Kinleith Forest on the Mamaku Plateau. This is a remote, elevated
area of land between Tokoroa and Rotorua.
Native
forest growing on Tihoi soils on the Mamaku Plateau.
Image courtesy of Dave Palmer.
The scientists
and research
student Dave Palmer compared
changes in soil properties in two
adjacent areas of land on which there had been either one or two crops
of pine trees, and a third area which was still under native forest.
The soils in each of the areas (named Tihoi series) were identical:
they were all acid, strongly leached soils that developed over the past
1770 years on soft pumice deposits from the ~233AD Taupo eruption.
Tihoi
sandy loam, a
very acid podzol
soil formed under native forest and developed on Taupo
ignimbrite (15-60cm depth) over a thin tephra layer on a buried,
brown-coloured soil below 70cm depth on the Mamaku Plateau.
Image courtesy of Dave Palmer.
The team looked at changes in soil phosphorus (an essential nutrient
for tree growth) and how much disturbance there had been in the soils
after harvesting. And the scientists also 'asked the trees'
how
they were coping with successive cropping by measuring the amount of phosphorus
in the needles growing at tree-top level.
25-year-old
pines (at back) growing on Tihoi soils. Image courtesy of Dave Palmer.
The results were startling. Although the soils in each area had similar
amounts of phosphorus, indicating that they had not become depleted
after one crop of pines, the phosphorus levels (around 4ppm) were well
below those needed for normal healthy growth (around 12ppm). To confuse
the picture further, the pine needle analyses showed that the trees had
adequate phosphorus. So the trees, including the second crop, were
growing happily with regard to phosphorus requirements, and yet the
soils seemed to be quite depleted in this nutrient.
Soils with
the 'X' factor
The scientists considered various explanations for this apparent
paradox and carried out further experiments. They found that these
Tihoi soils have the 'X' factor - they have an unusual ability to keep
releasing phosphorus because of a strong buffering capacity. This means
that despite plant absorption and losses to leaching, the
concentrations of phosphorus in the soil are maintained.
Their conclusion: so far, so good, regarding nutrient supplies for
continual cropping of pines on these soils. But further study is going
to be needed to see what happens after several more crops, whether the
strong buffering capacity is likely to be maintained, and whether
harvesting itself impacts on other soil properties such as compaction.
Harvesting pine trees using a skidder.
Does such harvesting affect soils and their ability to grow pine trees
sustainably?
Image coutesy of Scion Forests & Environment, Rotorua.
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Bush
sickness & the discovery of trace element deficiencies
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Land
settlement of the Tokoroa-Taupo-Rotorua area began in the 1880s with
felling of
the bush, clearing of scrub, burning, ploughing and sowing of pasture
(which
grew well. However, cattle and sheep brought onto the land soon lost
condition
and often died. The condition was nicknamed Bush Sickness.
Location
of the cobalt-deficient (Bush Sickness) areas
of
the North Island. Image courtesy of Chris Hendy.
You can download a
higher-resolution version of this image.
Veterinarians
were unable to diagnose any diseases, and it was thought that some
property of
the soil was responsible. Analytical techniques of the day (1900) were
too
insensitive to show what this might be, but trial and error showed that
iron
ore from some, but not all sources provided relief. Initial research
was directed
at trying to supplement the animals’ diets with iron. In 1934
Grimmett and
Shorland (senior chemists at the Department of Agriculture’s
chemical
laboratory) found that the iron ore which gave the best results
contained
significant amounts of cobalt, and went against popular wisdom by
dosing
animals with cobalt - with spectacular results. They then developed
cobaltised
super phosphate fertiliser, which has been applied to the affected area
ever since at a
rate of a few grams per hectare, and has resulted in the addition of
about
250,000 Ha of productive farmland to New Zealand’s stock.
This one discovery
has paid for all of the scientific research ever carried out in New
Zealand. In
1948 the discovery of vitamin B12 (cobalamin) showed that cobalt is an
essential requirement for red blood cell production.
The
cobalt deficiency is only one of
a number of trace
element deficiencies (eg copper and selenium etc.) in
New
Zealand. Most of these arise from the volcanic nature of
soil’s
parent
material. Many of the volcanic ash (tephra) showers, which cover the
central
North Island, are derived from rhyolitic
eruptions. This material is
rich in
silica, but has very low metal concentrations. The two very widespread
rhyolitic tephras with the cobalt deficiency are Taupo tephra (erupted
~233 AD from the Taupo caldera)
and Kaharoa tephra (erupted ~1314 AD from Mt Tarawera - see map above).
Tephra derived from the andesite
volcanoes (Ruapehu, Tongariro, Taranaki etc and from the basaltic
volcanoes
(Auckland etc) have very much higher metal content and provide fewer
trace
element problems. Elsewhere in New Zealand trace element deficiencies
can arise
from excessive leaching in high rainfall areas, or from soils derived
from a
single rock type.
Ironically,
the village of Lichfield, established in 1884 on the edge of the
cobalt-deficiency zone, struggled as a farming community, and much of
the land east of the village was converted to pine plantations. With
the solution to the problem of 'Bush Sickness', cows are replacing pine
trees and Lichfield now hosts the world's largest cheese factory.
The old stone
building at
Lichfield, originally built as part of the hotel.
Lichfield, the
world’s largest cheese factory
(built 1995)
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The Te Ara on-line encyclopaedia has two very
good soil-related pages:
Soil
Investigation,
which shows how an investigation into soil qualities solved the mystery
of sheep and cattle illness in the central North Island;
and Soils,
which provides a very wide-ranging resource on NZ soils.
If you are interested in soil biology, this
site at Iowa State University
has a series of short movie clips on various aspects of life in the
soil. But note that the material is best accessed using a high-speed
internet connection and requires macromedia flash player to view.
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