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Soils - the most complex ecosystem in the world

Why are soils important?What is a soil?How deep is soil? |  What is the best soil? | New Zealand's soils | Different NZ soils How old are soils?

Radiata pine production & NZ soils

Bush sickness & the discovery of trace element deficiences

Useful websites

Material for the sections on Soils and Radiata pines was provided by David J. Lowe. Chris Hendy wrote the article on bush sickness.

david lowe explaining a horotiu soil profile near cambridge nz
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.

 
     
 

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?

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.

buried soils & Mt Tarawera
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 is the best soil?

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?

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.

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.
  horotiu soil profile
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?

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.
hamilton basin geology & landscape
Hamilton Basin: landscape & geology. Image courtesy of David Lowe. You can also access a downloadable version of this image (753KB).

hamilton basin soil types & landscape
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?

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

 
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 on tihoi soils
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.

profile of tihoi soils
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.

25yr-old pines on tihoi soil
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
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

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.

bush-sickness-and-soils
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.

old_lichfield_hotel

The old stone building at Lichfield, originally built as part of the hotel.

lichfield_cheese_factory
Lichfield, the world’s largest cheese factory (built 1995)

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Useful websites

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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