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Special Edition: International Soils Day 2020

The 5th of December, as well as being 20 days away from Christmas - it is also known as International Soils Day.

Now, this may look like 3 different golf courses but they're actually historical Māori 'Borrow Pits'. These amorphous, and sometimes large, depressions found in geographically restricted localities are the result of sand or gravel being removed from the ground and added to nearby soils.

These Māori borrow pits are located along the banks of the Waikato River, between Ngāruawāhia and Kirikiriroa (Hamilton). Maori gardeners removed up to 800 mm of the more recently deposited volcanic ash and silty sediment on river terraces, in order to access the coarse gravelly sand layer derived from water-borne pumiceous material (pumice) carried down from the volcanic plateau and deposited in large alluvial fans. Quarrying of this sand and pumice material formed the distinctive, irregularly shaped depressions. The location of borrow pits is inextricably linked to expanses of modified soil used as gardens. These features, in the Waikato at least, are closely associated with other forms of settlement evidence, such as pa and storage pits. Borrow pits are the visible indicator that modified soils are present in the area; the material extracted from the pits was rarely transported more than 100 metres.

There may have been a preference for soils on the Taupo terraces, as borrow pits are more common here than on the higher terraces. The soils most frequently altered were the Horotiu yellow-brown loams, but the Te Kowhai silt loam has also been identified as a parent soil.

Pumice was added to these soils to improve drainage, while in other areas of Aotearoa, large river gravel stones were added to aid solar adsorption to make to soil warmer.

It is estimated that there may have been up to 2000 ha of modified soils situated within 3 km of the Waikato River. Kūmara gardens were located on lands and terraces along the Waikato and Waipa rivers. Modified soils are identified on soil maps as ‘Tamahere gravelly sand’ formed on Taupo, Horotiu, or Waikato parent loams, depending on their location. 

All of these factors combined to enable ancestral Māori to grow kūmara to maturity, the local growing season was otherwise too short and the climate too temperate to achieve this.

Te Parapara Garden in the Hamilton Gardens is an example of the once widespread kūmara gardens. The garden takes its name from the pre-European Maori settlement that occupied the site. The area was once home to the Ngati Wairere chief, Haanui, and was associated with sacred rituals regarding the harvesting of food.

- Matt Couldrey [geoid - digital geography]

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