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Ngā Tohuwhenua Mai Te Rangi: A New Zealand Archeology in Aerial Photographs

Physical conditions and soil fertility

Physical conditions and soil fertility

The most important physical conditions were sunshine, a degree of shelter and the need to avoid frost in the growing season. Moisture was also important, but critical only for taro, which is a species adapted to water. The New Zealand species is known as a 'dryland' variety but it still needed a good supply of water to flourish. It has survived by natural propagation in some localities from the East Coast northwards, but only where water is present, for example, near springs.

Sunshine, its effects enhanced by careful choice of site-aspect, cultivation and other modifications of the soil, provided the warmth needed for the tropical crops to thrive and the radiant energy needed for photosynthesis. Apart from the obvious digging and weeding, the most important cultivation techniques were mounding or the formation of raised rows, and the selection of areas that had stony or gravelly soil. Where there were nearby sources of gravel or coarse sand, these could be applied to the surface as a mulch, where they appear to have raised the temperature of the soil by a few critical degrees. Eventually, the gravels came to be mixed into the soils, improving friability where this was otherwise lacking. On river terraces or other terrace landforms and older dunes, it is not uncommon to see borrow-pits, irregular depressions ranged along the edge of the terrace 4 or on ridgelines. 5 These pits were dug to take sand or gravel from lower strata in the terrace, to add to the surface of the garden plots on the nearby terrace. The illustration here is from near Lake Karapiro on the course of the Waikato River, and Aotea Harbour examples are in chapter 8. The application of gravel or sand, and the blackening of the soil as the result of fire, meant that the soils would warm very rapidly. This would not be so important late in the planting season, but early in the season it initiated the sprouting of the leaves and roots of the kūmara. The overall effect was a lengthening of the growing season for crops that had not evolved or been bred for the cool, short growing season of southern latitudes.

Soil fertility in New Zealand was enhanced where land was forested. Forests, and swamps in which organic matter such as raupo has grown, are important page 62
Borrow-pits on the edge of terraces on the Waikato River near Tirau

Borrow-pits on the edge of terraces on the Waikato River near Tirau

The borrow-pits show as irregular depressions up to 20 m across, aligned along the edges of the terraces. On the hill above where the road cuts through the terraces, a pā shows (but not very clearly). On the furthest (righthand) extent of the terrace there is a ditch and bank, dog-legged in plan. The ditch was about 4 m across and it is likely to be a late pre-European or nineteenth-century fortification, exploiting an excellent tactical and strategic position on the terrace. The valley floor is now the bed of Lake Karapiro. The road is State Highway 1 where it leaves the Waikato River (flowing downstream from east to west) some 10 km from Tirau.

stores of the nutrients that plants have taken out of the soil or water. Nutrients in the form of topsoils and leaf litter also tend to drift down to the foot of slopes, or into small basins at the head of valleys within broad slopes. To make these fertilisers available to the soil, it was essential to burn the forest releasing essential nutrients for root crops such as potassium and phosphates. The burning was probably done in a manner that was consistent with other cultural objectives, as we saw in chapter 2. The pattern of this burning suggests that it had several purposes, only one of which was horticulture.

Swamps and river valleys generally were highly fertile and important in gardening. However, with the exception of taro, Māori root crops did not flourish with page 63 'wet feet'. Swamps therefore had to be drained, 6 and even where swamps were not drained deliberately, they may have been seasonally dry in some places, for example, around the broad fringes of infilled lakes. As the drains were dug and renewed over a span of time, the fertile organic matter was thrown up into beds, raising the surface slightly and increasing the thickness of the topsoil. Layers of sand may have been encountered and these would be mixed by this process with the organic matter. Once exposed to the air, the preserved organic matter in the swamp would rot, creating another source of some nutrients and improving physical conditions. The organic matter and manuka would also burn if fired, adding nutrients from the ash. Such beds would have been ideal for kūmara, and the bases and sides of the drains themselves would have suited taro.

This process was labour-intensive and appears to feature mainly in the far north around Kaitaia and the Bay of Islands, and in places on the lowlands of the Bay of Plenty. In these localities climate, particularly temperature, was not so much a limiting factor. However, there were other limitations on gardening in the wider region. In Northland the soils on the country surrounding the swamps are of very poor fertility. They had long had a cover of kauri forest, and the combination of warm temperatures, rainfall and the acidity of the soils under such forest had chemically removed the nutrients from the soil. The swamps on the other hand, particularly those formed on estuarine soils, had a cover of kahikatea, which demands good soil fertility. As a result, once the kahikatea was cleared, the fertility was high and gardening on areas of swampland was particularly highly developed.

Another important place for gardening was on dunelands. These gardens were on soils created on the sheets of sand, disturbed initially by human firing, driven by wind inland from the coast. Such soils were distributed over much of the North Island west coast from the Manawatū to Ninety Mile Beach. Other smaller areas were on the east coast of the North Island. Within a couple of hundred years of the soil's formation, they would have had good covers of nutrient-rich manuka, kanuka or other coastal forest. The nutrients could be released by further firing. What may now appear to be a 'desert', the coastal strip of Ninety Mile Beach from about 1 to 5 km inland to the east, was once a complex, if patchily used, gardening system. 7