Soils of Mohaka-Aropaoanui Area, North Island, New Zealand
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SOILS OF MOHAKA-AROPAOANUI AREA, NORTH ISLAND, NEW ZEALAND W.C. RIJKSE Soil Bureau, Lower Hutt Part of N.Z. SOIL SURVEY REPORT 55 New Zealand Soil Bureau Department of Scientific and Industrial Research R. B. MILLER Wellington, New Zealand 1980 Dfft:ICTOA CONTENTS Page Summary 5 Introduction 5 Parent materials 6 Topography 8 Climate 8 Vegetation 9 Soils 9 Recent soils 11 Yellow-brown sands 12 Yellow-brown pumice soils and related steepland soils 12 Podzolised yellow-brown pumice soils and related steepland soils 12 Composite yellow-brown pumice soils on yellow-brown loams 13 Composite yellow-brown pumice soils on yellow-grey earths 13 Steepland soils related to rendzinas 13 Yellow-brown earths and related steepland soils 14 Steepland soils related to intergrades between yellow-grey earths and yellow-brown earths 14 Steepland soils related to yellow-grey earths 14 Soil limitations for potential pastoral use 15 Soil limitations for commercial forestry 16 Soil limitations for cropping 18 Value for food production 20 Conclusion 21 References 22 Extended legend Explanatory notes 23 Index to soils 24 FIGURE 1 Location maps 4 TABLES 1 Soil-forming tephra showers 7 2 Legend of soil taxonomic units arranged pedologically 9 4 1'0 r'-Rualcituri PACIFIC OCEAN SEA. 111•e Figure 1 Location maps 5 SUMMARY Soils of the Mohaka-~..ropaoanui area are classified (NZ Soil Bureau 1968) and discussed in this report. Their parent materials, topography, climate and natural vegetation are described and the soils are classified according to their limitations for pastoral uses, commercial forestry, cropping and value for food production in terms of the Town and Country Planning Act 1977. Distribution of the soils is shown on the accompanying soil map at a scale of 1:100 OOO. An extended l~gend to the map summarises some soil and agricultural properties of the soil mapping units. INTRODUCTION N.Z. Soil Bureau Map 112, which accompanies this report, is based on previously published work (Pullar and Ayson 1965) and on unpublished district soil surveys generalised in the "General Survey of the soi is of North Island" (N.Z. Soil Bureau 1954). To compile the current map, soil boundaries and soil classification were revised following field observations made in 1970. Aerial photo interpretation was used to insert more detail in areas of steep land and to adjust the position of soil boundaries in relation to roads, streams, etc. However, a detailed examination of the soils was not made and, apart from the recent soils of the river flats, the soil names given in the "Genera! Survey of the soils of North Island" were used for the current map. These are "set" names and may in fact include several soil series, so that the soils in this area may differ in some respects from similarly named soils in other parts of the country. For example, classifications for land use for specific soils in this area may differ from classifications for similarly named soils elsewhere. The total area of soils covered by this survey is 118 400 ha (Fig. 1). 6 PARENT MATERIALS Parent.materials of the soils of the Mohaka-Aropaoanui area consist of a wide range of materials which may be broadly divided into 4 groups: (1) Sedimentary rocks (2) Tephra* beds (3) Alluvium (4) Wind-blown sand Sedimenta:r>y rocks consist of a wide variety of materials including mudstone, siltstone, strongly to moderately consolidated fine or coarse sandstone, limestone and conglomerate. They form a complex pattern as shown on the Geological Map of New Zealand, sheet 8 (Grindley 1960) , but are generally covered by tephra and are soil-forming only on hilly and steep slopes. Teph:r>a beds originating from the Taupo-Rotorua region mantle the landscape. Although erosion has removed most of the tephra from the steeper country, much remains on rolling and hilly land to form parent materials of the soils. The tephra which are important to soil formation have been described by Vucetich and Pullar (1964) and Pullar et al (1973) • A brief summary supplemented by field observations is given in Table 1. Beneath the oldest tephra listed in the table (Waiohau Ash), loess and older tephra such as the andesitic Tongariro Ash and the Kawakawa Formation (a.22,000 years B.P.) occur in some stable areas. Alluvium derived by erosion of tephra and sedimentary rocks has been deposited on the flood plains of the main rivers and streams. It occurs near the lower reaches of the wairoa River, the Mohaka River, the Moeangiangi River and the Aropaoanui River. Wind-bloum sand occurs in small areas along the coastline south of Wairoa and near Waipatiki Beach. * Tephra: A collective term for all elastic material erupted through the air from a volcanic crater. TABLE 1: SOIL-FORMING TEPHRA SHOWERS IN THE TINIROTO-WAIROA AREA Tephra Approximate age Occurrence Characteristics shower (Yrs before 1950) and origin Kaharoa Ash 930 ± 70 Very thin deposits (1 to 5 cm) White grains (glints) in the topsoil, Mt Tarawera in northwestern parts brown to grey sand Taupo Pumice: 1760 ± 80 Very thin (10-15 cm) deposits Black sandy loam with few pale yellow Upper Taupo Taupo Region soft large angular lapilli, highly Pumice* vesicular and fibrous Taupo Lapilli* 1800 ± 70 Thin (40 cm) deposits Pale yellow to strong brown coarse Taupo Region uneven-sized soft lapilli, highly vesicular and fibrous Hatepe Lapilli 1900 ± 60 Very thin deposits (10-15 cm) White evenly sorted sand, loose Hatepe Valley -.J Waimihia Formation: 3270 ± 200 Moderately thick to thin Consists of 2 distinct layers: upper Waimihia asht Taupo Region (30-50 cm) deposits layer is fine tephra, dark brown to Waimihia Lapillit light olive, lower layer is loose, uniform, rounded lapilli, usually strong brown to yellow with a band of dark vesicular rhyolite RotOl.1\a Ash 7330 ± 235 Very thin and patchy White and grey compact fine sand or Rotorua Region difficult to recognise sandy loam, iron accumulation often above or beneath Rotoma Ash, also iron staining and mottling within the ash Waiohau Ash 11250 ± 200 Very thin and patchy Yellowish brown silty greasy material with much iron staining * Collectively named Taupo Pumice t Collectively named Waimihia Formation 8 TOPOGRAPHY Topography is an important factor in determining the soil pattern in a landscape that has been mantled with tephra, for this determines the erosion pattern and thickness of the surviving tephra cover. The result of erosion is a complex pattern of soils formed from tephra, from the underlying rock, and from mixtures of tephra and underlying rock materials. Four topographical units can be distinguished in the Mohaka-Aropaoanui area: 1. Flat, and flat to easy rolling land. This is almost entirely restricted to the alluvial plains and some of the river terraces. There is little erosion, and on the flat terraces the tephra cover is intact in most places. 2. Easy rolling and rolling land with most slopes under 12° and many 0 slopes around s • Little erosion occurs on these slopes and the tephra cover is almost completely intact. 3. Moderately steep and moderately steep to steeg land with most slopes 0 0 between 20 and 30 and many slopes around 24 • The tephra has been removed from the steeper slopes by erosion, but is nearly intact on ridges and stable slopes. Soils on this land are mapped as hill soils and are more variable than those on flat and rolling land. 4. Steep and very steep land with most slopes between 30° and 45°, and 0 many slopes around 35 • Tephra has been removed or partly removed by erosion in most places except on ridges in northern parts of the area. Soils on this land are mapped as steepland soils and tend to be very variable. CLIMATE Mohaka-Aropaoanui area has a warm moist climate, with an annual rainfall of 1000 to 1500 mm near the coast and up to 2000 mm in northern districts. Near the coast summers are warm with day temperatures occasionally rising above 32°c; rain is unevenly distributed and unreliable in spring and summer; winter temperatures are moderate and winter rainfalls are highest. Further inland the climate is cooler and wetter, with very heavy rain at times from the south and south-east, and an annual rainfall of about 1500 to 2000 mm (Robertson 1959). 9 VEGETATION Pre~European vegetation in this area occurred in two vegetation belts. Ser.uh and fernlands dominated the coastal alluvial soils west of Wairoa and south of Mohaka River. A belt of podocarp-mixed hardwood forest occupied most of the country between the coast and the northern part of the Mohaka-Aropaoanui area (Holloway 1959) • At present most of the area is in pasture, except on steep slopes and valley sides where scrub and pockets of podocarp forest remain. SOILS The soils of Mohaka-Aropaoanui area are grouped on the basis of the New Zealand Genetic Soil Classification (N.Z. Soil Bureau 1968) in Table 2. The properties of each of the soil groups that occurs in the area are summarised in the text, and are set out in more detail in the extended legend, together with the ratings of soil limitations for potential pastoral use, commercial forestry, cropping, and value for food production. TABLE 2: LEGEND OF SOIL TAXONOMIC UNITS ARRANGED PEDOLOGICALLY RECENT SOILS from alluvium derived from tephra and sedimentary rocks rapidly accumulating non-gleyed Waipaoa series Esk series slowly accumulating non-gleyed Waihirere series moderately to strongly gleyed Awamate series Makauri series YELLOW-BROWN SANDS from wind-blown sand non-gleyed Opoutama series strongly gleyed Whakaki series 10 YELLOW-BROWN PUMICE SOILS AND RELATED STEEPLAND SOILS from Taupo