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SOILS OF TONGATAPU, TONGA

H,S, GIBBS

Soil Bureau, Lower Hutt now at Department of Earth Sciences, University of Waikato

Part of

NEW ZEALAND SOIL SURVEY REPORT 35 New Zealand Soil Bureau Department of Scientific and Industrial Research ~Je 11 i ngton, New Zea 1 and 1976

LIBRARY LANDCARE RESEARCH NZ P.O. BOX 69, LINCOLN, NZ.

R. B. MILLER OlkE CTC».

A.R. SHEARER, GOVERNMENT PRINTER, WELLINGTON, NEW ZEALAND - 1976

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CONTENTS

Introduction

Soils

Vaini soils Lapaha soils Nuku'alofa soils Sopu soils

Land Use

Summary

Reference publications recommended for further reading

Appendix 1. Soil Profile Descriptions

Appendix 2. Analyses of soil samples by members of Physical Chemistry and Analysis Sections of Soil

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INTRODUCTION

Tongatapu is the largest of the 150 islands of the Kingdom of Tonga. It consists of 26 100 ha of flat, undulating and rolling land mostly less than 50 m above sea level. The climate is humid subtropical and provides excellent external conditions for plant growth. The average annual rain­fal 1 is 1 700 mm, with a minimum rainfall of 80 mm per month. Average monthly temperatures range from 20°C to 24°C and the daily relative humidity is 70% or higher.

Nuku'alofa, the largest settlement, seat of port of the kingdom, is situated near the centre the island. The present population of Tongatapu the majority of whom live in or near Nuku'alofa. on other islands of the kingdom.

government and chief of the northern coast of is estimated at 55 OOO, About 40 OOO people live

This report and accompanying soil map are based on a reconnaissance survey carried out in 1968. Preparation of the report was greatly assisted by the analyses of samples by members of the Analytical and Physical Chem­istry Sections of Soil Bureau, and by discussions of the interpretations with Dr G.G.C. Claridge and Messrs J.D. Cowie and G.E. Orbell of Soil Bureau.

SOILS Tongatapu is a coral limestone island formed on the crest of a large

submarine fold west of a deep trench (known as Tonga Trench) on the floor of the Pacific Ocean. Geological pressure on the fold tilted the island so that part of the south-eastern end is now 80 m above sea level whereas the western end is only 6 to 8 m above sea level. Since the emergence of the coral the surface has been covered by numerous showers of volcanic ash and lapilli, up to 5 m thickness in some places. The actual source of the materials is not certain - some may have originated from volcanoes such as Tofua and Kao but most of the ash and lapilli have probably been exploded from shoal volcanoes formed during submarine eruptions. Many such "jack­in-the-box"islands have been subsequently destroyed by marine erosion, as recorded in historic times for Falcon Island, Metis Shoal and other centres of volcanism in the region. Pieces of basalt scoria up to 1 cm diameter occurring in the topsoils of western parts of Tongatapu are evidence for a fairly close and recent source for the latest fall of volcanic material.

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Farming tradition on Tongatapu recognised two kinds of soils -kelefatu and tou'one. An examination of a typical area showed that the kelefatu referred to soils derived from volcanic ash and the tou' one referred to soils derived from coral sands. Whilst this division had been sufficient in the past, it was too broad for modern purposes for which more detailed subdivisions were made into four soil sets as follows:

soils derived from volcanic ashes were subdivided into sets called Vaini or Lapaha soils

the free draining soils formed from beach accumulations of coral sands were classified as Nuku'alofa soils, and

the slow draining sandy or silty soils formed in marshy swamps or edges of lagoons were mapped as Sopu soils.

The subdivision of the kelefatu soils was made to separate soils derived from volcanic ash deposits of differing relative ages. Although deposits from the eruptions had accumulated in successive layers over thousands of years, they were not evenly spread over the island. The eastern end received little of the more recent eruptions, with the consequence that the soils of this end are formed mainly from older deposits. This difference was demonstrated in properties such as the higher content of clay, lower content of nutrients, more brown and yellow colours of subsoil horizons, and a general absence of fresh lapilli compared with soils in the western part of the island. Soils formed from the younger deposits are named Vaini set, and those formed principally from the older deposits named Lapaha set soils. As the thickness of the younger ash deposits decreases gradually eastward, there is a gradual transition between the two sets of soils, which is shown on the map by the areas with double symbols. Further evidence for the division between the soils is shown by the occurrence of Lapaha-like soils buried below Vaini soils in central areas of Tongatapu.

VAINI SOILS

Soils of the Vaini set cover practically all the western and central portions of Tongatapu and sizeable areas of the eastern portion. They have thick topsails of dark reddish brown clays that are friable and have fine and medium sized nutlike aggregates. Below 35 cm there is a gradual change to reddish brown clays, firm, sticky and with blocky aggregates having a thin coating of brown clay and with many weathered lapilli. Both topsoil and subsoil contain nodules of hard scoria, soft black manganese concretions, and small grains of weathering ash. Underlying these reddish brown clays there is a brown heavy clay which rests on coral limestone which occurs at very irregular depths between 1 m and 5 m from the surface. A dark brown clay occurring at the junction with the limestone may represent a soil formed on the coral before the ash deposition began.

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Laboratory examination of samples showed:

1. Neutral values for pH (6.4-7.6), and very high values for cation exchange capacity (50-60 me.%*), for base saturation (80%-90%) and exchangeable calcium (30-40 me.%).

2. High values for exchangeable potassium (1-3 me.%), phosphate retention (60-80%). High contents of total iron, aluminium, copper, cobalt and other metals. Total nitrogen is medium in the topsoil (0.4%) and low in the subsoil (0.07-0.2%).

3. Clay contents of 50% to 60% and consisting principally of halloysite and palagonite minerals. Hypersthene, feldspar and glass were common in the sand fractions of the soils.

4. Active populations of soil organisms that convert plant material to mull humus at a rapid rate, as shown by the low C/N ratio of 12. A moderate level of organic matter is maintained by the year­round growth of plants and associated organisms.

These analyses confirm the assessment of Vaini soils as nutrient-rich soils derived from basic volcanic ash.

Three subdivisions of the Vaini soil set are mapped to show areas of differing physical properties for detailed land use. Vaini clay (the most common member of the set) represents free draining soils on flattish land, Vaini clay, rolling phase, represents a similar free draining soil on the slopes of low ridges, and Vaini clay, mottled subsoil phase, the areas with periodic waterlogging in the subsoil resulting from slow drainage conditions.

A detailed profile and set of analyses of Vaini clay are given in the appendices.

LAPAHA SOILS

Soils of the Lapaha set occur only on parts of the eastern end of Tongatapu where the mineral materials of the land are derived chiefly from older deposits of volcanic ashes. The surface probably received shallow deposits of the younger ashes, but through the natural mixing by organisms

1 me.% = 1 m mol per 100 g oven-dry soil *

ion charge number

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and the cultivation by man, these additions are no longer visible in Lapaha clay or in the rolling phase of this type. Soils of the Lapaha set were divided and mapped as three members: Lapaha clay, a freely draining soil on flattish to undulating slopes and the.most common member of the set; Lapaha clay, rolling phase; and Lapaha clay, mottled sub­soil phase. Soft brown nodules in the mottled subsoil phase of Lapaha clay were assumed to be an indication of impeded drainage, but subsequent e~amination indicated them to be weathered scoria lapilli and most prot~bly evidence of deposits of the younger ash. Hence the mottled subsoil phase of Lapaha clay would be more correctly named as a shallow member of the Vaini set.

Lapaha soils have deep topsails of dark brown heavy clays that are friable and slightly sticky and have a fine to medium-sized nut structure. Below 30 cm there is a change to brown and yellowish brown clays, firm and very sticky, and with blocky aggregates distinctly coated with dark brown clay. A few nodules, mostly soft black or yellow, and pieces of coral occur in the profiles. The massive coral limestone is commonly found between 75 cm and 1 m from the surface.

A number of sites selected for examination during the survey had profiles with pieces of coral limestone in the upper 20 cm of soil. These are considered to be a consequence of additions of coral sand to the surface, either for purposes of housing, of paths> or of marking sites for identification of property boundaries or of graves. Such sites were omitted from classification. A detailed description of Lapaha clay and associated laboratory data is given in the appendices.

The laboratory examination of samples of Lapaha soils showed similar values for pH, and % base saturation to those for Vaini soils. Lapaha soils had slightly higher contents of carbon, nitrogen and vanadium, while overall values for exchangeable calcium, exchangeable potassium, phosphate retention are slightly lower, and for acid soluble phosphate distinctly lower, than those for Vaini soils. Total clay contents range from 70% to 86% and similarly consist of halloysite and palagonite. There is a notable absence of glass in the sand fractions. A similar rapid rate of humification of organic matter is evident from the deep mull topsoil.

NUKU'ALOFA SOILS

Nuku'alofa soils are shallow soils formed from accumulation of sands on beaches along parts of the northern coast of Tongatapu. The soils are subdivided on the basis of texture into Nuku'alofa sand and Nuku'alofa sandy loam. Both soils drain rapidly but the sandy loam retains more moisture and is less liable to serious depletion of organic matter under cropping than the sand soil. Both soils have an alkaline reaction and a low level of available phosphorus. Available potassium was not measured

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but is likely to be low. A detailed profile and analyses of Nuku'alofa soils is given in the appendices.

SOPU SOILS

Sopu soils are found in low wet areas of small bays and tidal marshes. They have formed from sediments, and are broadly subdivided into Sopu sand and Sopu loam according to the texture of these sediments. Their properties are dominated by excessive water, and by the composition of this water. Sopu sand is found along parts of the coast, where it is strongly influenced by salinity and the associated limited growth of vegetation and opportunity to accumulate organic matter. Small areas may be reclaimed by stopbanks and used similarly to Nuku'alofa sand. Sopu loam is formed from a mixture of beach sand, and silt and clay particles eroded from the associated Vaini soils. It receives drainage waters from the Vaini soils, the composition of which varies according to the uses being made of them by people. Hence the chemical composition of the soils differs widely from place to place. A common property of Sopu loam is a moderate to high content of organic matter, reaching peat levels in some places. The native vegetation consists largely of mangroves, with large rushes at some sites.

LAND USE

According to archaeological studies Tongatapu has been inhabited by Polynesian people for more than 3000 years, and for their food and housing requirements the successive generations have replaced the original sub­tropical forest with either different kinds of agricultural crops and shrubs, or with buildings and roads. Most of the farming land is subdivided into 3~ ha (8~ acre) lots which are allocated to individuals on long or short-term bases. The majority of the allotments contain coconut plantations with banana and/or vegetable and root crops grown either between the coconut palms or in separate 5mall plots. The small crops such as yam, taro, kumara and cassava are grown in succession over 3 years, and this part of the farm is then left in fallow for 2-3 years to grow shrubs and regenerate the organic matter, the structure and nutrient levels in the soils. Little fertiliser is used and only small areas are used for pastoral purposes.

The local system of shifting cultivation has been successful in main­taining food supplies for the inhabitants and a small surplus of copra and bananas for export. But the system is becoming less and less adequate to meet expanded demands on crop production for feeding the increasing popu-

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lation and for raising the export income. The present population of about 55 OOO on Tongatapu means an average density of about 2 people per hectare (or 500 people per square mile). Hence if this island is to remain self-sufficient in primary production, the co-ordination of land and human resources must be improved by a more efficient system of farm­ing. For the proper development of such a system, scientific information on the properties and needs of the soils is essential. Improvements in other aspects, notably land tenure, education and transport are also essential, but are outside the scope of this report.

The soil survey shows that the Vaini and Lapaha soils covering most of Tongatapu have excellent physical, chemical and biological properties for growing a wide range of crops. In the past the average production from these soils has been reduced by the nonproductive fallow period. This period could be eliminated by applying fertiliser, particularly nitrogen, to supplement the nutrient supply, and by using a rotation of crops to maintain a suitable level of organic matter and of structure for high annual production. This innovation, accompanied by improvements in agronomic methods and in plant breeding, as well as a reduction of plant diseases, weed competition, and storage losses could produce and maintain high yields of crops on the Vaini and Lapaha soils. The particular treatments required would depend on the crops to be grown and this in turn depends on the market requirement. Economic aspects of agricultural development are discussed by Hardaker and Maude (see references).

The Nuku'alofa and Sopu soils occur in small areas, and their distribution and particular properties limit their suitability for many crops. The Nuku'alofa soils are the most suitable soils on the island for residential purposes, and those areas not needed for housing and home gardens could be more useful for tree crops than annual crops. Under intensive crop production, water shortage is likely to become a critical factor on many areas of Vaini and Lapaha soils, and the Sopu soils could be used for wetland crops with high requirements for water.

SUMMARY

Tongatapu has extensive areas of deep friable brown clay soils that are very well supplied in nutrient requirements for plant growth. These soils have been formed and maintained by the combined action of the moist warm climate and vigorous organic life on successive deposits of volcanic ash and they have excellent physical, chemical and biological properties. Whilst these soils already produce reasonable yields of food and export crops, their properties indicate they could support a much more intensive system of farming which would help the people to alleviate their increas­ing problems of food production for local consumption and for export trade. More intensive farming systems would involve changes to the traditional

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methods of agriculture, land tenure, commercial and social life, but they can be adopted with the assurance that the soils have a high poten­tial capacity for growing a wide range of crops. In the absence of other natural resources, Tongatapu is fortunate to have such excellent soil resources to support development of the Kingdom to meet the needs of the expanding population.

REFERENCE PUBLICATIONS RECOMMENDED FOR FURTHER READING

BROOKFIELD, H.C. (Ed) 1973: The Pacific in Transition. E. Arnold, London.

CROCOMBE, R. (Ed) 1971: Land Tenure in Pacific. Oxford University Press, London.

GREEN, R.C. 1973: Tonga's Prehistoric Population. Pacific Viewpoint 14(1): 61-74

HARDAKER, J.B. 1971: Report on the Economics of Agriculture. (Report prepared for the Government of Tonga under the United Kingdom Technical Assistance Programme).

KINGDOM OF TONGA DEVELOPMENT PLAN 1965-1970. Government Printing Office, Nuku'alofa. 40 p.

MAUDE, A. 1970: Shifting Cultivation and Population Growth in Tonga. Journal of Tropical Geography 31: 57-64

MELSON, W.G.; JAROSEWICH, E.; LUNDQUIST, C.A. 1970: Volcanic Eruption at Metis Shoal, Tonga, 1967-1968. Smithsonian Contributions to the Earth Sciences 4: 1-18

SCHOFIELD, J.C. 1967: Notes on the Geology of the Tongan Islands. N.Z. Journal of Geology and Geophysics 10: 1424-8

STRAATMANS, W. 1954: Reclamation of Tidal Mudflats. South Pacific Commission Technical Paper 53. 18 p.

WALSH, A.C. 1970: Population changes in Tonga. Pacific Viewpoint 11(1): 27-46

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APPENDIX 1

SOIL PROFILE DESCRIPTIONS

VAINI CLAY

Location: Fahefa, on flat land, altitude 13 m, under grass fallow in a coconut plantation.

Climate: Annual rainfall 1500 nnn. Annual temperature 24°C.

Profile:

Ap 0-13 cm very dark grey (5YR 3/1) clay; friable; slightly plastic; moderate fine nut and fine granular structure; few fine yellow and grey nodules,

A12 13-28 cm as above except dark reddish brown (5YR 2/2) clay,

A3 28-41 cm dark reddish brown clay; moderate medium nut structure; indistinct boundary,

B1 41-60 cm dark reddish brown (5YR 3/2-3/3) clay; firm; very plastic; weak medium blocky structure; thin clay skins; soft yellow and black nodules,

B21 60-76 cm as above but with medium and coarse structures and more distinct clay skins,

B3 76-96 cm dark reddish brown (5YR 3/4) clay; firm; plastic; weak coarse blocky structure; patchy fine clay skins; many soft nodules,

C on massive dark reddish brown clay; firm and plastic derived from weathered volcanic ash and lapilli over coral limestone at about 2.5 m below the surface.

Classification: (NZ) Weakly leached brown granular clay

(USA) Udic Eutrandept

(FAO) Mollie Andosol

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LAPAHA CLAY

Location:

Climate: Profile:

Kolonga, 1.2 km west of village on gently undulating land, altitude 16 m approx. and under fem-grass fallow and broadleaf trees.

Annual rainfall 1600 mm. Annual temperature 24°C.

Ap 0-13 cm dark brown (7.5YR 3/2) clay; friable (moist), slightly sticky and plastic (wet); moderately developed fine nut structure; few gravelly nodules (1-2%); some roots; diffuse boundary,

Ai2 13-30 cm as above except horizon moderately sticky and plastic and medium nut structure; indistinct boundary,

B21 30-50 cm brown (7.5YR 4/4) heavy clay; firm (moist), strongly sticky and plastic (wet); moderately developed medium blocky structure with glossy (cutan?) coating to blocks; few yellow and black fine soft nodules; few roots; indistinct boundary,

B22 50-75 cm brown (7.5YR 4/6) heavy clay; as B21 except fine and weakly developed blocky structure,

B3 75-95 cm similar B22 except structure very weak tending to massive and numerous soft black nodules (manganese?)

on coral limestone.

Classifcation: (NZ) Weakly leached brown granular clay (USA) Lithic Hapludoll

(FAO) Luvic Phaeozem

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NUKU'ALOFA SANDY LOAM

Location:

Climate:

Profile:

Mapfanga, from flat crest of undulating land, atitude approximately 5 m, and under broadleaf trees and coconut plantation.

Annual rainfall 1700 mm. Annual temperature 24°C.

A 0-20 cm very dark reddish brown (SYR 2/2) sandy loam; friable; strongly developed fine and medium nut structure; many roots; some yellow sand grains,

B 20-40 cm dark brown and brown (lOYR 3/3 and 5/3) sandy loam; friable; moderately developed fine nut structure; many roots; many yellow sand grains,

on yellow and very pale brown (lOYR 7/6-7/3) sand, loose, some roots, and overlying coral limestone.

Classification: (NZ) Subtropic yellow-brown sands

(USA) Lithic Rendoll

(FAO) Rendzina

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APPENDIX 2 ANALYSES OF SOIL SAMPLES BY MEMBERS OF PHYSICAL CHEMISTRY AND ANALYSIS SECTIONS OF SOIL BUREAU)

DSIR

Soil Name Vaini clay Lapaha clay

Horizon Ap Ai2 A3 B2 Ap Ai2 B21

Depth (cm) 0-13 13-28 28-41 61-76 0-13 13-30 33-51

c 1 ay % 53.1 53. 8 57 .6 39.8 68.0 71.5 79.6 silt % 20.3 20.0 18.9 23.0 8.6 6.6 2. 7 sand% ' 19. 2 18.4 16.6 16.3 11. 3 10.6 4.8

pH (H20) 7.6 7.2 6.4 6.7 7.7 7.7 6.8 CEC* me.% 57.4 50.3 57.0 55.9 55.6 42.4 ! 36. 3 BS % ** 87 94 88 81 95 92 i 80 Ex:.!: Ca me.%

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33.8 33.4 34.6 30.9 43.5 31. 4 I 21. 0 Ex K me.% 3.1 1. 7 1. 8 1.0 2.3 1.4 1.6

N % 0.42 0.20 0.17 0.07 0.59 o. 23 0.10 c % 4.9 2.5 2.3 0.8 5.8 2.7 1. 3 P(O. SM H2SOti) 40 39 25 17 15 12 6 P retention % 57 62 64 77 51 53 56

Tamm Fe% 1. 37 1. 38 1.18 1. 72 0.50 0.45 0.33 Al % 0.48 0.52 0.50 0.73 0.29 0. 32 0.23

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Mineralog}:'.:

B22

58-76

82.7 2. 1 3. 1

6.8 37.6 69 18.7 0.0

0.09 1. 1 9

61

0.33 0.28

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Sand chlorite, glass, feldspar, quartz, feldspar, augite, hypersthene hypersthene

Clay palagonite and halloysite halloysite and palagonite with trace of gibbsite

Nuku'alofa sandy loam

Horizon A B

Depth (cm) 0-20 20-30

pH 7.9 8.3

N % 0.85 o. 35

c % 8.5 2.9

1 ~ 1 m mol 100 d ·1 me.a= ion charge number per g oven- ry soi

* CEC = Cation exchange capacity ** BS = Base Saturation ~-Ex = Exchangeable

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