Pacific Science (1992), vol. 46, no. 1: 35-45 © 1992 by University of Hawaii Press. All rights reserved Soils of the Laloanea Farm, Northwestern Upolu, Western Samoa! R. J. MORRISON 2 AND M. ASGHAR 3 ABSTRACT: Soils ofthe Laloanea Farm (40 ha), in the uplands ofnorthwestern Vpolu, Western Samoa, were studied by an examination of nine pedons composing two toposequences, one running S-N and the other W-E across extensive portions of the farm. Over short distances considerable variability in the soils has led to their classification into two soil orders of Soil Taxonomy (Entisols and Inceptisols), two suborders, three great groups (Troporthents, Humitropepts, and Dystropepts), four subgroups, and seven families. Major factors contributing to the variability were depth to basaltic boulders or flow rock, presence or absence ofa cambic horizon, amount oforganic carbon in the profile, particle size distribution in the control section, and occurrence in some pedons of andic properties. Soils all had an oxidic mineralogy class and an isohyperthennic soil temperature regime. Relationship of the factors affecting variability to topographic position is discussed, together with an overview ofthe physical, chemical, and mineralogical properties of the soils. Similar variability might be expected in other humid tropical situations on young basaltic landscapes with steep, rolling, and benched terrain. WESTERN SAMOA CONSISTS OF four volcanic Much ofthe terrain is steep land, and these islands and series of islets lying between 13° areas have a thick vegetative cover ofprimary and 15° S latitude and 171-173° W longitude. or secondary forest, or dense scrub. Some The two main islands, Vpolu and Savai'i, are agricultural development has occurred in the composed of a mass of successive olivine rolling and flat areas with the production of basalt flows, and the landscape is the result of root crops, copra, cocoa, bananas, and more subaerial erosion of two major lava domes. recently passionfruit and pastures on flat ar­ The age of the flows ranges from the Fagaloa eas of Vpolu. flows ofthe early Pleistocene up to the present Early studies of soils of Western Samoa century, with the last major event (on Savai'i) were those of Hamilton and Grange (1938), occurring in 1905-1907 (Kear and Wood Seelye et al. (1938), and Birrell et al. (1939). 1959). The climate is humid tropical with The first comprehensive maps were those rainfall varying from 2000 to 7000 mm annu­ of Wright (1963), published at scales of ally; some areas have a weak dry season 1 : 40,000 (Vpolu) and 1 : 100,000 (Vpolu and during June-October but there is no true Savai'i). Wright did not discuss the variability leeward coast. Temperatures at sea level aver­ within series or mapping units (apart from age 26-27°C, with a drop ofabout 0.65°C for separation into phases). Schroth (1970) at­ each 100-m rise in altitude (Curry 1955). tempted an analysis of the variability of cer­ tain characteristics, but examination was con­ fined to a limited number of pedons and did 1 This work was funded in part by a grant from the University of the South Pacific Research Committee. not include an evaluation ofvariability within Manuscript accepted 2 February 1991. the established mapping units. 2 Institute of Natural Resources, University of the The Laloanea Farm ofthe Vniversity ofthe South Pacific, P. O. Box 1168, Suva, Fiji. South Pacific is located in an area typical of 3 University ofthe South Pacific School ofAgriculture, Private Bag, Apia, Western Samoa. Present address: many in the uplands of northwestern Vpolu Queensland Department ofPrimary Industries, P.O. Box (see Figure 1). This farm thus represents a 201, Biloela, Queensland 4715, Australia. suitable resource for future agricultural re- 35 36 PACIFIC SCIENCE, Volume 46, January 1992 172°W UPOLU, WESTERN SAMOA CJ LALOANEA FARM o 10 20km I I I FIGURE I. Location of Laloanea Farm. search. Because no detailed soil information strated the changes found at the northern end was available, a study of the area was under­ of the study area. taken to provide valuable resource baseline Pits were dug in each of the representative data. At the same time, the survey permitted soils, and the profiles were described accord­ an investigation of the variability of the ba­ ing to the procedure used by Taylor and salt-derived soils located within the single Pohlen (1979), except that the term "nut mapping unit (the Avele stony silty clay and structure" was replaced by subangular blocky related very stony soil) in the 1 : 40,000 map structure. Horizon designations were accord­ of Wright (1963), which characterizes the ingtotheFAOjUNESCOLegend(1974). The farm site. Data necessary for classification by soils were sampled and analyzed as described Soil Taxonomy (Soil Survey Staff 1975, 1990) by Blakemore et al. (1981) or the Soil Conser­ were obtained so that the Western Samoan vation Service (1972) ofthe U.S. Department soils could be better related to those of other of Agriculture. tropical areas, thus assisting the transfer of Mineralogical studies were made on whole soil-based agrotechnology. soil samples « 200 mesh) and on the clay (< 2 J.lm) fraction using X-ray diffraction analysis. This analysis was carried out us­ ing an X-ray diffractometer system (Philips MATERIALS AND METHODS XRG-3100) employing a long-fine-focus cop­ A preliminary auger survey was made ofthe per tube operated at 40 kV and 40 mAo By use farm area to indicate the general soil pattern ofa "theta compensating" divergence slit, the and variability and to identify dominant soils. same sample area was exposed to the X­ Two toposequences were selected to illustrate radiation over the entire angular range of the soil pattern and range of variability (see the diffractograms (2-66°, 20). The diffrac­ Figures 2, 3, and 4). The S-N sequence tometer employed a wide-optics, curved­ covered the major reliefunits ofthe farm, and graphite monochromator in lieu ofa Ni filter. the W-E toposequence, running from the Sample preparation consisted of bulk air-dry top of a small cone (LNA 1) across a ma­ powder for the < 200 mesh material and of jor portion of gently rolling terrain, demon- water suspensions that were air-dried on Soils of Laloanea Farm-MORRISON AND ASGHAR 37 r ---- I -- ---- ---- I ---- ---- I I BUS H I \ \ o 100m 200m I I 1 CATTLE >N WATER II II ----:A~ ,,,I LNA 2 I II RESEARCH I I AREA BBB II I II BBB I I I I BBB_____ ""J I ,..-- --"" I I LNA8 I BUSH I PTP T PTT TTT PTP TPTTTTT I P 1 P TPTTTTT PTP T PTT TTT I PTP TPTTTTT ~APIA P = Passion fruit )()()< = Fence T = Taro --- Farm Boundary B = Banana ---- Track FIGURE 2. Laloanea Farm: land use (1985). 38 PACIFIC SCIENCE, Volume 46, January 1992 w E I I I 1 I t I I I I I I I I 1 I I I 1 I I I ..,1< ~30m-¥-35m .... 1" S5m I GOm I LNA 1 LNA2 LNA3 LNA4 LNA5 A A A A o. 0 o D o ~ '" C' C> ~ 0 0 0 C C 0 '0 0 Bw 0 Bw 00(J0 o 0 (J o ° ti DOaO <)0° 0 9 • 0 0 0 1 m a Bw C f) {) " ~ 0 C 0 0 0 olJo R R R DO;:/ FIGURE 3. Laloanea toposequence no. 1. molybdenum slides for the clay fractions. occurred in the Samoa group as recently as Because it is not possible to obtain ac­ 1907, additions of recent ash must also have curate quantitative soil mineralogical data taken place. The farm is at 500 m (1650 ft) from X-ray diffractograms alone, the quanti­ elevation, so the average annual temperature tative estimates are in the form of ranges. is estimated at 23°e with a small variation The soils were classified according to Soil between "winter" and "summer." No rainfall Taxonomy (Soil Survey Staff 1975, 1990). data are available, but annual precipitation has been estimated at 4000 mm, with a very weak dry season June-October (Hydrology Section, Apia Observatory, pers. comm.). The RESULTS AND DISCUSSION soils therefore have an udic soil moisture The Laloanea Farm soils were formed from regime and an isohyperthermic soil tempera­ Pleistocene or Holocene olivine basalt flow ture regime. The farm area is generally well rocks or weathered products derived from drained, and no evidence of sustained imper­ them. Because major volcanic events have fect drainage was observed. The present vege- Soils of Laloanea Farm-MoRRISON AND ASGHAR 39 s N I I I I I I I I I I I I I I I I : I I I I I IE-~ --~*«--- gOm x 200m--~' ---130 m I I : I I I LNA9 LNA6 LNA 7 LNA8 A 0 A A l?c3° o • 0 0 0 ° Do Bw 0 o °0 a0 Bw <;)~; 0 <;) Qo <0 0 () 0 o 0 .j)0 0[> <) C> &' .lJ,. Bw 0 1 m oCS 0[;, 0 f?o Q 0'6 0 C o <J tJ 0 R 00 c R R FIGURE 4. Laloanea toposequence no. 2. tation pattern is shown in Figure 2, while Toposequence No.1 (W-E) some indication ofthe reliefis given in Figures 3 and 4. Five profiles in this sequence were studied, Although the soils in the two toposequences moving from the summit ofa small secondary showed some similarities, the soil patterns in cone eastward to a relatively flat area close the individual sequences were quite different. to the bottom of sequence no. 2. The whole Profile features and laboratory data are sum­ area had a dense covering of grass (including marized in Tables I and 2.