Soils and Agriculture of the Palau Islands'
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Soils and Agriculture of the Palau Islands' A.J. VESSEL and Roy W. SIMONSON2 SOILS OF THE PALAU ISLANDS range from shown in Figure 1. Lying 7° north of the deeply weathered Latosols- to shallow, rocky equator, the Palaus are in the same latitude as Lithosols, and include Regosols, Alluvial Colombia and Venezuela. Soils, and Organic Soils. Most extensive in The Palau chain stretches appro ximately 50 the islands are the Larosols, ofgeneral interest miles, from Kayangel at the north to Angaur because of their wide distribution in the at the south. From east to west, the dimen humid tropics . Associated soils are much less sions of the chain range from 3 to 25 miles, extensive but are more impo rtant to local as measured to the outer barrier reefs. food production . The latter also illustrate The total land area of the group, which effects of local factors in soil formation . Data consists of some 350 closely spaced islands on characteristics, distribution, and utilization and islets, is 175 square miles. Babelthuap, of the soils and a preliminary evaluation of largest island in the group, has an area of al their agricultural potentialities are given in most 140 square miles. The bulk of the re this paper. More complete descriptions of the maining land area is distributed among eight soils are part of a comprehensive report on additional islands. Most of the 350 islands the geology and soils ofthe islands now being and islets of the chain are tiny reefs or bars, prepared for publication as a professional barely above sea level. paper of the U. S. Geological Survey. Physiography and Topography GEOGRAPHIC SETTING The four types ofislands in the Palau group Locationand Extent are reef and atoll islands, platform islands, high limestone islands, and volcanic islands Most western group ofthe Caroline Islands, (Tayama, 1953). The reef and atoll islands, the Palau chain is almost directly south of by far the most numerous, are Rat land sur Tokyo and due east of Mindanao.Location faces only a few feet above sea level. Ex of the group in the western Pacific Ocean is amples of this type are Kayangel atoll at the north end of the chain and the many small 1 A reconnaissance soil survey of the islands was unnamed reefs and atolls north of Peleliu. made in 1948 as a parr of a cooperative program of soil and geo logi cal mapping of islands in the western The platform islands, also mainly Rat, are a Pacific Ocean carried ou r cooperarively by rhe Corps of little higher than the reefs and atolls. Angaur Eng ineers, U.S. Army, and U. S. Geological Survey, and Peleliu are chiefly of this type, though Deparrment of the Intetior. M anuscripr received June 12, 1957. part of each consists of limestone ridges. The 2 Soil Scientisrs, Soil Survey, Soil Cons ervarion high limeston e islands rise steeply out of the Service, U. S. D epartment of Agriculture. Th e aurhors sea and are mostly narrow and elongated in were on assignment to rhe U. S. Geological Survey and rhe Corps of Engin eers for rhe mapping of rhe Palau shape . A few have the form of mushrooms. Islands. The principal high limestone islands are 3 Larosol is a term proposed within the past few Urukth apel and Eil Malk; these and the nu years to include zonal soils formerly called Laterites, Reddis h-Brown Lateritic soils, and Yellowish-Brown merous small ones all lie between Ko ror and Lateriti c soils (K ellogg, 1949). Peleliu, approximately 25 miles apart. Parts of 281 282 PACIFIC SCIENCE, Vol. XII, October, 1958 l OS' 150 0 165 0 YOKO HAMA 30· .Bonin Is 10 " r- Marianas ~ G : : m 15 0 MAOILn;~.. e . 010/ Jj: ~~~ 0 /0 1. 1''/ .P.?j ~.. <; /YoPC . .r- J. ~· ~'/.H n d a n a o ~,/ " a ..r. 9 J i, n ,~ ~ ~1J" 525 PA LAU IS. e d " I s, 0' Di stances in nautical miles 10 50 120 0 150' FIG. 1. Map showing the location of the Palau Island s in the western Pacific Ocean. Peleliu, Angaur, and Koror and mu ch of one ofthe ridges in northwestern Babelthuap. Auluptagel are also high limestone ridges. The volcanic islands (Babelthuap, Arakabe The maximum elevation of the high limestone san, M alakal, Auluptagel , and K oror) are all islands is 686 feet above sea level on Uruk closely grouped in the northern part of the thapel, Slopes of these islands are generally chain. As mentioned earlier, Auluptagel and steep or very steep though a few have small K oror are partly volcanic rock and partly central depressions with mild relief. limestone. The volcanic islands, so named because of Soil Parent M aterials the un derlying basement rock, are generally hilly to rolling with local relief commonly The variety of rocks from which soils have measurable in tens of feet .Slopes and ridge been derived is relatively small in the Palaus . crests are rounded and sub dued. Babelthuap The platform and high limestone islands con has three prominent ridges running roughly sist of former coral reefs. The present surface parallel to the long north-south axis of the of the high limestone islands is largely a mass island . Th e highest elevation in the Palau of rubble of assorted sizes. Much of the land Islands is that of 794 feet above sea level on surface of the platform islands also consists So ils of P alau - VESSEL AND SIMONSON 283 of rubble, commonly small in size. The princi vananon is about ten degrees. Prevailing pal volcanic rocks range from olivine-augite winds are from the northeast and east from basalts to silicic hornblende dacites (Corwin, November to June and from the south and 1951) identified as andesitic agglomerates by southwest for the remainder of the year. J apanese geologists (Tayama, 1953) and also known as basaltic and andesitic volcanic brec Vegetation cias. Associated with the flow rocks and ag The native vegetation (Fosberg, 1946) in glomerates on Babelth uap are some basic vol the Palau Islands was rain forest, which re canic tuffs. Interbedded sedimentary clays mains in only a few places on Babelthuap. and lignite comprise the surface formations in The rain forest consists of large trees, includ a few places in western and southeastern ing Parinarium, Campnosperma, Couthovia, Babelthuap. Cynometra, Dysoxylum, Ficus, Semecurpus, Ran The distribution of rocks on the Palau dia, Fagraea, Pittosporum, Schelftera, Horsfieldia, Islands is strongly reflected in the distribution and many others. Growing also in the rain of soils (Fig. 2). Rocks of volcanic origin are forest are such palms as Pseudopinanga and parent materials for most of the Latosols. Exorrhiza; an occasional slender Pandanus; Some latosolic soils have also been formed such climbers as Preycinetia, Canavalia, Piper, from interbedded sedimentary clays. Deep aroids, and Ipomoea; also ferns, orchids, and soils with distinct horizons have' not been other epiph ytes. On both the low and high formed from limestones, most of which have limestone islands, vegetative cover consists rubble surfaces. Where the volcanic rocks are mainly of small to medium-sized trees and exposed on steep or moderately steep slopes, shrubs because moisture condition s are un shallow rocky Lithosols have been formed. favorable for plant growth. Deep unconsolidated materials of recent ori The most extensive vegetative type today gin have given rise to Regosols, Alluvial Soils, is anth ropic savanna consisting of coarse and Organic Soils. grasses, weeds, and occasional shrubs. Com mon genera in the savanna are Ischaemum, Climate Paspalum, Digitaria, Miscanthus, Lycopodium, Nepenthes, and Pandanus (Fosberg, 1946). Typical of the hum id tropics, the islands Soils of extremely low fertility support almost have heavy annual rainfall and high mean pure stands of a fern, Gleichenia linearis. temperatures with small seasonal differences (U. S. Dept. ofCommerce, 1953). The islands SOILS also have rather steady winds, lying as they do in the trade-wind belt. The mean annual The general character and distribution of precipitation is 148 inches, with a maximum soils were determined through a reconnais of about 19 inches in July and a minimum of sance survey, according to procedures already nearly 8 inches in March. On the average, at described (Simonson, 1953; Soil Survey Staff, least 15 days of each month have 0.04 inch 1951). This reconnaissance survey was part or more of rain, whereas 7 days in July have 1 of a program of soil and geologic investiga inch or more. The average relative hum idity tions in the western Pacific Ocean (Simonson, is 82 per cent, with a low of 79 per cent in 1953). How the soils were being used was March and a high of 83 per cent in July, No observed during the field mapping from vember, and December. The mean annual March through August, 1948. temp erature is 810 F. The maximum and mini Distribution of the soils and land types of mum mean monthly temperatures are but one the islands is shown in Figure 2. The map degree higher and lower, whereas the diurnal units are either soil associations or miscel- 284 PACIFIC SCIENCE, Vol. XII, October, 1958 BAB£LTHUAP t 'i' ANGAVR Legend Soils and Land Types of Palau Islands Latasols and Latosolic Soils E3 Latosols from Brecc ias ITIIIIID Bauxitic Latosols f rom Breccias ~ Lalos ols from Tuffs f2222:l Lctosolic Soils from Cloys Regosols, A'lIuvial Salls , and Organic Soils l2!JQ0J Shioya Sand (:=::J All uvial soils ~ Muck and Peat Llthosols, Stony L and, Roc k Land,and Mangrove Swamp . [=::::J Lithosols f rom Volcanic Rocks ! ARAKABESAN _ 1:'''':'';:''·.1 Smoot h St ony Land IIllllIIIIIIIJI limestone Outcrop -~~~~~:~~;g.