SOILS 1N HAWAIT

Mnny differen1 t~pe.7 o p l s are found on olI.tl~c lsl~mds, each wirh charactens!rCp ysrca ond chonicalproperties. The soils of Hawai'i ore quite d~flerenrfrnm those of the continental Unired ..Tmrrr.

John R. Street

The slooes of the active volcanoes of the island of ilawai'i. with their lava ~~~ ~- - flows of various and known ages, iurnish the soil scient~st wjrl,:? natural Inh- oratory for the study of soil fortlmtion--a laior:~tory with the iurther advan- tages of a great range of temperatures as one goes-from sea level to over 13,000 ft (3,900 m) elevation and of rainfall as o r ~ e lollows the contour from the wet windward flank of Maulla 1-03 to its desert leeward slope. Indeed, the state of Hawai'i has been a major center for the study of tropical soils. Manaeement of the island soils has chanced markedlv over the vast two cen- - - ,~ ~ ~~. furies, a, Polynesian suhsisterce agriculture gave rvav to cattle ranches and they in turn i n many areaswere d'splaced by pla~tlat;ons ofsugar c m e (Sncclfanrm officinar!onl and ninexvle (Anunur con~orusl. llow retreatine , . - before the m&trous giowth bf subu'rbia.

In several respects the soils of 1Iawai'i a re quite different from those of the continental United Slates. They are in many places very deep, very rich in iron and aluminum. almost free of unartz sand. and extrsordinarilv oermeable

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and resictant t o er&un. ' I l~ese q u z i i t i e s d l e c ~ the conditions unde'r which the soi:s tlevelopcd--the year-romd warmth of the Tropic$ comhinetl with abundant rain filling upbn beds of dark, porous l a w a i d deposits of vulcanic ash.

Soils show the infl.~encc of the factors of soil formation: parent material, climate, lopogruphy, : h e , plants and animals, and tnallagerncnt. For the most part Hnwa~iun soils have develooed front volcanic rocks. cinders. and ash of a hawltir ur andesitic nature. h m e coastal soils havc'formed bn coral limestone and coral sands. while vnllcy-floor an(! co:tslal-plain soils are commonly on alluvium (sedimentary material tlepos~ted 5v water). With the Islalds' immense diversity of climates-.Iron1 desert to the wet:esr on earth and from perpe~ooll.; warm to just sliort of eternal snow-there is a mmparahle multipiicity of soils. Terrain feitu~res are also u r i cd , including such lnnd- fmms a< crater?, cliffs, gently sloping volcan:~ domcs, and nlluvial plains. Phnt cover ranges from simple lichens to tall, dense rnin forest. Since it takes m e for soils to develop from rock, volcanic a$h, or beach % m i l . and since

soils differ from one stage of development to anofher, we fitld different soils on surfaces of varlous ages.

GEOGRAPHY OF SOIL TYPES

Vast expanses of the flanks of Kilauea and Mauna *a on Jfawi'i Island are covered with lava flows so recent that little or no sot1 bas formed upon them. But even here. one must distingttish between the rough jumble of clin- kers of an aa flow and the smooth, ropy, massive surface of a pahoehoe flow. In South Kona at ahout 1,000 ft (300 rn) altitude, thin, stony soils have devcl- oped on somewhat older flows. It is instructive to follow thesc flows down to the sea, from the humid uplands to the semiarid coast. The vegetation he- comes sparser and the soil thinneruntil there is almost barren, fresh-lookin!! lava ne& the shore. Moirtttrc, working together with the plants it nurtures.- is required to comer[ rock intoroi:. Muck ofthe i-.terior of the !alntld of Hawai'i and the lecivard co:~stal lands north of Konn and on the other 14ands is cit:tracterizc? by stony desert and semi-desert suilr. S:cep slopes tllat can support only a thin soil, and are prone to scvere soil crosion if dtsturhed, characterize the innuntins that occupy much of West Mnui, sastern Moloka'i, 0'ahu. and Katra'i. For the most part, these heavily diwcted up- lanL:s are managed as protected watersheds (areas ccntrihuting to collection of common runoff of water).

Patterns of occurrence of the different tvDes of Hawaiian soils are also relatedto ~%&rature and sunlight. ~x t en&e virgin forests remain in mounxin areas that are very cloudy and wet. such as the windward flanks of Mauna Kea. hIauna Loa, and Haleakala at an elevation of about 3,000 It (900 mk their s l o ~ e s mav be moderate and their soils reasonahlv deeo and . ~ ,. stone-free. In the' cool tiplends ulth moderate slopes ar!d $. reci6itdt& -, ' , ' and . fairly deep soils, such as are fot~nd i n Kula nn Maui and u n e a on Hdyat'i, eeo!oeicallv recent volcanic nsh has develo~ed into dark. hum:!s-r:ch, frlable - (crum%~y),borous soils with a high nutrient content.

NATURE OF HAWAIIAN SOILS2

Although nearly nll Hawaiian soils are volcanic in origin, many different twes of soils are found in the I4andr. each wrrh character~mc ohvsical and . ,r. . - ~ - ~ ~ ~~~ ~ . , chemical properties.

Zonal Soils p~ -

Zon:~; soil5 h e x a strong imprint from the climate in which they have developed. In lands of moderate slope with rainfall in the 20-40 in. (SOU- 1,000 m n ) range and altitude of leu than 2.00C f t (600 m), :be most common

consirvatinn scrvicc,\Iras~in~tci D.c., t ~ i ) . ~ h c tcrnls in rhc furmcr arc msicr to rcmcmbrr, facilitate thc recognition of sirnilarilics bc1wcc.n soils, and Icnd themselves well to the explanation of roil formittion.

soils, except on the geologically young island of Hawai'i, are of the group& humic latosols. These soils are deep, with well-aggregated, friable topsoils that remaln veyfirm when wet. ('l'his physical strength permits the oft- observed Hawallan custom of parking automobiles on lawns--a practice that would have disastrous consequences on weaker Mainland soils.) Quartz sand is almost absent from the soil since Hawaiian volcanic rocks contain no quartz crystals to be liberated upon chemical breakdown of the rock (chemical weathering).

With warmth and moisture, which promote chemical weathering, the oorous. nermeable lava has, to a depth of manv vards. undereone a comdete iransfoimation of its constituent minerals. In ihk process, c&tain elements were carried away in solution, with the most soIuble taking precedence. Bases such as sodium, calcium, magnesium, and potassium were the first to go, followed bv silica. Due to this selective removal. the low humic latosols con-

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tiin le~ser~~roport iocs of silica and bases than tlle rocks from which they were deriver! and display rek~tive enrickmert by the !csc co111'-le elcments iron. aluminum. titanium. and manganese,

ln t x i r natural st;tte'tl~e lo\r 11;mic lalosols range from neutral to mildlv acid. Texturally 1k.e~ are largely composcd of cia?, and the cluy.s:zed partr- cles are ?redominantly kaolinirz. 3 clay mineri :'la1 has only 3 rno<lcst capacity to store water and nutriens. Organ'c matter contrnt is fairly II,\\-- 3-JYc, as mi85 he expected :nn warm, fairly dry environment.

Hnmic Intosols occur i f ) climatically wetler :ireas ll~on do the low humic latosrh atrd arc nftcn upslope of them. They contain leissilir.a and aluminn and more of the oxides of iron and titanium. The texture is still cloy, but the proportiln of kaolinite in the clay has decreased whde tlra: of :he oxides of iron and titanium has risen. Humic larosols are characterlzetl bv cood granular structure and a high capacity to store moisture and nutrie'nk thanks, at least in part, to their organic matter content of 8-10%. As is usual, with increasing rainfall comes higher acidity.

h m i c ferruginous lattrsols cltaracteristically occur upslope of low humic latosols on the geologically old surfaces of Kaua'i and of the Wai'anae Range ofo'ahu. At a depth of a few inches they have a dense horizon (distinct layer of soil) of high specific gravity containing very high proportions of the oxides of iron and titanium. These soils have poor structure, high erodibility, low Capacity to store nutrients and water, and low permeability. They are re- garded by a number of soil scientists as the * soil--the end product of soil development in Hawai'i. Underneath these old soils occurs a residual material so high in alumina and low in silica that it could be mined as an ore of aluminum.

Hvdrrrl hlin~ic latosols have developed in arms of very heaq rainfall, rtm?r~ly on \olcanic 3rn on the windward slopes of Hawai'i 1s.and and East 5' : ' '. aul. Very h~gh orpnic matter content an? a phenomenal capacity to store

water characterize ti~em. When they are dried they shrink drastically and cannot he rehyilratcc! In their former state.

Red desert soils, located on geologically young materi:~ls in dry places Such as Kawaih:~~ (on Hauai'i lcland) ard Koko Head (Oahui. have under- gone very little uteathering or leaching. They exhibit wh~te cdcium carbonate deposits and tend to hc stonv. \Vuh increasing molsture the red desert soils blend into reddish hrowr soils favored with a friable, humus-rich topsoil of high -. . native fertility. They itre sinilar to the soils of the southern Great k'l'lains on the U.S. Mainland

Progresting farther upslope along a continuum of increasing moisture on the ioutheastern flank of Halcakala on Maui or ahove Kaweihae on Hawai'i. reddish brown soils merge into &&t~ ~&i& &s. High in hares. wirh dark, humus-rich topsoil, the reddish prairie soils contnln no lime deposits and bdve lost some silica through weitherine and leachinr. Latosdk brown .. --- .fox2 ~ 0 1 5 , in a slJl muis!er en'vlrocnlent, hive lost evm more si:ira and h c s &.id :end to be sl~ghtiy acid. Unth types of soils ore frlahle and have a high capscity for storing moisture and mltrients.

Intrazonrl Soils The soils of Hawai'i are those whose cl~aracter has been

strongly conditioned by impeded drainage or a high groundwater table. They may he considered nnder two categories: mineral soil5 and oreanic soils. All of the mineral soils are plastic clavs that are easily converted into a structureless, imperrneahle paste when wet. They have a high content of cxchangeahle magnesium (that is, magnesium that may be extracted from the soil by plant roots), and the higher the magnesium content, tlic more plastic the soils. They resist penetration of irrigation water, air, and plant roots, and their high magnesium content interferes with the uptake of potassium. These soils occur naturally on coasral plains, valley floors, and adjacent hasal slopes at low altitudes in areas of low to moderate rainfall. Humans have also created plastic clays by flooding and cultivating fields for the culture of wetland taro (Colocasiu esculmta) and rice (Oyza .miva). The plains of southern and western Oahu and western Kaua'i encumpass the largest expanses of plastic-clay soils. Water drainage from higher slopes over and through lava with a characteristic high magr~esium content has enriched the lowland soils with magnesium.

Water-logged organic soils in the form ofpeat boss exist on the cool. wet upper slopes of the mountains. The nearly level summit of O'ahu's highest peak, Mount Ka'ala, is a bog (see Cutldihv, this volume), as are the highest parts of Moloka'i, West Maui, and Kau$ By far the largest bog, the Alaka'i Swamp on Kaua'i covers 15 mi2 (39 km-) of the high interior of Knna'i.

Azonrl Soils Soils that are very young with weaklvdeveloped topsoils are called

:]zonal soils. They include the :~lluvi:~l s d s that lwve formed on streatn- deposited sediments; the that have developed on volcanic ash, cin- ders, and coral sand; and the sh:~llow, stony m. The alluvial soils occur in vallev bottoms and coastal ohins. Reeosols are so sandv in texture that rhey retain link wdter or nui;:cnt$. ~oil;devclo~ed on s o h sands form a transverse belt in the isthmus region of Maul, are also fairly extensive in the ' wcstcrn Darts of Moloke'i and Kaua'i. and underlie the settlementi of Kaihla and ~ a h a n a l o on O'ahu.

l 3 e utcgory of llihosol, includes those areas that are stony either he- cause they r e covered hy recent lava flows or are verystcep. On m ~ i t t , steep s!ogei. s:wh as those at !.he head of Manon Vallev on O'chu. soil forms verv rap~dly, but once it attains 3 depth of 2 ft (0.6 m)'i! hecomesunwble, und' heaw rains trigger lanhlides ctlvying ihe soil downslope i!lto stream chan- nels and tow3r;l the sea. I t has beep eslim3ted l k ~ t this process of soil iormatiun and lois takes place ar !he remarkable rate of 1 f t (0.3 m) in 400 )ears in rain-drerched upper Manna.

SOIL AND WATERSHED MANAGEMENT

Archaeological evidence-layers of water-deposited topsoil mixed with charcoal and ashes--from Makaha and Kahanavalleys on O'ahu and Halawa Valley on Moloka'i strongly suggests that Ion before the arrival of Captain James Cook, Hawaiians cleared and burned f orest on the slooes to ~ r e o a r e for planting eanlen% thus cauring soil erosion (see Stone. ~&ai'i's'W&- lands.. . , t h ~ s volume). Ancestral Polynesians brought domecticated pigs (Sur ~crofa) to the Islands, some of which escaped into forests and caused some damage to plants and animals. However,-it is clear that damage from over a millennium of Polynesian land use was small as compared with the impact of continental humans in the past two centuries, The causes and effects of some of these changes and the impacts on soils and watersheds are discussed below as well as elsewhere in thisvolume (Gagne and Gill; Gill; Holt; Stone, Non- Native Land Vertebrates; Burrows).

Grazing Animals Eurooeans who arrived in Hawai'i in the last auarter ofthe 18th centurv

introduceh domes:icated grazing animals: cattle, tiorses, sheep, goats (BO; ' taurus, Equus whnllus, Ovis u k s , Cupru hinus). and domestic European pigs. The Islands--essentially free oidisenses and predators, with a flora that hud evolved in the absence of large herbivores--afforded an idyllic environment for the rapid proXeratlon of the introdgced animals. Within a few decades hundreds of thousands of the hoofed creatures roamed wid, hrn~vsing and tramplirg vegetation largely lacking in the usual defenses of toxins, spines. acd offensive taste and smell. Uot even the soils were adapted to resist the assad1 o i the interlopers: earthworms had not vet ccdonized the remote Hawaiian Arehioelaeo and thus were not resent to restore the oorositv. permeability, c r h a i e , and aeration of sdi~s com aced by the h'ooves.'~arcd g . - : . of 3 proTectlve mant:e of plant5 and resist;int to t e int ltrat~on of rain, rich topsoil uas uashed awav bv the wa:er that coursed duwn the sloues during storms. Before long, stirilk subsoil was exposed, creating red scars--some scars have resisted recoIonintion by plants to this day. Destruction of forest also precipitated landslides in soils no longer anchored to the slopes by the thick, strong roots of trees-a form of devastation garishly illustrated in the windward valleys of O'ahu, where removal of the land's vegetation and soil has revealed great bIotches of pink, rotten rock. The splotches of red in Waimea Canyon on Kaua'i (vaunted in the tourist blurbs) are the creation of the feral goats that roam the cliffs. On West Moloka'i, a sheep-raisingven- lure by an Hawaiian uli'i (memher of ruling class) led to formation of a had- land landscape of deep, red gullies. Erosion caused by grazing also led to the siltation of the Hawaiian fishoonds on the south shore of Moloka'i and to the . . transformation of the coastnl'wa:ers between the ~ e f s and the coral sand beaches into r vile, muddy me% Desolate Kaho'olawe, strip(:ed of its top- soil. with its hardened suhsoil hakine in tie sun. is in its oresen: sorn state as iconscquencc of its exp~oitation>s a sl~eep ram11 m h oi t,ruusink by ferd goats, contrary to common belief that it i\ 3 result of prac:iie hombiny by - . ihe Navy. .

Gradually the landowners and rulers of Hawai'i realized that indiscrimi. nate prazixg was ruining the land; feral animclr were hunted and sharply reduced in numbers. and ranchers cut their herds to more ncarlv fit the carryingcapaeity of the range. However, as recently as 1960, cattle were still

grazin on severely eroded lands on the T o n g Kanch in the Wai'anae Range of W a f u and on Mo1oka.i Kanch in West Moloka'i. And even todav, in some localities. ranees are overerazed and feral animals continue to damiee the ~~. land. 1h/s is gotably t m e h lands owned by t h e state of llawai'i, w$ch to this day are almoit devoid of government surveillance while lessees who pay a small fee have s t r i o ~ e d the la6d of its cloak of soil. For examole. some slopes df the ~a'u'district on the I3igl\land were exposed for few poor crops ofsugar cane. H u n t e d Inbhies have prevailed upon the State to maintain d e s t ~ c t i v e herds of feral s h e e ~ on Miuna Kea and feral eoats on the Na Pali coast of Kaua'i, but Federal c6urt rulings should result in'ihe removal of all ieral sheep and mouflon (Ovir musinton) from Maum Kea. Feral p ~ g s descended from domebric E u r o ~ e a n stock. althoueh not as evidcnt as herds of shcep,clttle, and go:o,ts, u~nt inuk to "rototiil vast &eas of native wet and mesic forest and open pasture.

F o r e s t a t i o n Thc influence of destructive grazing upon water resources became a con-

cern of thc su dr industry as it was develo ing irrigation w o r k toward the end of the 191 f' century. The planters reaEzed that a forest cover protected the soil and effectively promoted the infiltration of rain water that nourished springs and groundwater. Therefore, thev supported control of feral stock, establishment of Forest Reserves. and afforestation of denuded lands. In the early 1900s the Hawaiian Sugar I'lanters' Associat~on huilt a station (now the I.)on Arboretum) in upper hlanoa Valley, O'ahu, for the purpose of introduc- ing and testing tree species to be used ifreforesting Hawai'i'i mountains. Consequently, O'ahu is much more forested (with introduced species) than it was a t the turn of the century. At present, an introduced plant that covers much semi-arid land, the koa haole or ekoa (Leucaena leucocephnla), is threatened bv an insect that is causine some scientists to fear it will kill ewugh to expose the slopes tr;crocion.

Natural forest communities provide the surest protection against ero- ston, since soil is stabil i~ed by ground rover of grasses, herbs, and shrubs a5 well as trees. Introduced s e m s such as cucalqtus (Eucalypftcr spp.) and even-agcd stands of trees 'f o nor su port diverce ground cover needed for sta- hili/.rion, especially in wet areas. Brotection o f remaining native forests in Jlawai'i is a good soil and watershed conservation practice. Unfortunately, many of the nat:ve woody plants of semi-arid districts have hecome extinct or rare. By saving what is left we also help protect soils and watersheds.

Important References

Armstrong, R.W. (ed.). 1983. Arlas ofHauaii. 2nd edition. Univ. Hawaii Prcss, Honolulu Burrows. C.K.P.M. Ithis volumel Hawaiian conservation values and mactices. Cline, M.C., et a1 1955. Soil Stmqv, Tcmro~ ofHawoii . . . . U S. Soil Conservation

Service, Soil Survey Series 1939, no. 25, Washington, D.C. Cuddhhv. L.W. Inhis volumel Vcnclalion ?ones of lhc Hawaioan islands. . . . " Cia&, W.C., and L.T. Gill. [this volume] Conservalion education in Hawai'i: history,

problems, and needs. Gill. L.T. [this volumel Persaectivcs on environmental education in Hawai'i. ~ o l t . ~ . [[his volumel'~rote~tionof natural habitals. Morgan, J.R. 1983. Hawaii:A Geography. Wcswiew Press, Boulder, Colorado.

Steams, H.T. 1985. Geology of tlte State of Hawaii. 2nd edition. Pacific Books, Publishers, Palo Alto, California.

Stone, C.P. [this vulume] Hawai'i's wetlands, streams, fishponds, and pools. Stone, C.P. [this volume] Non-native land vertebrates. street, 1. 1983. Soils. Pp. 89-97 (Chapter 9) IN J.R. Morgan, Howaii:A GroppIty.

Westview Press, Boulder, Colorado.