No, 22 S1uumal.y of b.formation on Atoll So i l s by E, L. Stone, Jr, Issued by THE PACIFIC SCIENCE BOARD National Academjr of Sciences--National Research Council Washington, D, C. by E. L. Stone, 31,. Our icnoc:l.edge of atol.1 ? o i l s ; s s t i l l f r awen ta ry and. t h e followi.ng r e l a t e s p a r t i c u l a r l y t o met iltol.ls, nhick. nave been gore e::te:lsi:rely s tudied than dry a t o l l s . The na tu re of & t o l l s o i l s i s dose3.y' l i n k d t o t h e ge~!.ogj.ca?. inc~.terial on which they fo~med. Thrse s o t i s a r e eu;;rwely imrratvre, ueual ly having A--C p r o f i l e s aomioa td ? ~ y calci.ug anG magneslurr. c ~ r o c g a t a s and wj.th . texture but l i t t l e cod i f i ed .:ro:,i t h a t .of t'ne oc!.ginal n a f c r i a l . This imc,aturity i s i n p a r t a cmsec,uence of t h e y l.ia!i td ace, v+iich i s prrsun,abl.y l e s s than t h a t of t h e xerothermic p e r i 4 , ?ad of t?ir ccn-cinuel 6isturbance by st~rms aiid -C,yphoons. The f reauent rcji1ven;'cion by btoi-ms i s one oi' the riiajor 2ec.hres s f the l and surfa.ce. T h s t?.er+ h?s hsen l i - b t k s o i l d o v e l o p ~ e r ~ t i n t h e usua l se r se , tied t h e charac+erist:.cs of t h e v;-31.1--iirzi.n~d s o i l s , i n so f a r a s t l l e ~ d i f f e r from geolcgica l mtiLerials, a r e 6uz c h i e 3 y +o o r g a i c mat ter a c c u ~ ~ u l a t i o n and 8,s- s o c i a t e e ciiemxal cilenges. I General: -- Desp;.te imi?aO~ri.ty, the d i f fe rences i.n s o i l s and piq%sicgra,pny a r e s u f f i c i e n t t o gj.ve r i s e -50 a v a r i e t y of risbitaks. The e::pression of tkese i n t h e :?egatatiou, ?oweve;', i s i n p a r t cmcealed by t h e l i m i t e d f lo ra . T1,i.s sometimes Leads t o a spec io i ;~ appearwoe of uniformity. I1 Fhjsical. Natme of tk>e F a r i n t Materia&: Tile priinary sources of !.and ma- -- t e r i a .1 we, of course, t h e m e 8 mgmis f i s 'but i n a t l e a s t a few h s t a n c e s fore ign meteyial nay occur. 011 Rose Ato l l b a s a l t has been found and puntice has beer, repor ted common on a number of a+.ol!.s. The mode of land fom:ation has been t r e a t e d inore fully by geo log i s t s and need nclt be repeated here I;L+,hough it i s obvio!.?~ that p a r t i c l e s i z e di.stribu- S o n and e levat ion of t h e lanci surfaco s t rongly inf luence s u b s e q u e ~ t s o i l de- velopment. Frequently material; deposited along t h e seaward shores a r e coarse- textured arid porous. Conaequen-tl.y, a s the d a t a of Cox ind ica te , t h e ground water l e n s may have s. lower head an?. g rea te r sa2.init.y on t h e saaward s i d e a s cont ras ted with t h e ].agoon shore, xhich i s commonly composed oL" f i n e r na+,erials. Although d i f fe rences i n tgxture of t h e deposi t s a f f e c t ground water move- ment an2 p l a n t growth, a l l a r e su f f i . c i en t ly coarse t o be f r e e l y pervious t o a i r am? va te r . Tie moisture holding capaci ty of t h e mineral s o i l i s ].ow, increas ing t h e e f f e c t of r a i n f a l l d i s t r i b u t i o n and ground 3a te r . Vihere f r e s h % a t e r is ava i l ab le a t shal lov~ depths t e x t u r a l considera'kions obviously do n o t kave t h e s ign i f i cance f o r deep-rooted p l e n t s t h a t thsy oft,en hzve el.se?ihere. Further- more, it i s probable t h a t t h e poros i ty of weathered cora l and r e e f rock increases t h e i r moisture-holding cagacity, a s compared ~ i t h t h a t of s o l i d fragm&nts of s i m i l a r s i ze . I11 Chen~ical Mature of the Parent Illaterii.: Anal'jses of the r e e f organisms, - a s by Clark and i.het:ler, i n d i c a t e t h a t calcium carbonate, 'vhile predominant, i s by no means t h e only zomnound of importance i n t h e i r composit,ion. Some of t h e Lithothamnicln group may contain up t o 25% magnesium carbonate and some of t h e Foraminifera up t o 11%. Although phosphorus i s .seneral ly iow, some of the Crustacca may contain up t o 27% calc iun~ phosphate i n t h $ i r sks le tons and the re a r e appreciabie tmounts i n some o f t h c Lcyonar ian cora ls . Kine s a p ] - e s of non-phosphatic subsoi ls from Arno contain from . O 1 t o .0%% phosphor,us. SoLub1.e potass%um i s found i n moderate amounts whenever appreciable exchange capactty i s present . Traces of most elements a r e of cour.se t o be expected by reason of t h e i r presence i n sea water. A s a source of p l a n t nutrien-Ls these mator ia ls have c e r t a i n apparent advia- tages and disadvantages. The calcareous mdium tends t o be favorable f o r some ni t rogen-f ix ins legumes and Azotobacter, Hovever, i t limits a v s i l a b i l i t y of' c e r t a i n n u t r i e n t s such a s i ron , of whtch the re i s a conspicuous def ic iency when- ever orgar1j.c ra- t ter c o r t e n t i n t h e s o i l i s low. IV S o i l Formation and P r o ~ e r t i s : Theee can no t he considered a p a r t from .- c l ima t i c influences, n a r t i c u l a r l y those of r a i n f d l . Not only t h e a:.loud but t h e constancy of minfa l : is of prime i.mportance, viith tke e f f e c t of va r i a t ion being more dres-Lic the l e s s e r t he m o m - t . The e f f e c t s of v a r i a t - i m a r e of p a r t i c u l a r consequence on narrow i s l a n y s xhore the na tu re of t h e ground water l e n s i s more r e a d i l y a f fec ted by s h o r t droughts. Ra in fa l l obviously a f f e c t s t h e composition of t h e ground water and the r a p i d i t y of leaching of s a l t s formed i n t h e s o i l and those added by spray. To date conduct iv i ty measurements seldom show s u f f i c i e n t concentrat ion of s z l t s in the surface s o i l t o be in ju r - ious t o the plax~%s, although t n e s e do n o t represent t h e temporary condit ions t h z t may r e s u l t ' from flooding o r heayy spray during severe storms. Kers desc r ip t ions and analyses of a v a r i e t y of p r o f i l e s a r e o f t e n of l i m i t e d value u n t i l t h e s o i l s c m be c l a s s i f i e d and arrenged i n t o an a c t u a l o r in fe r red sequence r e l a t e d t o time o r developmental s tages. Many of the da-La f o r mell- drained s o i l s on Arno and Onotoa can be described by such a sequence: I t Fs evident t h a t the i n i t i a l development of vegetat ion i s somewhat analo- gous t o l i f t i n g one ' s s e l f by t h e bootstraps; p l a n t growth.:is required t o c r e a t e organic matter vhich i n turn suppl ies n i t rogen, renders c e r t a i n n u t r i e n t s avai l - ab le , e t c . , t o permit addi t ional growth. In t h e e a r l y s t ages following colo- nizatj-on by hardy p lan t s , each gain i n mount of organic substance tends t o favor. g rea te r and more d iverse vegeta.tiona1 development. UI.timately t h e ex ten t of t h i s developzent i s r e f l e c t e d i n tine s o i l p ro f i l e . Thus a sequence may be observed extending from the wholly unaffected beach sand o r boulder r a m p a r - t ~ f the i s l a n d margins t o the' dark su r face s o i l s beneath t h e lush vegetat ion of t h e i s l a n d i n t e r i o r s . Although i n any one t r a n s e c t such a sequence i s usua l ly as- socia ted witn time of development, o the r f a c t o r s such a s exyosure, r a i n f a l l , groundwater, e tc . , su re ly inf luence Yne r a t e and presumably t h e maximum s tage of development a t t a inab le . The conventional s o i l type de s i ~ p a t i o n s employed t e n t a t i v e l y on Arno can he used t o designate various bands i n . this hyyothe-tical developmental sequence on moist a t o l l s . Figure 1 shows some f e a t u r e s of pro- f i l e s represent ing successive pos i t ions i n such a sequence. Fi@re 2 i l l u s t r a t e s tho ic fe r red sequcnce from shore t o i n t e r i o r and the effects associated with increasing organic mather contenb. In the synthetic t r m s e c t the OnoCoa prof i le appears t o agree generally with those from Arno a to l l . Tine collsideration of organic matter i s almost insez~arable from t h a t o f ni- trogen since the t v m a r e ].inked i n .i;he s o i l wj.th au OX/N rtition of between ,% 20:1 and 30 : l . Apart from growth e f f ec t s , i n tile absence of nitrogen additions of organic su'3stance alone do not r e s u l t i n formation of "humus" and hence or- .gmic mat,ter accumuiation. 'The very conside?able a.ccuniulations i l l u s t r a t e d by F i g ~ r e 2 d i r ec t a t tent ion to the sources of nitrogen. Ayart from r a i n f a l l four are !mom: (1) F lo t sm i~;d desd marine organisms a l e prew~v~ably of signif- icance only -to the early stages of s t rand vegetation. (2) Legumes, Sophora, Ce.naara1?, and are often common in the ea r l i e r stages of the sequence but only zn&s,& pe r s i s t s in q m n t i t y iii the dense forest . (3) The contribution of nitrogeu-fixirig blue-green algae i s unlmomna1t"lu~h certain t.errest,ial. algae are abunclant. Azotobacter is coma;! i:? certain hrno collecL,ions (Lochhead) end presumably vrould be favored by the highly organic calca.reous substrate. (L ) Observations sugges+, .that sei-birds may add apprecisble amomts of nitrogen to tk.e land surface generally, apar t from th? marked guano or phosphate areas. High amoun.ts of nitrogen and phosphorus found i n sparsely vegetated areas on dry Canton is land are a t t r ibuted t o nestiilg birds (?&ac.Daniels, Hathemay). There a r e no systematic studies of organic matter deconposition but, vigorous faunal a c t i v i t y by kermites, snai ls , earthv~orms, crustaces, etc., (Usfnger and LaKisers) rapidly reduce organic rasiSues. Incorporation of organic matter may aiso be accomplished by grasses and ' t3e dense root mat of the coconut. Kell- drained a t o l l s o i l s seem -to be war!cea by a sharply delimited zone of mixirg, -vi.th s l i g h t organic s ta ining of the deeper layers only i:t the o?.der profiles. Darker layers occurSng a t 2epth usually indicate prof i le burial , a s by dunes, typhoon debris, e tc . Deep profi:Les may or iginate by continuous additions, as of wind-blown sax? t o a vegetated surface, and probably a lso i n cer ta in imper- f ec t ly drained s i tuat ions . A s indicated by Figures 1 and 2, the depth of the organic layer tends to in- crease n i th age but the mst marked e f f e c t i s in percentage composition. Rather rough estimates indicate t ha t the dnrk s o i l s from Arno contain a t l e a s t 2,000,000 Ibs. organic matter per acre. 1t appears l i k e l y t ha t a feW centuries may have been required f o r such accumulation and there i s no evidence of any rapid de- c l ine i n the long occupied areas of Arno Island. In non-saline a t o l l s o i l s reaction commonly ranges betveen pH 7 and 9, with the h igherva lues presumably influenced by magnesium carbonate. Moderate but consistent s h i f t s in s o i l reaction a r e associated with the e f f ec t s of organic! matter. Species unsuited to an a l k a l k e substrate may be favored fur ther by localized pockets of organic matter, decaying vzood, e tc . , of lower reaction. A s l i gh t ly acid reaction also characterizes some pbats. Of the minerai nu t r ien ts phosphorus occupies a special position because of the probable significance of sea-birds in the phos?horus cycle of land areas, as ell a s i n numerous l & a l i z e d accumulatiozi~.' A poss ib le r o l e of phosphorus def ic iency i n t h e "laora" d i ' s t u l h i c e of coconut has keen conjectiirecl but ev-i- dence i s as y e t incomplete. An e f f e c t of coprzi cropping on phosphorus removal from the land has been suggested. Fosberg's st,udy of the eco log ica l f a c t o r s izivolved in phosphorus accunmla- t i o n may solve a number of ques t ions on phosphate "rock" and "hardpan", and perhaps on n i t rogen accumulation i n t h e dark s o i l s also. In add i t ion t o t h e i r e f f e c t s i n si1-m, such deposi t s n i g h t ??e l l be reworked, hotably enriching n w beach mater ia ls . Ekchangeahle ( o r ext rac table) potacisium tends t o incl-ease ni.th t h e exchmge capaci ty of o ~ g m i c mat ts r but it i s markeGy a f fec ted bzr leaching and. by con- t a c t v;ith s a l t nate?. Umer d r y c o n d i t i m s evaporeticn nay. concentrate potas- sium, a s wel l as n i t r a t e niLro&n, a t t h e s o i l sur face 'CLt t h e high values of such sanlples (e.g. F ik in i ) a r e n o t ind ica t ive of t h e e n t i r e s o i l . Of t h e o the r p l a n t n u t r i e n t s calcium and mabmesium a re , of course, abundcnt, end l i k t i e i s knovh of' t2e :,recis* s t a t u s of many "minor" e:.ernents. i r o n de f i c i ency i s colmon i n many p lan t s , and s~ynptoms on introduce2 ants suezes t $he i r s u s c e p t i b i l i t y t o oth.er de f i c i enc ies . ; Observations on s o i l organisms frola Arno A-toll show appreciable nurcbers m d t h e e x y c l e d ~ e l ~ a t i o p s h i p s between numbers and organic matter o n t s n t . Ce+,er- mination of s p e c i e s and groups (Lochhead, Nart in) ind ica te t h a t t h e fungal and b a c t e r i a l f l o r a a r e cosmopolitan. . . . Losses from lea.ch.ina cgn'be assumed except on t h e dr ies t . , a t o l l s . Some re- ccvery from t h e groundwater by vegeta:bion i s probnble but i s l i m i t e d by t h e l e v e l of rnaxi~num evaporationg (presumably about 60") and t h e s e l e c t i v i t y of p l a n t roo t s . A c o n t i ~ u i n g l o s s of caiciurii and magnesium carbonates i s obvious b u t v i s i b l e so lu t ion seems l i m i t e d t o t h e su r face layers . S o f t e n i n g and re- ductj.011 of l a r g a fragments i s marked i n t h e surface l a y e r s of t h e o l d e r s o i l s . Although t h e undisturbed well-drained type of p r o f i l e i s takein as a model, numerous exceptions and&~omol.ies occur as a r e s u l t of typhoons, shore l ine activi'zy, e t ? . Freshmiter and mengrove pea'ts, phosphate rook, exposad beach rock, e t c . provide numerous specia i izeZ s i t u a t i o n s f o r vege ta t iona l deveSop- ment. S-tudents of a t o l l s o i l s and ecology must add t o t h e i r i n t e r e s t of ie:eta- t i o n a l succession a recognit ion of. t h e f requent and d r a s t i c a f f e c t s of {ktastrophe, of tfii influence of p r imi t ive man and .his r ecen t congeners, and of t h e self- l imit in(: n a t u r e o f many formative process, , . . . . The economic aspects of a t o l l s o i l s a r e obvious, a.t l e e s t i n ou t l ine . Wth- i n the l i m i t s s e t by cijmate, groundwater and exposure ( a s t o spray) t h e hro- ductive c a p a h i l i t i e s and responsi.veness of s o i l s a r e s e t by t ex tu re , organic matter s t a t u s , phosphate i n f h e n c e s and perhaps by certa", effefec:s o f age o r h i s t o r y such as a r e associa ted v i t h th&.-"laoraf ' disease. Many of t h s s e a r e subjec t t o mapping, descr ip t ion o r i n t e r p r e t a t i o n f o r use i n any appra i sa l of a-toll l a d productivi ty. Figure 1.--Characteristics of Some Ato l l Trof i les ......................................................................................................................... Bacteria per gram x 100,000 Bacteria per gram x 10@00 Fungi per gram x 1000 Fungi per gram x 1000 iC C Total Nitrogen - % ...........-. ...... i Bacteria . . - -I,--+ _ . _ - - - - ............... ! C ! 1& .jC+ ! /I I i I I i f 24 1 B. Shioya sand ( ~ r n o ) ! Total Nitrogen - $ .... 45 ~ 3 L . 4 5 . . . .6.0 7 . - 9 k - . /' .+' + + g . . . . - . . . . . . . . . . . . . + OM + + ' , 4'. - . - Bacteria /-.----.> A . - :,4- - . - D. Brno loamy sand ( h m ) Figure 2.--S;-nthetic seq>Jence of s o i l deve1opnen.t and s o i l properties. - -- ............................................................................................... Shioya Shioya loamy Arni, loamy Brno gravelly sand sand send loamy sand Beach ! sand 1 I ! Prof i le VI, 27 GOC 1 Profi le 25 Prof i le 6 ~ ! m o Onotoo Arno Arne i ! ! Total .25 -, Nitro- gen - .so : % . 7 s i i- 15 inches