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AGRICULTURE & CONSERVATION . IAMES MERRYWEATHERSECRETSOFTHE SOILThe key to healthysoii is the vastweb of complexsymbiot ic elatonships.

HE GARDENER'SOY is k)straighten up after a da1'spent i l l i rg thc soil, Logc ntly nrassageis or her achingbackand heave saLisficdigh,surveling the prepared and, norv ready forplanting. The s'eeds havc all beencomposted nd tlreseedbeds ready oIrur tLrreln\ \er \ ^r lc, . ,d. qrralr )atr rfied, the farmer leartson tlte ficlcl gatcconfidrrt o{ rnaxirnumy'ield rom thecrop. n both cases fi lc i ' ield can bea..r r red h) t l re addi t i . , r r , f f r rL. l r r r r

26 r"",.g"n." f,iu ll5 N1&([/,\J,r ]00r,

and some rvell dlrectedpest control.W.r1 rr .t rrr, \ \dr) r !Jadd arr l l t i r rgelsel Becarrse onelhing is missing:tlrr tttitr.L,r' t lrt llrrdidtr in llJt] liunand biod iversitl ', and red uce theirnpactofpestsald diseaseo lrvelsanecosystem an olerate.Il fion earliest times, we hadunderstood horv soils rvorked andhad developedagriculture n sFnpa-thy rvith soil o.rganisrls, rve u'ouldnever have chosen to use the intcr-sive agricultural practices hat have

done so rruch harm t o soi ls andlandscapes. The plough rnakes lifeeasy,but it doesn't account fo r th eua1 plarr t r r .a lJ1 gr , , r r and l i l rngsoils funcrion, undisturbed. Tillagehas created he necessity or contlnualintervention and chemical assistance.Before humans, u.ho plougl.red theland arrd addqd frr t r l i . * r and pesl icides? Wild ecoslrstems are selfsustaining, rvith a social structurebased on co operative interaction-, . , t , -, , . , , -, -r . -. . . . 1,-. .

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SYMBIOSIS S A major driving forcefor everl living organism on the planet. All living creatures animals andplants, acteria,ungi and others areinlolved in a worldwide, multi layeredn-ebof co-operation. ymbiosiss notan occasional uriosity: it is absolutelye\er)\"here. It is usuall y deffned ast\\'o organisms combining for mutualhcnef i r a r a. t orer ' implr l lcat iorr .First, drop the idea of murual benefit.Prof i t nd o.r in s)mbiosrsan rar) i r rany tempnraland 'pat ia l dimension.Benefit to its componentscan be moreor lessequal, but it is frequently a one-sided affair, at least for a while, afteruhich benefit may swing to anothermember of the association;a previousconsumermight becomenet contrib-utor. Also, forget about involvement ofjust two organisms.Symbiosis cano(cur bet ! \eenany numbero[ organ-isms greater than one, at any scalefrom what we rnight consider o be anindivrdual o con inen -wlde organi-sation. Symbiosis s infinitely variable.The soil ro which trees,shrubs andherbs are attached contains at least50% of the biomass of any ecosysternand several times the number ofspecies found above ground. Intactsoil is the most diverse nteractivewebof interdependentorganismson larth,but it is solid, dark and impenerrable.Since we cannot look into soil to seewhat is going on and understand it,we discount its importance anddestroy ts very life as we removetreesand clear away all the other nuisancevegetat ion o lhat ue ma1 "p]oughthe fields and scatter he Sood seedonthe land. Sor l organirms. n mrndbogglingly complex interaction, support the above-ground flora and fooduebsuponwhich ue rel) or our e\ i .tence, whilst the same above-groundflora support soil communities.Vycorrhiua s a symbiot icassociation of a fungus and the roots of aplant. It is arguably the fundamentallife process on land. Its most usualfunction is to facilirate plants' supplyof phosphate. This essential nutrientgenerally occurs at low concentrationsin natural soils and is mosdy heldtightl) by soil particles, unavailable.\fr corririza provides the remedy. Forin:tance, rhe roots of Britain'sfavourire ll,ild flower, the bluebell(Hiilcinthoidrsonscripta) perate in anenlr r ,n lr iFrr rrhere pho\phaLe isavailable n soil solution at less than0.1 part per million. Bluebells annotsurvl \ ' ( f nc,n r rycorrhizal ,icr theirshort, thick rooLs are incapable ofexploring the soil for inaccessible

nutrients. EvolutioD and symbiosishave taken care of the problem.Bluebell roots are colonised by at leasteler n di l f r renr mycorrhi /d l lungi .nost of rvhich ar e unculturable,unidentifiableand nerv or unknown toscience.They range our beyond theroot \y\ tem, sorneof them gdlhcr ingotherwise inaccessiblephosphate onbehalfof their plant partners; n rcturnthry reteive carhohydrate, ha.r tfoodstuff they cannot produce them-selves.Around 500 million years ago,ancestral plants found phosphateacquisit ion uncomplicared in theirprimeval, aquatic habitat. t $-asnot soeasywhen, rootless, hey experimenled with life on land.They collaboratedwith fungi as mycorrhiza, whichenabled hem borh to live on land anddiver l r fy.rom rhe slar t . o operal i t - ,nu i t l r soi l ungi ua\ l l r r nonnalwa) ollife for land plants,and it still is for anestimated 90-95% of planrs in allecosystems n everycontinent.From the out'er , runrani ty ' rnter-vention sel in motion a series oldisasters or naturally self susraining,symbiotic communities. Yes, worms,rabbits and earthquakesall disturb thesoil, but only in isolated patches rharall the constituent speciescan rapidlyrecoloni 'e. l r is sor t of d i , turbanL isbuilt into soil ecological processesorit releasesocalisedbursts ofnutrients,which promote soil heterogeneityandc( os)slem biodirersir l . In len. i !etillage affects vast areas, repeatedlyexterminatingsoil organismsby expo-sure and also, n the caseof the fungi,nhich form delrcare,wide rangingnetworks, causing fragmentation.Destroy mycorrhizal fungi and,though some ca[ adjust, plants thatare dependentupon them will die.Thefungi rhemselves re not so adaprable:i f .eparared rom rheir p lanr". rheironly source of carbohydrate, hey can-not survive. Therefore, f you removethe fungi, plant populations suffer,and if you remove the plants, you killthe fungi. Whether you take he view-point of the planrs r rhe fungi , i l issymbiosis that keeps them alive andsymbiosis hat is disrupted by humans,who must share he consequences.MYCORRHIZAI PARTNERSHIPSANbe highly speci f i rand also rery specialised f Lomponenrs f a communirlare removed, community structuremay soon be compromised. Removealot of them, and community structurewill collapse. Ecosystem collapse isoccuring with the progressof defor-

estation and intensive agriculture, andl lot iuslbe!au\en i ld plant . aree,r terminated, but also because omplicatedwebs of symbiotic parrnership aredisrupted.Agricultural land, particularly n the'developed'r"'orld, is probably a worsestalting point for ecosystem restora-t ion than bare rock thanks rc)!ontaminal ion, and not ju ' t b1 pe. t r -cide residues.At Iow concentrations,soil phosphate s a nutrient in biolog-icdl rommunir ie\ thar tan recy, . le tefficiently. In agricultural circum-stances,where popularions of mycor-rhizal fungi are mpoverished, we findlve have to keep adding phospharerosoils so that crops will grow.f l rerefore. , rhen r is added n largequantities as an artificial fertiliser andis not utilised or recycled, phospharebecomes a pollutant. Only phosphatein soil solution is available to rootsunaided, and even then it does notflow in soil in the way that, say, itraresolution does. Excess becorrresanached to soil particles - stuck sothatunl) : ,pe( ia l istungi tan gat l rer t- or is out ofreach to the root system.Phosphate ich soils favour non myc

Few hingsmattermore o humancommunitieshan heir relations rththe sorl.The brology f sorl sof fun-da-ental .r 'porLdlceo The usta;n-d,oir)t ot l;{eon Larll- soil emdinsr l -e edst . ]de's looo.ndpemdps hemost abused, abitat n Earth,

R chard Bardgett Thehologyof SotL Aa5)

orrhizal plants, and therefore ecosys-tems become permanently changed,usually not for the betterWhen we add phosphate to crops,even he few tough, generalistmycorrhizal fungi that have survivedmechanical assaultare physiologicallyexcluded from roots by their plant

Rr\ur8"ncP . 2 ls Md(h Apr i l 20ob 27

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hosts,cutting off th e lasr remainsolt l rc plro.phatc (qLr\ i t iL,nrr \ i (F t l re)lorild receive free of charge in anintact natural communit),. Thereafter,plants must gather dreir own phosphate.The majority cannot, but somecan: pioneer, weed and crop species.Grassesare perhaps the n-rostsuccessfulplants in the world. They haverefined the highly branched, fibrousroot system so that a single plantmight have kilomerres of fine rootsthat can explore dre soil for phospiraterery ef f i t ient l l . Crasst-s an galherphosphate unaided. That is whatmakes them such good crop plants.When humans exterminate plant-support microbes, just adding rvatcrand fbrtiliser to disfurctional soilenables a grassmonoculture such aswheat, barley, ye, maize, sorghum orrice to thrive. For the same reason,other librous rooted crops, such aspotato, sunflower, flax, alfalfa and

soya,also seem o do well in monoculture as they too are able to gatherphosphate unaided.Many u.eeds and pioneer plantshave to be non mycorrhizal becauserheir life cycle is necessarily unpredicrable and they cannot rely onsymbiotic fungi arriving at the sametime as their seeds.They too havefibrous roots or alternative phosphateacquisition strategies. he most su ccessful veeds are in the non-mycor-rhizal cabbage, beet and goosefoorfamilies,r'hich also happen o includesome of the most amenablecrops -forexample, rape/canola, cabbageand itsmany variants, sugar beet, quinoa,green manure .Imagine the consequences or soilorganisms of non mycorrhizal plantcroPsover an entire season r more. Inrhe pa' t . t rup rurat i , ,n nd [al lorningassistedsoils to recoler from unsym-pathetic crops, but nowadays larmers

repeatedly plant rape or rape/u'heat,for instance,without resting he soil.This effectively deprives mycorrhizalfungi of carbohydrate for a year ormure, r ' r r r , r is latal or t l re-n. t ! \cwant biologically unctional soils,wemust str ive to maintain diverseplant-microbe soil communities,rcturning biodiversity ro agriculturalland.If we want to reconsrructnaturalisticecosystems,why not add mycorrhiza'w.henwe plant? If only it were thaleasy.Like plant seeds, mycorrhizalinoculants are available to purchase,but they contain only generalist,easilycultured fungi essentially ungal'weeds'- which rnight shou, benefitin simple cropping regimes,but dc)not represent the majority of mycorrhizal fungi found in the naturalworld, ol which most are unknownand marry that are known cannot becultivared.NATURE HAS PROVIDED us withfood production processesprovedover millions of years in collabora-tion \\,ith evolution. Yet \\,'echoosc LL)ignore, interfere u,'ith and damagethem,a roga r l i mak ng remedatronrvhtclLs er ,pen. i re n t ime, morrr l ,transport, raw materials, environmental distress nd common se nsethe basis of our agriculture, ratherthan co operation. It seems thatbefore we begin to grow food, wernust first deliberately make the landunf i t for thal purpu.r . HL,\ \ ( \cr , f $etook just t\r'o measures we couldeliminate the need for the toxic evilsof modern agriculture. he f irst is tocease crop monoculture and followth e mixed cropping methodsused nFa.tern rounlr ier ( rvhrch mimrcnature). The second is to reducel la8e o a pract icable.( ) i l - ronser\n8lrlntmum.

It would be rather ike'co-opting'ourselves nto existing symbioses, ollaborating $'-ith and assisring theorganisms hat support the plants thatprovide us lr,'ith fcrod.When lve cultivate he land, we can acknowledge rhenatural history and purpose of elegantnatural nechanisms such as mycor-rhiza, tailoring our methods to suitthem rather than exterminate thelrand then have o devisecrude alternatives to replace them. Symbiosis rl/illprosper Our environment lvill benefit.We will thrive. oTo discoyermors obout mycouHzo pleosevisit .4pine ediin0Brorvn cnveenlcss lotes,llustrotingow heyoung ootsysterns extmdedvidely y theectomycorrhizoiungus ith which t hdsbeennocuioted couRrEsy: Do.t(rred(r,rycorh'a.e(>

28 n" .u,gence io 215 March/Apr i l 1006