HUMUSAN PROO TACTIVITY. - Dr.L.K.Y/iersum- Institut e for SoilFertility ,Groningen .
r Introduction Theroo t system ofth ecommo nhighe r plantha s themai ntas ki n supplying itwit hwate r andminera l nutrients,whil e itma yals ob e concerned inth euptak e ofothe rdissolve d substances. Sincevo nLiebi g l)discarde d theolde rhumu stheor y the large array ofresult s obtained insolutio nculture s (e.g.Sach s 2), Hoagland 3), Gericlce 4), Hewitt 5)t Homes6) )an d evenmor e strictly thoseobtai ned,i nsteril eroo tmedi a (Gerrotsen7) ?Nilsso n 8))hav e established that onlywater ,carbo ndioxid e and anumbe r ofminera l elements are essentialt ogrowt ho f theplant .Th e soil,however ,i ngenera l contains avaryin g amount of organic matter fromfres hplan t and animalremain s to themor e or less stabile endproductso fdecomposition ,th ehumus .I t thus contains an enormousarra y of themos tvarie d organic substances, someconsistin g of small,easil y solublemolecules ,other so f large polymermolecule s and ofteninsolubl e inwater .A sresearc ho npermea bility ofcell san d tissues 9) ha.3 showntha t some of themediu m sized / organic molecules canpenetrat e into theplan t andresul tobtaine d with antibiotics havedemonstate d that substancesu p toa molecula rweigh t of about.60 0ca nente r theplan t (Winter 10)),i t iseviden t thatw e must reckonwit h theuptak e ofman y smaller soil-contained organic substances. Thequestio nno warises :ar e these substances tob e considered as noxious,inoccuou s orfavorabl e forgrowt h ofth e plant.Partl y they mightb eo f importance asaccessor y nutrient and partly the possibility of-a stimulating activity onroo t growthan dmetaboli c activityexists . It isth e latter aspect ofth e influence ofhumu st owhic hattentio nwil l bomainl y drawn.I norde r tob emor e concise the term humuswil lb euse d tódesignat e themor e or lessstabile ,brow ncoloure d end-products of decomposition.Al l otherno nminera l substances inth e soil,withou t regard to their stage ofdecompositio nwil lb edesignate d organicmatter . Effectso nroo t growthan dbranchin g Inorde r tob e able toextrac t a sufficient ora large amount of mineral nutrientfro m the soil peruni t of time theroo t system must have alarg e surface of cellswit h absorptive capacity inintimat econ tactwit h the substrate.Th e absorptive capacity ismainl yrestricte d to theyoun g parts of the roots,usuall yhavin groo t hairs.A s the amounto f soilwit h theyoun groot sar e inintimat e contact ingenera lwil lb e less than lfo ofth ewhol e soilvolum e occupied by theroots ,loca ldepletio n is likely to occur.S o theyoun groot shav e tob ekep t growing tob e able to tapne wnutrien t sources,especiall ywit hregar d to the lessmobil e ions. One ofth emai nfactor sgovernin g rootgrowt h inth e soil ismoistur e content.A s humusan d also organic matter can inbibe large quantities ofwater ,it soccurrenc ewil lresul t ina larger stock ofmoistur e inth e soil,although ;a considerabl e part ofthi swate r mayno tb e plant availa ble.Humu san d organic matter also favorably influence soilstructure . Thisals oresult s ina rais e inconten t ofavailabl ewater .Th econse quences ofdraugh twil l thusb e minimized as theroo twil lkee pgrowin g for a longer time.Als o indr y soil theroo tma yb e induced tosufceriza - tiono f the epidermal tissue 11), afac tb ywhich ,afte rremoistenin g of the soil,renewe d uptakewil lno t set inbefor e growthha sagai n started. - 2 -
As occurrence ofoxyge n isals o anecessar y prerequisitefo r growth thefavorabl e influence ofhumu so n soil structure (Deuel 12),Kononow a 13), Q,uastel 14))wil l inthi smanne r surely enhanceroo tgrowth . The soil inth efiel d isb yn omean shomogeneou s inregar d to its contents ofnutrients .Fertilit y differences usually occur-"betwee nsoi l layersa tdifferen t depths and eveni nth e same layers local differences innutrien t concentrationma y exist,e.g .granula rfertilizers ,place d fertilizers.S o itwoul d be importantfo r the plant if itsroot swer e able to takeful l advantage of thesedifference s by athoroug h exploita tiono f these layerso r spots.Thi s canonl yb e achieved bygrowt han dbran chingregulatio n inreactio n to stimulusfro m the substrate. Canw a imagine thathumu s plays arol e ininducin ggrowt hregulatio n ofroot si nconnectio nwit hfertilit y level? The answerraust b e inth e affirmative asgenera l experience innearl y all soils shows thatmos t rootsar eusall y tob efoun d inth e soilhorizon swit ha hig hhumu s content (Goedewaagen 15))«Becen t investigations concerning factors inducingroo t branching have shown thatcontac twit h peat enhanced formation of seconda ryroot s incompariso n to sand andresinou s Lan exchangers.The•explanatio n putfort h istha t thehig hconcentratio n ofadsorbe d ions inth e surface of thehumi c substance l6)act sa s stimulus (Wiersum17)) « Itwa s also demonstrated that of the separate ionsnitrat ewa s themos t active in inducingdens e branching.A shumu san d organic matterca nb e amai n source ofnitrat e asa resul t of their decomposition,thi swoul d be a second way inwhic h these substanceswoul d influence root distribution inth e soil. As densebranchin g and extensiongrowt ho f amai naxi sar e inversely related,a searching root striking afertil e spotwil lautomaticall y react ina manne r soa st o takeful l profit of thiscircumstance . Attentionwa s alreadydraw nt o thefac t of the innumerable organic substances occurring inth e soil,man yo f thosewit h smaller molecules should beabl e to penetrate into theroot .Thu s thequestio narise s if humusan d organic matter contain substances influencing extension growth andbranchin g ofroots . Inrecen t times increasing interest hasarise n inproblem s concerning thepossibilit y of afavorabl e influence ofhumu so nplan tgrowt han d crop yields.Som e of these investigations have beenconcerne dwit hroo tgrowth . Effect onroot s instimulatio n ofroot-hai rformatio nha sbee nmentione d byvo nZeschwit z l8)i nusin gwate r extracts of peat.A favorabl e influence onroo tgrowt h of soluble'humu san dNa-humat e ismentione d by Chizhevskii andDikusar « However,th e sameresult scoul d notb e obtained under sterile conditions 19).Humi c substances derived from shale and discarded material ofth eoi lindustr y ina concentratio no f 0.0004P and ahumi c acid ex tracted frombituminou s rock at aconcentratio n of0.004/ aincrease d the lengtho fwhea t rootsb y 100-200^accordin g toGuseinov ,Edigarov a and Kasimova 20).Accordin g toSouku p andMatou s 2l)humi c acidsfro m several leafmould s stimulated seedlingroot^growt hstronge r thanhumi c acids from peat.No t onlygrowt h inlengt hwa s influenced but also branching habit.Becen t own experience has taughtu s that increased rootgrowt h in theLepidiu m root testma yb e achieved byapplyin g humusextracts .Th e resultsvar y according to the type ofextrac t used -fulvi c orhumi c acids- anddegre e ofpurification .Boo t inhibitionma y alsob e observed.Sligh t indicationswer e obtained by addinghumu s extracts to themediu m in sterile root-tissue culture,tha t althoughgrowt h of themai naxi sma yb e retarded, branchingma yb e more profusive and increase indr ymatte r slightly enhanced. -3-
The«bviou sdifficult y inth e interpretationo f theseresult s isth e poor specificationo f theapplie d substances and their impurity,espe cially theusua l highminera l content. InEraunschweig/Vb'lkenrod eFlai gan d coworkers,however ,hav e"kee n testing synthetically produeed pure substances,whic h are considered to te analogous to intermediates inhumu s metabolism. Quite a»umbe r of ortho-ehinoneshav ebee n shownt o enhanceroo t growth ina cres sroo t test.Positiv e resultswer e also obtainedwit h «xy- anthrachinones,althoug ha fe wnegativ e resultswer e nsted.Thymo-hydro - chinone was tested insteril e root-tissue cultures and enhanced root growth.22) ,23) ,24) ,25) . A stimulation ofroo tgrowt h should have consequencesregardin g nutrient uptake and growthb y the plant.Tha t thisca nindee d be the rase isshow ni nresult s obtained inhydroponics .Penningsfel d 26)mention s the advantageous effects of adding toth e inert substrate,whi« h indeed pr*motesroo t growth.Simila r experience hasals o teen«btaine delse where 27). We are stillfa rfro m a «ompleteunderstandin g of thevariable , th»ughmos t oftenadvantageous ,result s of these investigations.Flai g and coworkers28 )ar eactivel y engaged ininvestigatin g themetaboli c changes induced byadministratio n ofmode l suhstan*es. Part »f the observed favorable effectsma y"b erelate d to the chelate forming ability humus compounds possess.B ymean s of thisabilit y several minerals,especiall y themicro-element san d notablyFe ,ma y enter the plantmuc hmor e easily 29). Ironi s indispensable forr«o tgrowt h30) . Burströman dTulli n 3l)als o discuss therol e ofchelate s inroo tgrowth . Occurrence »f chelatingagent s inmanur e and other«rgani cmateria l is demonstrated byMille r andOhlrogg e?2) . A largemas so ffact s isaccumulatin g onth e influence ofvariou s •rganic substances onroo t growth.I ti seviden tfr« m criticalexperi ments thatman y substances may influence rootgrowt h either asaccess«r y nutrient ora s astimulatin g agent.Favorabl e affeets aregenerall y limited to specific concentrationrange s and excess isofte n twxie.Othe r sub stances aredefinitel y deleterious.I tha s alsobecom e increasingly »lear that the organic matter inth e soil,bein gmainl yderive d from plant remains,consist s ofa ninnumerou s number of substances.Mos t ofthese , however,onl yoccu r inminut e amounts asthe yar e continuously consumed andmetabolize d by the soilmicro-organisms . To summarise anumbe r offact s atabulatio nwil lb egive n of sub stances,whic h inpurel y physiological researchhav ebee n shownt o in fluence rootgrowth .A comparison canthe nb emad ewit h substances,whos e occurrence insoi lha sbee ndemonstrated . -4-
Table 1.A comparison of substances influencingroo tgrowt han d their occurrence in the soil. physiologically active found in soilan d humus chelating ability saccharose sugars h acetic acid h,o citric acid h formic acid o lactic acid h,o + 1 malato malic acid h + 1 oxalic acid 8 succinic acid 0 tartrate m tartaric acid 1 aspartic acid a aspartic acid a,n glutamic acid Ó glutamic acid a,n alanine alanine a,n arginine a,n asparagine n someamin o acids cysteine n 1 glycine glycine a histidine n leucine n lysine n tryptophan tryptophan a valine n adenine d adenine r 2,6-diaminopurine d guanine d guanine hypoxanthine d Bj_ thiamine 0 B2nicotini c acid e (some vitamins) h B6 pyridoxine j (possibility of C ascorbic acid .1 formationb y folic acid f microbes) b indole acetic acid e (formationb y bacteriaan d fungi) c,p actidone P actidone P azaserme P azaserme P polymyxin P polymyxin P + see references The conclusion canb edraw n that ina soilric hi norgani c matter or soilt owhic hmanur e orcompos t hasbee nadde d theroo t systemma yb e specifically influenced.Som e substancesma y effect theroot s asacces sorynutrient ,other sma y influence uptake ofmino r elementsvi a chelating ability and some,notabl y anumbe r excreted bybacteri a andfungi ,coul d act asregulatin ghormones . Of coursemos t substances beingdiscusse d here canwor knoxiou s as well,dependin g onth econcentratio n inwhic h they occur.Bu t practical experience,suc ha s that presented byKortleve n 33),seem st odemonstrit o that enrichment of the soilwit h organic matter has specific advantageous effects.A part of this effect canb erelate d to influences as just des cribed,althoug h it isb y nomean s quantitatively assessed. -5 -
Effects onrat e ofuptak e Thefeedin g power of tlioroo t system isfo r apar t governed by its extensionan d ramification.With. 'a wel ldevelope d root system a thorough exploitation of the soilca nb e achieved if the conditions inth e medium arefavorable .Bu t uptakeb y theroo t isno t onlygoverne d by its surface but.als ob y therat e of intake.Th erat e ofuptak ewil l partlyb e governed by therat e of supplyfro m the soil,a facto r onwhic hhumu san d organic substancesdefinitel y have afavorabl e influence.Bu t this effectwil l hardlyb e considered here.Bu t if the supply isno t limitingrat e of nutrientuptak e isregulate d by the intensity ofmetabolism ,a saccumula tionca nonl yb e achieved under expenditure of energy.Eas e of transport inside theplan tma y alsob e of importance. Inmuc h of the literature the resultsmentione d are only those con cerning theoveral l end-effect onplan tgrowt h oryiel d and this isespe cially thecas e inth e older investigations,e.g .Camero n 34)an d also Bottomley 35)» Stillw ema yusuall y conclude that enhanced root activity was implied. Inrecen t times several investigators havebee nconcerne d with in fluence onmetabolism , especially respiration.Christew a 36)give s a summary ofresult s and explanations in1953 « Bothhumi c acid andbitumi nousproduct sar econsidere d to enhance oxidation processes.Fo r anex cellentrevie w ofal lRussia nwor k on humusreferenc emus tb emad e to therecen t book (195$)b yKononow a 37)«A sGuminsk i et al. 38)fin d a reduced oxygen consumption of excisedroot swhe nhumr.t cha sbee nadded , theyconclud e that the humate partly replaces oxygenuse d inroo trespi ration. Intensive researchha sbc ?n conducted inVölkenrode .Saalbac h 39) concludes that themode l substances tented enhance tyrosinase activity of theroot ,an d also that enzymes sucha saldolase ,amylase ,sacchara3 0 and phosphatase are stimulated.Effect s onminera luptak e are alsore ported.Differen t kinds ofhumi c acidsal l induce ahighe rN-content , whileK-conten tma yb e smaller.Th e effect on phosphate isvariable , althoughgenerall yfavorable .Result s obtained byBlanche t 40)» however, clearlydemonstrat e increased uptake forN , P,S andK .Humu s ispostu lated to enhanceK-absorptio no nroo t colloids inth efirs t phase ofup take.Bu t ina later stage of theuptak e accummulationi sstimulate d as well.Par t of the effect isals o ascribed to enhanced permeability.Th e influence ofhumu s extracts inincreasin g permeability had'alread y pre viouslybee ndescribe d byChaminad e andBlanche t 41) • Further advances inresearc h of the influence onmetabolis m by sub stances considered as intermediates inth eformatio n ofhumu shav e been summarized byFlai g 42). That the substances occurring inhumu s extracts do indeed penetrate into the root is stressedb yAleshi nan dTyuneev a43) . Both inth eRussia nan d German investigations themai n effect ofth e active substances isrelate d to the chinoid groups (Flaig 42), Guminski 44)) One of thedifficultie s encountered inth e explanation of theobser ved effects istha tman y of the active humic substanceshav e chelating ability.Saalbac h45 )alread y mentions theresemblanc e of some effectt o those obtained under amor efavorabl eFe-suppl yan dresearc hb yd eKoe k46 ) points to the same possibilities. Ingenera l the advantageous result3 of applying humâtest o roots haveha d themos t attention,bu t a scanning of the literature soonshow s that inman y casesdecrease d activity hasbee nmentione d aswell .Mos t substances tested have aver y low optimal concentration in stimulating activity andma ybecom e noxious athig hconcentrations .Man y secondary factors are al30 of influence and preliminary experience already learnt us that theresult s expected are not always achieved. - 6
Theroo t as centre ofmetaboli c and of synthetic activityha s in later timesreooive d morerecognitio n (Mothes47),Kursano v 48))»Als o we have becomemuc hmor e aware of thefac t that numerous organic substan cesca nente r theroo t (seeth e previousdat a concerningroo t growth). The organic matter inth e soilan d excretionproduct s ofth e soilmicro bes canthu sals ob e expected to influence root activity toa greate r or smaller extent.Som e effectsma ybe,favorable ,other sdefinitel y noxious (Patrickan dKoc h49)) . To conclude ourremark s attentionma yb edraw n toanothe rpossibi lity.Additio nt o the soilo f largeamount s ofdecomposabl e organic matter will surely inincrease d microbial andfaunisti c activity inth e soil. Itma yb e conceived thata sa resul t toxic excretion products, produced by the plantsthenselve s 50)» will thenals o be easilydecomposed .Thi s indirect activity mightals oac tfavorabl y insom ecases . Summary Anattemp tha sbee nmad e togiv ea concis e review concerning our knowledgewit hregard s to someway s inwhic hhumu san d organic matferma y influenceroo t performance innutrien tuptake . As soil exploitationdepend s ontota lactiv eroo t surface and soil volume occupied,attentio nha sfirs t beendirecte d toroo tgrowt han d branching.Bot h true humuscompound s andman yorgani o substancesfoun d in the soilhav ebee nfoun d able to exercise stimulating influence.I ngene ral acausa l explanation ofth emanne r inwhic h the effect isbrough t about is still impossible.Increase d penetration ofmicro-element sa sa result of chelation,hormon e activity,direc t influence onmetabolis m and use asa naccessor y supply ofnutrien tma yal lb emean s to effect the result.Furthe rresearc hwil lhav e toprovid e abette r insight,whic h can onlyb e achieved inworkin gwit h purified substances andunde r strictly defined conditions. Althoughman yfavorabl e effectshav ebee ndemonstrate d itmus tb e clear,tha t these areusuall yobtaine d ina certai nrang e ofconcentra tionsan dunde r certainconditions .Advers e effects are no exception. The soil;alsoma y containman ynoxiou s substancesa swel l init s organic matter. The performance of theroo t system also depends onth erat e ofuptak e of ions.Substance s oforgani c nature occurring inth e soilma y enhance ease of penetration of someminera l ions,e.g .chelatio no fFe .Man y other substances can induce enhanced metabolic activity,whic hresult s in' anincrease'i nenerg yabl e to effect accumulation. Although thenorma lhighe r plantsca nthriv ewhe nbein gfe d with mineralnutrient s onlyan d organic substances are inn owa y essential,ye t infarmin gan dhorticultur e much importance isattache d to them.Beside s the supply ofminera lnutrient s theycarr yan dwhic h isfree d during the mineralization process,humu san d organic matter havebee nshow nt oposses s specific stimulating capacity.Thes efact sma y partly explain some of the favorable influences attributed to ahig hhumu s content,althoug h influon- coorolate d to inprovenonto f physical coilfnctor a crover yimportant .
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