1: IlCCNT DEVELOPMENTS IN UREASE AND N ITRIFICA'I'ION INHIBITORS*

K. I., SAHRAWAT

I . J G : I I I ; ~ lio11~1 Crops Kcscarcl~ lustitutc for the Semi-Aritl 'Tropics, (ICIUS A?') I'dtancl~cru, Andhra Prodesh

ABSTRACT

T l ~ c cirrrci~t irlformation on the control of urcasc a c ~ ~ v i t y and nitrifrcatio~~ in soil by chemicals is rcvicwcd. A 1.rryc array of compounds have bcet~ propoeed as urcasc and rlitritication inhibitors. Compounds capable of retur- ding nilrillcotion aud urcasc activity arc available but lhcir uc in practical ~griculture is not very popular. I'hc yroblcr~l uppcars to be that the soil and cnviroon~cntal conditions that stimulates nitrificat~on and urcasc activity PCO also conductive for thc instability and ineffectivcncsi of urcasc and nitrification inhibitors. For inhibitors to hove potential value in practical ogriculturc, they must bc incxpcnsive in additition to bcing cffcc~ivc at rcilsonablc rates of application and environmentally safe. Future research irr necdcd to dcvclop con~pounds/matcrials to control urease activity and nitrification from rcsourccs iodigcnous to a rcgiou or a country.

Urea is k e mast knpxtiarut uitrogesr fwtirlizer *in WOFM agricul~urc. Ids use is stcladlly baas ing a d this t r ~ n d .is like- ly b awYthje, (15, 20, 29). The Nlst imipolrtarvt lwtcci'c of urea is W it ig a chenuioal nitrogen femizw wbsc ;tvcui.hbility 40 plands d&s vesy much m the activity of urtwse enzyme in sail.

In table soils, uraa is rlapidky converted lo c;pnmolliurn whm~a~te by mil urnasc, &A& results in swenal pl-ablems cn- cou,ntcJ.ed in Wle use of urea as a fertrilizer. ~~ itwlude in- orease 'in sail pH, ammunia d ~idlrit e cmcenhlttians and gas- erurs - .d N as ammb d oxides a£ nihogm (14,17,18, 18,24,49,62,'18). Chue approach Eor findnng solu.2;h to these p & m bets . ~ ~ ~ U i n g urea hydrolysis in mils by using

&lgd 'urt.ase irhibiffors' can velard urea hydrp- lw and : ( b r M y reduce w l w t ile lans of nir moll as ammolrh

0 . ' , o . - *ApprovedT lCRlSAT C. P. No. : 219..

I f I 0 4 1 , I I I l l ? i I i * , I ! i~lld .1111li 1 1 ~ L I 1 . 1 1 1 I ~ ~ I (y 1 0 yO(llll( secdJiugs (*10, G.)). ThC ~ W I U ~ O ~ U U I I I I'ut~ntad 1 1 . ~ 1 1 1 11r~'it in soiP is canverted to niCrulte via njitnile thr- I ; I I i t I N I I ~ B I J ~ , is s w e l l i Wc to hsn ihrow,tl lea- l I I t l I I ~ i l i i . TJlere is mnaidcl~blc irJorcut in coii.;tli vi i i~ i I 1 1 l-upc~i in the atmanitun bm by usjag c]lem.jculs oollccl '1li11 t i ( : dim idl~ibit01~' k t o m retard nitrifi~fllrion alld l-cclltcc ltc . o l ~ ~ i l t ~ ~ o g t i ~ in giti.l.ua4tio,na \Mhare 106s of nitrate v . i ~ lc ;lt611i, ;:: . ~ l u l tl~~nilnifimtion is wvore (40, 62, 76).

l'liu td~jeot~lve of t h i s paper is b review the current hx- 1 0 1 1 1 1 d i o ~ ~ L ~ U ~ ~ A ~ I I I ~ to the contlral of ur- aobivity and niitrifi- ci~1ii)ll jtl ~ 0 i 1 *by Ohemicals. FOP earlier reicramces m the to- pics. Ilic wilder Q refixred to the reviews by S&wwat (62) atld hftrl r larlcy aiul B r m m (40).

11 1Ci*::\Sli !:iC'l'I VITY IN SO1 LS

'L'he y ~ w s e ~ 9f ureriise in su& w w fimt mgge&x.i by l t c ! l ~ i l ~ ~u 1U35. Simmer, the poineering work by Cuiluad (12, 1;) lcfit JUO doubts tihr;Lt u r w e is rergpamiblc for m~~vcl;sion of U ~ L M to at , . +I tre@ed with urea Beaausc ureo is lhu mnwl =& 8 f@izar, ure& hqs meived much more resealdl cuuktlirul Wran all dher soil enzymes combined. Tllc r ~ u d e r is referred to excelled review by Bremner and Mudva~wy (a), which aomprehensively covers extensive lit era- t u r c ~*claR~ia~g to mil ulrwe.

U~~uasc aohivity in soils genemlly inoreases witlh mcrcac UA t 3 ~ c suWrate orrrllcd~~latb~~. i.e. urea WU the amount of urca is suffiuie~d to ~dtuzalte tihe enzyme. Resulk perkLlulg to the t*llcd af soil \vatcr candeal on urewse aobivity are mlllictung. I1 k ye~~utul ly found that umse activity in soils usually increa- x s with increase in wder oanlenl upto field capacity but Qr- !her h r c w c in w u k - oonknt m y mt influence or mlay eve11 cleoroase the tlotiviiy.

0 ,

+ Hcmmtly Scuhawrat (67) found that urease wtlivily hr some hudriran semiwid tropical mils 'increased wir& increase in teznrperu.Lwe f ~ o m 10 ta 60 or 70°C aPld Ulen dewwed with fudiler inor- in temjpawture @o 10o°C whew i d was nearly coxniplately W t e d (Fig, 1 laPld 2). Both buffer and m buffer mdhodrs af ur- asay gave 'srimiiar tread in resulk Theso re~~lts point to the pdecliot~~ of urewe in tlhese soils even at

1111111 l t - n r j z ~ d ~ UI-L'S ( t ; ~ - l ( t ) . Corllrol 01 urca r l y \ l l ~ l ~ r . ~ . r - - - such SUjl ~ i l u ~ l i w ~ s in the iroljic:, wl~tbrc sail tta~Tl' la$~l;rl~rt~:i nW ),igh will p s c a few p1u~blc.blcms in controllbim urtba i~ydrolysis

- :- en W) -0 kon buf fer

1 iy. 2. 1 . 1 l c ~ l O( ~CIIIJ)CI.I~UIC on urcnsc aclivily in Vcrtisol.

CONTROL OF U11W HYDROLYSIS IN SOILS

c ~ r m p u n d ~ b v e been proposed to rdwrd uroa hy- dn,yl& h mil (Tbble 1). In a reced study, Martens and Bre- rnner (40) found that p h e n y I p h ~ M ~ i & t y ! (PlPD) wws el- fedive in ratarcling urea hydrolysini to varying degrees in 15 &iveu'se sails. The inhibiiory effect of PPD an urea 'hydrolysis

. . - -- --- --. . - . . ..

I 1 1 1 I ' J 1 i t I : ( 1 ) ; Ill El i l l lCr i , l l t l I )LIIIII,II d l ~ ~ J l l ~ ' ~ l * . ~ ~ i l ~ ~ ~ l t ~ ~ ] l l ~ l l t ~ l ~ ~ ' ~ i i i ~ t l (19*/1), l ) ' l~Jy and ~ ~ ~ 1 ~ 1 " ~ ~ (1973b); IILII',, , ' ~ ' , l : a P I , , , , rMi5hr-d i111d 1:laig (1973), Mulv,t~~cy uod

llrcn~trcr (1978), blishrrr e i al. (1980). - ! J ' I t * j r 1 1 . j . ~ . . ~ ~ i ~ i d c i c c ~ M,lzrl et ul. (19711); Vlek el at. (1980);

I I I I V I I J i ; * l t \ ~~I~oro t !~ ,~ t~ i i t l ,~ t c Uyrr~cs et al. (1983); Kampfe 1(1')83); (1 '1 '1 ) 1 hlnr~tcns and Urcnlner (1984).

UKC~IIIICI. and L)ouglas (1971); gill^ 311il ,411.il1s (1969); ICuraalii ct (11. (1972); I'i~gl~ d n d Waid (1969 a, h), Wi~itl (1973).

8 1 . 1 '11 .14 .1 1 1 1 4 4 . 3 . I I I ~ s ~ ~ l ~ ~ l i l ~ l l c ~ l K i ~ ~ i ~ ~ ~ w s L y ~ 1 1 ~ 1 Q / ~ i l ~ ~ ( 1 1 ) S J ) ; Shill 111 C, I ;. and Haval (1961 9 .

(P. . ! I L I I I I , L 1.~11t~I11c c t ; i ~ ~ p ) i ~ ~ ~ t J ~ Mulvency and U r c n ~ ~ ~ c r (1'177).

S. Noli ctlil)lL. oil $r.cd crhzb arrd Ualasubratlraninn c l nl. (1972); tlicir' cor~stiluetlls, organic Fcrnu~ldo and Pnbcrts (1376). nlarltll.cs, rcsiducs and o r l ~ c ~ planis prorlu~lo

. hlisc'cllui~cous ~o~icyounds i111d Gel \,clli ct al. (197.5, 1976); tntllcrids (cl1cltriing con~pot~nds. Lcthbridge ant1 1iu1 nu ( 14)70); S,tlil iiwdt solwlrls, biurct, pesticirlcs ctc.). ( 1977); SitI,, i ~ ~ i i t ( 15 X a , 17. 19tiU;i);

I3rcnlner i111d I)ougla~ (1971). -.-- - . - ' - .- I- .----. _ - __ _ _ _ - _ - . --- _-.--

.*- - s ~ ) l l ~ ~ ~ ' ~ \ \ I ~ I ~ ~ I c ~ u i i ) t ~ i Sl i r :~i t i~~ ( l l ) $ O ,I).

i l ~ c ~ * ~ ; ~ s c t l \snihll the arl~olullt of PPD added and dt.clvascd nsrirk- c d y witti t,illlc aud ir~creasc hl tcrnpenatw-c 'frwn 10 10 40°C. The cffcwtlvt.rms of PPD to retard ulra hyrflrdpis was colnejri- tieu~ra~bly lower ab .30°C aold 40°C than at .lower 'temperalures and this 'hct slwrJd:b borne in milld while develaping compounds or rr~i\'~&-hls for J& ao urewe i&hibm 'in the 'Grqics. It was fouind thiiut tile ~~Ml'ilfy of RPD 10 r e h d urea 'hydrolysis w si&uLfirarutIy &&ed wilh organic C (r= -0.68**). (olnl N (r== -0.74**), CEC (r= -0.65**), sand .(r=O.GG'*), clay (r= -O.G4+9 and swke:'erea (r = -0.60.) but was not signjfi~mlly oarre!drd with pH, &,Y cudad, ur- aotivily or 'OaCO, equi- ~ I c w ! . ;bl&iptle regressiron analyses &owed #at the efleahive- n w of =I) 'to .r&d urea hyd'rolys'is in 90ih izxmwed with dmt*a& in : s d 1 ~ organic matter mnltent. SimUa~Jy, the eflecti-

v t b i i ~ ~ ~ ~ ; oS ~ ) - ~ l ~ t ~ ~ ~ x o ( ~ ~ ~ i i ~ x ~ ~ ~ t ~ ~ ~ ~ ~ c ~ ~ hy~lroqui~~~mic (1 1Q) f k ~ * I I ,i.rtw I I %ti ( h i vc r:ic f i t I * hrt. rxri IR UI:; r t l l i l l b - led by o ~ w n ~ i c mutter codeat, CEC, .u;l~,d, ?rill1 ancl u11i.1~ cot111*~111, t n i - ~ l r t c t b clii.trt ill~lrl ureilst: wki vit y o I ,wills. TIN? ~(tnerii l wu)d,lt - sion w i s llllrill lhc cfruct~ivcn~.:~ of IJUCJ alul I I(;) f(hr I c*l',~ 1.t I i r J t r H I of urea h y c b l p ' i s ill mils incscosed wiitt~ dmr0a.z. in sy: l ())*:?it-. nic n1atitt.1. mn!tc-r~~t (Mulvancy and Brcrnner) '(48).

I n ucldittion to sail propert:ies, penhap temperature is the most i s n p o ~ t m t factor that greatly affeds the efledivcn- o P ~ ~ r c u ~ s c inhibi torrs b r re Larding urea hydlqlysis.

Idc~restt in the use u l ura7.w i.nbibltcrrs y t m fwnl 1,1142 fioding t h a t rdiardatiion of urea hyrlrolp'is reduces thr pi.ohlems suah as vohadli,Ie 'lous of ammoniga assoclidcd witih the lwpid hy- drolysis 'and conwuniknt rise in sail pH, spx5aIrly in light-tex- tulred p n l y buffered mils (49'62). ' For exlam'plle, Bundy and Bremncr (7) dhowed t:hat tihe effective uraase inlmibHors such as suhsli lntud p-benmquinones redwed tihe volatiilr Loss of om- moniiw Irom 62.8% (in the conlt1.01) to 0.1!';1 from a r:indy so''\ ~e~~~bilizccl w:dJ1 urea. C o m p r e h c n s i v c review by Mulvdn(ey 1;111d B r e w s (49) suggests that several inorgan,k 'and or@;lruic coin-' pulrrls previously isohated ~Srorn microorgiand or p h b have Lllc a&~li,ty to reduce the gascuu& loss d urea as ammonia from sr>:ls when 011cy tare appbied in xlmlixture wish u:.".

Earllie,r work cover& by n U.S. (sew MuIva11ey and Ulunmer) (49) suggest& thl sm~n'oni a vohlt i himtion 40s f r m

: urca-irLaibcd sails aan be redwed by addition antirnalo'blic c0- ~~~popou~~rls. '3h.e caqmund i s when added to soils4 rdiard urea hy-

/ drolysis not by inhibit:ing ureve aotaivlly bull by inhibiting I pnrduoLion of uxense by rn ic roo rpa ru i sms . H o w e v e r , subseq uenf j st ucly with lhme an~tilncttnbabi~tes clearly dlowed t.hud t llesr* co-

mpuJlds (pyridine-3s~toruic a c i d , desWobiatlin and oxythia- mine c!hlorida) nailher roblrded urea hydrolysis rwr reduced 1 he vohti)e 1- of an-u-nonl from a sandy soil lcrtiliztrl with u,rea.

\ Hydloqdnm as however, found to be very elic~(ive i n re- : -ding urea hyddysis 8s well as in reducing nmmonb volilt3i- lizslion loss ~n>m t t ~ soil (47). 1

I

Anxm,g the fiaeton: that aReot ni*rificacJlhn ill soils, pH, moisture regime, tempctwtu~e and oxygen supply are intpor-

- - - . - - - . . -------- .-- A --. ---- -c - .-----_

1 2 3 - . - - -4--

-- - - K:II I I I J J \, I , I L C Grorind Ldrnnja accJs arc dchttsd Si~lgh (1966)

by extraction with boiling pctro- lcutli ctllzr untl tllc seed uscd for cvalualion.

K , I I , ~ I ~ ~ I I c . I ~ . , l'hc LCJ\*CJ arc d j ic~ i and ground Saf~rawat et a!. bcrorc use. (1974)

h I 1 I Thc ground seeds arc first dcfattcd Sahrawdt ct .I. with pctrolcun~. cthcr and tho (1974) tcsiduc (cake) is then exrr acted with boiling ethanol (95%). solvcnt removed to obtain alcohol extract, which is usod for rcstirrg without fui I~ICZ. pt~rificiition.

1 I 1 A l l Predl balk uf lhe tree is grottnd Salrrr,v.bl rl "1. i11lJ extrctcicd with 40: 60 (v lv) (1974) n~ixltrrc: of pcrrolcum cthcr : acctonc, aolvcnl ren~ovcd to ohti~in thc cxtract.

Kilrar~jitr, ii C~ir.;riio- Ka~a r~ j in , rr cr ystall~nc solid wrth S.IIII;IH,II ILrrmuLI T I ~ U I I L * I ~ I I ~ , ~ niulc~ular formulu C 1 a 1 1 1 ~ 0 ~ and (1973) seed chemically 3-n)clhoxy furatlo-2, 3, Sa111.awi1~ acid

7, 8-flavonc i s pi.cyarct1 f~.om Mukc~jcc (1977) kd r~n ja S C C ~ S . - .- --.- - - - - - - - -_

A dcioikcl study of s ' t rucl~re%it~oJl( i rn~ inhibil;io~l acli- vily \ M ; ~ P A k a t u n j i u l , a i-vonaid f m kar,anja seeds, 1us dablitihcd b a t ' (Jae,lfum ring in 4he molecule is css~ti~ti l krr

, . * r a 1 1 ,

41". aqi,$fty of'. ka~wjin (58, 64, 69). Iat vnas formd Uwt st+uc- . . . tyla a 1 r m . l ~ ' oi'lginapjin (jwynj ketone, kanicnjm.01) w h r e furan ling was present in.the molrrule showed wryjw deg~ces of inhibition of. ~~rlfiotulian' while in the w e of dihyd~m&mji~

- - we!'@ finan ring. was mtuxwted by hydrogenatioJ1, s h o w no i$vi!f (m . , 3)::. h l l ~ Up d~udlies 'with a number of ccnl-

- ~-

poirpds &vw !&,:fnulan m p ahched b either alkyl or aryl * i n g . , b e d t&ut, aC,jhis group imparts nitldfioa%n irrhibitcuy acti- vity '0 m&unds varying degrees (38, 70) mble 4).

Tnbld 3 : ElTccts of kai.anj in d its structural anrrlo~s on inhibition of nit1 1- ficiltiol~ in rr salttly I J sc)~:.. .

--- Cornporind

9 F\lrijn ing rn t(lr: ~ ~ ~ o l c c ~ r l e ':L rnl~rbiiir)n of nilrlficih\ion

p~cscnt of nl>svl\\ a t I S Ja>s - +-. - - - - - - -. -

Kit1 .IIIJIII l'rtbscn t 47

Xaranj hclt>fic I ' ICXCIII 4 4

Kar,~~rjonot I'rcscn\ ?,X

Drhy J IO hatavj~n Abi~11t U ____- -_ -- - - I --- -- _I - . _ I - - &Sot1 s .~~~ i i> l c s wcrc ~ ~ l c r ~ l ~ ; r i c . t l 11i.rlc~ .ICI ohic co~~ i{ i l~c . r~s a f i c ~ i ~ t l d i ~ ~ g 2 0 0 i)p111 01

1 1 1 ~ ; ~ N ,111d 5',',, (of N ,r~IJctl) 01- tllc c t~i~:p~luni ls sl)ccificd.

'I'ahlc 4 : I:llkcts of furiu~,;rl ;kr>rl fi111\1jlI alcolrol O I I ~~~iiil ic,riio~b 111 a h~ibdy clay icxtrn fcr tilircd with utc;~.

___I______ ___._ _ _ _-__ _ - - --- - ---..- _ _ _ _ -- ..&I__ ?'rc,~tnicnt Nrli ificallon ta lc (':;,I .~f(cr clays _ ___-.--

15 30 45

Urea+ fur fui,~l Urea t I ' I I ~ ~ I I } I alcoliol

- -. _ _ _ _ ___ - . ..---. .Soil ranlplcr wclc t r r u ~ r d u ~ t h 2W ~ P I I I of urc.1 N J I I ~ u ~ ~ h 11 s t rolnpoondr ut

t l ~ c r;llc or IO':;, of N addctl i r ~ i ~ l incub,~tc~l at 30 C' untlcr ircrol)~c contlitionS.

Fu.rther research is needed to devclcjp nilr~licilt:on inlri- biloxs from ind*igcnoua nualerials suoh as nm-cdi:ble oil s c n l cakcs (64, 74, 75, 80) and pbant prod~uots.

EFL'ECTS OF NITRIFICATION RETARDATION O N GASEOUS LOSS OF NIT1ZOCII;N

Comb~L91 and Ch~asncy (IF) and B d y and U r c n w ~ e r (8) Sound t b t ni~t~7if'loat'bn i,nhiblb:tors enhanced volatililu 1- of an- mmia from urea-fertilized soiL specially - urea was sur- fvcc appl$ied. Iihcld cvuluath of thnre T ~ B U H S in the presence a: growing p1en1L~ nee& to be invd&~tanod. Sane recent studin llave also shown h h a l retia&lim of niJrifiwtion reduces niatrom oxide cvoluliion firom soils fcrlilizcd with nilrifvlblc nitrogen sources (9). M c E m o n and Mi& (42) found chat nitro~pyrin signifiaantly reduced ixi4mus oxide emhisbians Imm ah: 1s IrrU- lized with ammonium or niiltmlte falm of nitrogen aml planted with sweet m in a 2-year field study. Probably nii~.npylrin reduced N20 evoluiim zismaiated with M h nitrifioalian oI am- mo~lium and deni4~ification of nilrate. Nit rapy r irl is reporled lo reduce &wen lorn by daxitriflcahan in si~uatiu~w: wilere readily oxidizable c a ~ h n substrate is available; for example in

In acbIii;lot~ lo cllc a b v c c f I w l s , nitrjfirnti;an imh&ilors niuy illso edwi~ce i m m o b i l i m t i o n of

nitwen: For ex*unple, 'Jun1ta iuld Pad (34) found that 4 % b l , 2, 4itrhzole (ACT), rilic;d,ion inhibitor enlhawed h & i x & i o n of 15N us well il~: alrcctcd ilk mbsqued release $wn ~ n - e x c ~ w e - ablc amn~on.ittm and r n i c ~ ~ b l b-.

CONCLUSIONS

1)cspLIu a @-wl dral uf rcnwrch part~ic~uhrly 011 nilrilica- Ukn Ld~ilrilors, a l ly a few mporwrds have bee11 adapted lor ~ g ~ l i c u l ~ u r a l use. For exanuple, ni~kapyrh is registered and upproved br we in the United Stlet= and dicqnandiamide h produced mld rrvarkeled iar use in Wes( Germany. The main y ~ . o b h sc-I to be (~i) Uw h a mst invalved irr lrhe develop- men4 iuul rcgid~ution d effrotive urease a d nitrification huhi- biters, ( i i ) the t m i a s of their use panticuhrly in low input ;~[~~cul(ure and (G) b, tad tibat (hey have given variable re- ~ A l t i . Tixis slhdd nd, however, deter research efbrts jn d c vebping effedive u r m and nitrfica~ticm ir&ibjtors from indi- ~ : L + I W ~ S luwurces so thraR their use is cxatmPnrical.

S m of the Indian work, where nan d~ible ail seed cakes . - ~

(pafi~U&rJy neem) hes been found useful in incl.m.ing the efiiciency of ler9iYzer N h m e si tuat im m o t be wahoUy YUributed b nitnifimth inhibition beQause immobilimtion e m - . . - ~

k d & w d e d in si4-1s, where these ca rhaceous ma- tuiab are dded in M e r ha& amounts. Use of nitri fiabion h l h i b b ~huuhd be rrmllned lo specific silualdans where they wdid red- hus of ni'brified N by l a a m a u j denarilicaition 4 rW&dim is nd affcoled by high ammo- ru ivn nil- (68). . ,

kxfardl om ease hhiihib~~ is gaining momentum be- of M ~ r d u6e of urea as N fertilizer and the need lo

hnd ,saluUms to m e p m b h esooided with the use of urea miculqrly anumaiq. toxicity b plank, genniaating seeds and

I .- Pnd :v~la(ile b ai amrnmh. Huwwer, the grmiryl b m urease W i t i o n owLd be oflsed by

1- ad urAa in soure &ulaAiians.

a I . Alox:~ildcr, h.1.. Introduclion to soil 11lic.robio1ogp. John \Vilcy and sons

Inc., New York, U. S. A. (1961). 2 . B.i l ;~subra~l~i(niai~ ;lA,. Siddara~n;ippa, R ; ~ n d Rn~iws\\~nrni. G I Plr~ l t Suil

37, 319-328 (1972).

3. I)r.cniilcr, J. M. a i d DIi~ck~llcr, hl. A. 111 D c l ~ i ( r i f i c ~ ~ i u ~ ~ , ~l i t r i f i~i~t inn a i d a1111osp11cric I I ~ I ~ O U S oxide. Dclivicl~c, C.. C. (Ed.) J O ~ I I I tYi1c.y h Sons Illc., Ncw York. pp. 151 -170 t 1981).

4. Urcmncr, J. M. nntl Doug\;~s, L. A., Soil D i d . Riochem. 3,297-307 (1971 1.

5. nrc~nncr . J. M. and Mulvancy, R. I.., Urcase activity in scils. In Soil

Enzyme,, Rurns, R. G. (Ed.). Ac;rdsnric; Press, London, pp. 149-196 (1978).

6. Uundy. 1,. G. and Brcna~er, J . M.. Soil Sci. Soc. Am. Pros. 37, 39f-398 (1973n).

7, UunJy, L. C j . allJ U~C"~IICI , J. b l . , Soil L)iol. I ) IUCIICI I~ . 5,847-85) (197)b)-

8. Uundy, I*. G . alld Urem~lcr, J. M., Suil Uiol. U i v c l ~ e ~ ~ ~ . 6, 360-376 (1974). - . 9. Byrncs, U. 11.. Savant. N. K.. and Cras~vcll, li. 7.. 1913 boil Sci. Sur.

Am. J. 47, 270-274 (1983).

10. Ccl.vclli, S., Naalipicri, P., Giov i l~~~ l i~ l i , G . and Pernil, I\., l'cstric. Iliucllc~ei. Physiol. 5, 221-225 (1975)- . .

I I . Cervclli, S. Nailllipicri, P., Giov;~i~i)i~li . G . 'i111d l'cr11i1, A., Weed 1 1 ~ ~ . 16. 365-36s (1976).

I?. Coilva J. J. P,, Soil Sci. 49, 253-2b3 (1940). 13. Conrad, J. P., Soil Sci. 54, 367-330 (1942). 11. Cookc, I, J., Nature 194, 1262-1263 (IgG',), 5 Cookc, G. W., Uilll Doc. No. 5 t r Stipcrpl\orphi\\e and

Compoutd Manukuxumrtr' Association, L'aris, pp. 1 - 13 (19601. 16. Cumlorth, 1. S. and Chust~cy, A. U., plan; Soil 39,491-501 (1911). 17. Coutt, M. N., S~cp l~cn , R. C. and WaU. J . S., Nature 9 1263-1265

(1962). 18. Courl, M. N., Stephetl, I ( . ('. and Woid, J . S.:U+*r+J. Soil. Sci. IS,

42-48 (19Urr).

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20. Enyclstod, 0. P. ~ I I J Clauck, 1

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