Rt penetratin prfiles f wheat and barley as affected by sil penetratin resistance in field cnditins Daniel L. Martinl and Carl F' Shaykewich Department f Sil Science, IJniversity f Manitba, Winnipeg, Manitba, Canada R3T 2N2' Received 28 April 1992, accepted 15 December 1993' Martin, D. L. and Shaykewich, C. F. 1994. Rt penetratin prfiles f wheat and barley as affected by sil penetratin resistance in field cnditins. Can. J. Sil Sci. 74: igz-zoo. e stuoy was cnducted n three Manitba sils (Marquette heavy clay, Frtier silty clay lam, and Suris lamy sand) with the bjectives f: (a) assessing the effects f cntrasting tillagesystems n varius sil physical prperties; and O) ielating rt penetiatin f cereals t these prperties. The zer tillage () and cnventinal tif^fale (Cf) treatments hadbeen esta'blished between 3 (Marquette and Suris) and 1 (Frtier) years prir t the initiatin f the stridy. Penetrmeter resistance (PR), bulk density (BD) and pre size distributin (PSD) were measured at varius depths and iimes during tw grwing,"urnr. The effects f tillage systems n sil prperties were generally small, particularly in the prly strucirred S-uris l6amy sand. PR in the tp 1 cm f sil tended t be higher under than^under ^C1.. pr uarleo mart<eoty *ittr time and was clsely related t changes in iil water cntent. The prprtin f macrpres ( > 1^1m in diameter) near the sil surface tended t be higher under ihan under. There was n evidence f any detrimenlal effect f pr aeratin n the final rt penetratin p.ftl.r f wheat. The prprtin f rts penetrating the sil was inversely relatec t ir. Sil strength critical fr rt penetratin was determined t be 2 MPa and was independent f sil type. In mst situatins' hwever, rts were able t grw ini sil with mechanical impedance greater than 2 MPa, pssibly by making use f bichannels and spatial and tempral hetergeneity in the sil stmcture. Key wrds: Wheat, barley, zer tillage, cnventinal tillage, rt penetratin, penetratin resistance Martin, D. L. et Shaykewich, C. F. 1994. prfils de p6n6tratin des racines du bl6 et de I'rge seln la rdsistance du sl ir la pen6tratin.n.nditirm r6elles au champ. Can. J. Sil Sci. 74: lg3-zffi. Une 6tude a t6rdabs e sur tris sls du Manitba (argile lurde Marquette, lam limn-argileui Frtier et lam sableux Suris) dans le but de: a) 6valuer les effets de systbmes de travail du sl diff6renis sur diverses pr-pri6t6s physiques du sl; et b) 6tablir Ia relatin entre ces prpri6tds et la -p6n6tratin des racines des c6r6ales. Les rdgimes iemis direit iso) et travail classique (TC) 6taient install6s depuis tris (Marquette.et Suris) et dix ans (Frtier) avant*la mise en place de l'6tude. Des mesures di la r6sistance au p6ndtrmbtre (r'p.) de la densit6 apparente (d.a.) et de la r6partitin des calibres des pres (r.c.p.) 6taient prises?r diverses.prfndeurs et ir diverses 6pques duiant deux saisns de v6g3tatin. Les effets du r6gime de trav;il sur les prpri6t6s du sl 6taient, dans I'ensemble, de peu d'imprtance, particulidrerient dans le sable lameui Suris, peu structur6. Lar.p. dans les dix premiers cm du sl paraissait plus frte en r6gime SD qu'en r6gime. La r.p. fluctuait de tagn marqu6e en fnctin du temps et, en plus, elle 6tait 6tritement re1i6e auihangement de la teneur.n "uuiu sl. La prprtin de macrpres (> 1 pm de diambtre) )r prximit6 de la surface du sl 6tait plus grande en r6gime SD qu'en TC. ie manque d'a6ratin du sl ne semble avir eu aucun effet n6gatif sur les prfils flnaux de p6i6tratin des-racines Ou Ute. f-a prprtin des racines p6n6trant da11 le sl 6tait inversement reli6e d la r.p. La valeur critique de r6sistance du sl ir la p6n6tritin des racines a 6td 6tablie d 2 MPa quel que sil le type de sl' Dans la plupart des situaiins, cependant, les racines ataient capables de pusser dans un sl pss6dant une imp6dance sup6rieu-re e 2 Mp;, viaisemblablement grdce ir leur aptitude d utiliser les bichenaui ainsi qu'i expliter I'h6t6rg6n6it6 spatiale et temprelle de la structure du sl. Mts cl6s: 816, rge, semis direct, travail classique du sl, p6n6tratin des racines, r6sistance d la p6n6tratin Sil physical cnditins are affected by tillage, and the effects vary accrding t the implements used, sil type and climate, and many ther factrs. Fur sil physical prperties directly affect crp prductivity: water availability, xygen diffusin, temperature and mechanical resistance (Letey 1985). The determinatin f ptimum levels f these variables is made difficult by their cmplex interrelatins and their variatin in time. Mechanical resistance ffered by sil t grwing plant rgans is a functin f the strength prperties f the sil and can be empirically measured as sil resistance t lcurrent address (D.L.M.): INIA La Estanzuela, CC 39173, Clnia 7 Uruguay. metal-cne penetratin. Plants may grw thrugh existing pres and channels large enugh t accmmdate rts (Wie.su- 1957), r by enlarging ther pres within the structure f the medium. It is in the latter case that strength prperties f sil becme imprtant. - The maximum pressure that rts can exert is restricted t abut.7-1.3 MPa in the axial directin and t.4-.6 MPa in the radial directin (Pfeffer (1893, cmpiled by Gill and Blt 1955), Misra et al. 1986). N grwth is expected when the resistance f the medium surpasses thse limits. Hwever, the actual prcess is much mre cmplex due t the prus nature andhetergeneity fsils. Althugh rt enlargement is the direct cnsequence f the axial Dressure, the frce in the radial directin als has a number 193 -
194 CANADIAN JOURNAL OF SOIL SCIENCE f imprtant functins: (a) radial enlargement f pres (Dexter 1987); @) tensile failure f the silahead f the rt tip, thus reducing pint resistance (Abdalla et al. 1969); (c) radial thickening causing increases in the ttal frce applied in the axial directin by expanding the crss-sectinal area (Barley et al. 1965); and (d) the skin frictin, which prvides anchrage t the axial frces and is an additinal cmpnent f the frce exerted (Stlzy and Barlev 196g). Sil strength, as measured by a penetrmeter, iras Ueen shwn t be uniqueiy indicative f resistance t rr peneratin thrugh a wide range f sil misture cntents and bulk densities (Taylr and Gardner 1963) and sil types (Taylr et al. 1966). Cnsiderable attentin has been siven t critical cne indices that cmpletely inhibit rt grwth in sils r artificial media. Fr a wide range f sil fypes, plant species and experimental techniques, values reprted fr critical penetratin resistance vary between 1.5 and 5.6 Mpa (Grimes et al. 1975; Gerard et al. 1982). Resistance t cne penetratin increases expnentially as sil dries, the effect being mre marked at high bulk densities (Ayers and Perumpral 1982). Since bth misture cntent and bulk density are widely variable in agricultural sils, it is expected that crp rts will nrmally be expsed t changing levels f mechanical resistance during a grwing seasn. Cne indices are usually higher in the tpsil under zer tillage than in cnventinal tillage @all and O,sullivan 1992). Very frequently the ppsite is bserved belw the tpsil wing t a plugh pan r a naturally cmpact hrizn (Ehlers et al. 1983). Tillage systems can als affect pre vlume and size distributrn. Under zer tillage the activity f the sil fauna is enhanced, as indicated by larger earthwrm ppulatins than thse under cnventinal tillage (Ehlers et ai. i9g3; Francis et al. 1987); the channels left by ld rts remain intact after they decmpse; and shrinkage cracks have been bserved t be mre persistent and deeper than thse under cnventinal tillage (Ellis and Barnes 198). All these factrs cntribute t frmatin f a predminantly vertical, cntinuus pre system (Chan and Mead 1988; Francis et al. 1988). Als. nntilled sils usually have reduced ttal prsity and retain mre water than tilled sils (Gantzer and Blake 198), ttrus increasins the likelihd f aeratin stresses. A study was cnducted during the 1989 and 199 grwing seasns n three Manitba sils representing a wide range in texture. Different tillage systems had been practised n these experimental sites fr a number f years. The srudv had the fllwing bjectives: (a) t identify and quantiff the effects f tillage systems n the sil mechanical resistance and sme ther physical prperties, and their spatial and tempral variability; and @) t relate thse effects t wheat r birley behaviur, in particular the final rt density prfiles. MATERIALS AND METHODS Experimental Sites Sunrs. A randmized cmplete blck experiment with fur replicates was established in 1987 n a-suris lamy sand near Brandn, Manitba (Legal descriptin -1-18W). In a split-plt design, tw tillage rreatments, cnventinal () and zer () tillage were applied t 11 by 17-m plts. cnsisted f ne pass with a heavy-duty cultivatr in fall plus tw such passes in spring with simultaneus harrwing every time. Fertilizer was incrprated in bth and, causing sme sil disturbance in plts. Spring wheat was seeded in 1989 and spring barley in 199. bth with 2-cm rw spacing. FRlr,n. A similarly randmized experiment with tw tillage treatments ( and ) was established in 1979 n a Frtier silty clay lam near Prtage la Prairie, Manitba. Plts were 15 by m. The experiment was under a cntinuus cereal rtatin frm 1982. Spring wheat was seeded in bth 1989 and 199. Since the tillage treatments at this site have been in existence cnsiderably lnger than at the ther tw sites, this site prvided the mst reliable data n effects f tillage system n sil physical prperties and their influence n crp grwth. MaRqurrre. A farm field n a Marquette Red River heavy clay lcated near Grsse Isle tiegal descriptin NW31-12-lE) was divided int tw 3.6-ha areas, with each area receiving a different tillage system starting in 1987. cnsisted f tw tandem-disc peratins in the fall plus a heavy-duty cultivatin and harrwing in the spring. T spread crp residues, received tw harrwings each year, causing sme disturbance t the sil surface. Spring wheat was seeded in bth years, and all measurements were dne adjacent t the bundary between treatments. Prcedures Penetratin resistance (PR) was measured by a hand-held recrding cne penetrmeter (Andersn et al. 198) t a depth f 52.5 cm in 3.5-cm increments. The number f replicatins per plt varied accrding t sampling date, and was between 16 and 24 (Suris), 2 and (Frtier), and 5 and 6 (Marquette). The cne used had an included semi-angle f 15' and a diameter f I2.8 mm. The penetratin rate was apprximately 1 mm s-'. The sil misture cntent was measured by drying sil samples taken at the same time as each PR measurement t cnstant mass at 11'C. At the Suris and Frtier sites, measurements were made three times in every seasn: at the fur-leaf stage, at mid-tillering and at crp maturity. At the Marquette site, PR at the fur-leaf stage was measured at pints I m apart alng a transect perpendicular t the brder between treatments in 1989, and at pints 2 m apart alng three transects in 199. The lcatin in the field f each measurement was recrded in these cases. PR was als measured at crp maturity in 5 randm pints within each sampling area in 1989 and 199. Bulk density (BD) t 6 cm depth in 15-cm increments was determined by the auger methd (Zwarich and Shaykewich 1969). Cylindrical hles apprximately 1 cm in diameter and 15 cm deep were made with an Iwan-type auger. Sil taken frm the hles was weighed and its misture cntent determined by ven drying t cnstant mass at 11"C. The diameter at the base f the hle was measured with a calier and the mean height with a ruler. The number f replicates
MARTINO AND SHAYKEWICH _ SOIL RES'SIANCE TO ROOT PENETRATION 195 sampled was fur in 1989 and tw in 199 per plt in the Suris and Frtier sils, at the fur-leaf, tillering and maturity crp stages. In the Marquette sil, eight and fur sites per plt were sampled at the mid+illering stage in 1989 and 199, respectively. the range, nugget and sill parameters were used t estimate the PR and its variance fr a 6-mr atea ar the bundary between the treatrnents by blck-kriging. Estimates frm the and treatments were cmpared, and the variance estimates were used as experimental errrs. Pre size distributin (PSD) was determined n undisturbed sil cres taken in cpper cylinders 4. cm in diameter and 1.5 cm in height frm a depth f 1 cm at the mid-tillering stage in 199 nly. Three samples per plt were taken at Frtier and Suris and 12 per plt at Marquette. The cres were placed n tensin plates and water cntents at 1, 3, 9, 27 ard 81 cm f water tensin were determined, crrespnding t 148, 493,164,55 and 18 pm pre radii, respectively. Rt penetratin was determined by the cre-break methd (Bdhm 1979). Sil cres 7.5 cm in diameter were taken at the end f the grwing seasn at each site, and the number f rts visible n the expsed faces after breaking the cres at the desired depth was recrded. Rt cunts were made at7.5-,22.5-,3'7.5-, and 52.5-cm depths. In the Frtier and Suris experiments, six sites per plt were sampled in 1989 and eight in 199. At the Marquette site the number f samples per plt was 12 and 15 in 1989 and 199, respectively. In 1989, samples were dried at 6"C fr 48 h and stred in plastic bags. Rt cunts were made 6-9 d later. In 199, cunts were made directly in the freld. As discussed by Bland (1989), the cre-break methd has its limitatins and prvides nly a "lw precisin estimate f rt length density". It was used in this study because it was the nly rapid and, therefre, feasible, methd available fr estimating rt penetratin. Statistical prcedures Data frm the Frtier and Suris sites were analyzed by cnventinal statistical prcedures. Where different depths were invlved, the mdel used fr analysis f variance crrespnded t a strip-plt design (G6mez and G6mez 1984), with depths being the nn-randmized surce f variatin. Due t the lack f experimental design, cnventinal statistical methds culd nt be emplyed t analyze the Marquette experiment. PR data at the furleaf stage was analyzed by gestatistical prcedures (Trangmar et al. 1985). Semivarigrams were prduced fr all depths crrespnding t the and areas and were fitted t a spherical mdel. When the mdel was statisticallv significant (P <.5), Bulk density (Mg --3) RESULTS AND DISCUSSION Sil Physical PrPerties The effects f tillage system n tpsil (-15 cm) bulk density and misture cntent at the beginning f the grwing seasn are shwn in Table 1. Data fr the Marquette sil are presented nly as a reference. N differences were bserved in the Suris lamy sand. Lack f differences in these tw sils may well have been due t the shrt time - 3 years in which the tillage treatments had been impsed' - On the Frtier silty clay lam, prsity tended t be lwer, and water cntent higher, under thanunder. Despite lwer prsity, had a higher prprtin f pres in the size class between and 1 m than did (Table 2). Penetrmeter Resistance On the Marquette hearry clay, PR at the beginning f the 1989 grwing seasn was spatially crrelated' The semivarigram iange varied between and 36 m, depending upn the depth (data nt shwn). Kriging estimates shwed that PR was higher under r:i.y at 1.5 cm depth (Fig. 1a). Mechanical impedance increased with depth, reaching levels ver 3 MPa belw - cm. Similar trends were bserved in 199, althugh n spatial crrelatin was detected (Fig. 1b). There are tw pssible reasns fr this: (a) the lag spacing used in 199 was 2 m, cmpared t 1 m in 1989, and (b) the measurements were mre cncentrated near the brder between treatments. This wuld have increased the difficulty f detecting any crrelatin, althugh the magnitude f the ranges bserved in 1989 (up t 36 m) suggests that a lag distance f 2 m shuld have been adequate. In additin, the values fr PR at the shallwest depths in 199 were substantially lwer than thse in 1989. This was prbably assciated with the higher sil misture cntent in 199 (Table 1). It is likety that the spatial crrelatin bserved in 1989 was actually due t spatial variability in misture cntent rather than t ther factrs gverning mechanical impedance. Variatins in micrrelief may have resulted in znes with varying misture cntent, particularly in relatively dry Table 1. Tpsil (-15 cm) bulk density, ttal prsity and vlumetric water cntent at the fur-leaf stage f the crps 1989 199 Z.87.88 Marquette C Frtier SiCL Suris LS.88.92 1. 1.6a 7.9a I-l2a 1.47 a 1.44a 1.45a.4la Prsity (%) Water cntent (% vllvl) 1989 199 1989 199 / 67 -) -l 36 z : cnventinal tillage, : zer tillage. a,bmeans fllwed by th- same r n letter within each year and site did nt differ significantly (P <.5). / 65 33 38 6a 58a 36a 32b 57b 56a 38a 37a 43a 45a Ila 17a 44a +a lia tr,a
196 CANADIAN JOURNAL OF SOIL SCIENCE Table 2. Pr size distributin (V f ttal vlume) at l cm depth in samples taken at the mid-tillering stage f the crps Marquette C Frtier SiCL Suris LS Pre size class (pm) Z > 1-1 1 1- -1 1 3.7 z. 2.8 8.6 3.6 2.5 3.5 9.4 z : cnventinal tillage, : zer tillage. a,bmeans fllwed by the same r n letter within each size class and.84.6b 1.3a 2.a.8a 2.a 1.8c 2.a site did nt differ significantly (P <.5). 3.a 1.5a 2.5a 13.a 4.5a I-3a 2.7a 73.2a 1 2A Ff,U O -c _ ) 1 2C 5. Peneiriin Resislnce (MPc).5 1. 1.5 2.O 2.5 _<. 3.5 f, *-=-,r,/ Q.. cr ZI a i aa 9) a? O(} a?t Fig. 1. Penetrmeter resistance prfiles at Marquette, fur leaf stage. (a) 1989. Kriging estimates frm and areas f treatment means (curves) and standard deviatins (hrizntal bars). (b) 199. Measured values. cnditins such as thse in 1989. Varius reprts have shwn that the structural features which determine sil strength are usually independent at scales ver I m (O'Sullivan et al. 1987; Perfect et al. 199), while ranges f several meters have been reprted fr sil water cntent (Warrick et al. 1986). On the Frtier silty clay lam, PR in the tp 15 cm f sil was higher under than (Fig. 2). The reverse tended t ccur between 25 and cm, althugh the difference between tillage treatments was statisticall! significant nly at the 1.5-cm depth. The same behaviur was bserved in bth years, and these sil strength prfiles were similar t thse btained in the Marquette experiment. These results revealed the actin f wheel traffic and sil slumping at the surface fnn-tilled sil. In additin t the eventual presence f plugh pans under, the reduced subsurface strensth under may have been due t several factrs: increaled I 1 2 E)U T @ 1 2A 5 Penelriin Resisince (MP)..5 1. 1.5 2. 2.5 3. 3.5 <}}\=-- --'-\ -O-.\- + LSf 5% --Q r". e? cr?i QE *,a.q Q t? <x) <rc -l Fig. 2. Penetrmeter resistance prfiles at the Frtier site, fur leaf stage. (a) 1989, O) 199. sil misture cntent, aggregatin by natural agents (wettingdrying and freezing-thawing cycles) and the preservatin f bichannels. The latter was prbably an imprtant factr in this experiment, as suggested by the PSD data (Table 2). On the Suris sandy lam, n the ther hand, bth treatments shwed essentially the same behaviur with respect t sil mechanical impedance near the sil surface at the furleaf stage (Fig. 3). Differences ccurring deep in sil in 1989 were likely the result f differences in sil misture. The magnitude f the mechanical resistance in the tp 1 cm was lw cmpared t the ther tw sites. This was attributed t the single-grained structure f this sil, which wuld permit primary sil particles t be pushed aside easily as the cne prbe advanced. At greater depths, the verburden pressure, which is high in this sil due t the high bulk density caused by a large sand cntent, became larger, and the particles culd nt be easily displaced. In this case, the resistance t
MARTINO AND SHAYKEWICH _ SOIL FES'SIANCE TO ROOT PENETBATION 197 cl s _ O 1 2 Penetriin Resisince (MP)..5 1. 1.5 2.O 2.5 3. 3 5 4.O 4 5 2 I 5 Q. a Fig. 3. Penetrmeter resistance prfiles at the Suris site, fur leaf stage. (a) 1989, (b) 199. l E2 c OJU 5 Peneirtin Resistnce (MP)..5 1, 1.5 2.O 2.5 3. 3.5 A-..-a:.-.-a- \\ -n -^. \\A,^' \\ a*^- A l' I l\^' /l A^. \\ A \ \" A Refre A,^' l\a a,., \\ I'a /\ Ai Tr/ AA Befre Af ier.,"itr",,il. " rt l Fig. 4. Effect f a 32 mm rainfall n the penetratin resistance prfile under zer tillage at the Frtier site. cne penetratin depended upn sil cmpressibility, which, due t the lw prsity f the sandy sil, resulted in large resistances. In bth tillage systems, there was a sharp increase in PR between the 1- and 2-cm depths, resulting in values exceeding 2 MPa, which remained relatively cnstant with depth. Table 3, Relatinships between penetrmeter resistance and sil misture at the Frtier and Suris sites Depth Intercept Slpe Sil (MPa) (MPa _2 (cm) %-') Suris '7.5 15 22.5 37.5 45 52.5 7.5 15 22.5 3',1.5 45 52.5 2.1 4.9 2.8 3. 3.r 3.2 4.5 4.2.3 2.3 4.9 5.4 5.3 5.3 4.9 -.5.68* -.1 1.9* -.4.89* -.5.47 NS -.5.81 x -.5.73* -.9.68* -.9.49 NS -.1.1 NS -.6.8* -.23.72* -.26.8x -.27.94* -.25.89 E -.21.84* *Signifi cant at th e l% lev el f prbability ; NS, nt significant at the 1 % level f prbability. The decreases in subsurface strength under bserved at the ther tw sites were still nt evident by the furth year after treatments were established. There are tw pssible reasns fr this: (1) the pr cnditins fr rt grwth in this sil wuld cause bichannels t frm at a very slw rate, r nt at all; and (2) the lw clay and rganic matter cntents f this sil wuld limit natural aggregatin and wuld cause the cllapse f newly frmed aggregates and bichannels. Als, it may well be that tillage treatments need t be applied fr much lnger than 4 yr in rder t have any effect n physical prperties f this sil. As the grwing seasn prgressed and the sil became drier, PR increased in all six site-years, reaching values that likely reduced r prevented rt grwth. The effects f sil water are illustrated in the cmparisn f PR befre and after a32-mmrainfall n the Frtier silty clay lam (Fig. 4)' Since BD changed very little during the crp cycle, it was cncluded that all tempral variatins in PR were due t differences in sil water. Data frm all six samplings at each f the Frtier and Suris sites were pled and linear regressin equatins f penetratin resistance as a functin f sil water cntent calculated (Table 3). (Data frm the Marquette site were nt analyzedbecause the number f samples was t small, and because sil misture determinatins were nt made at the same place within plts as penetratin resistance measurements.) The stress induced by the penetrating prbe is cmplemented by the pre water pressure (in saturated sils) r Ly a fractin f it (in unsaturated sils) in causing the sil t fail (Barley and Greacen 1967). In a sandy sil, because f its relatively lw prsity, that fractin increases very rapidly as the sil is wetted, and this markedly reduces the fice necessary t penetrate the sil. Als, ne wuld expect a larger angle f internal frictin in the sandy lam sil than in the silty clay lam sil. This, alng with the reductin in the frictinal cmpnent f penetratin resistance as the sil water cntent increases, wuld have been the reasns
198 CANADIAN JOURNAL OF SOIL SC'E'VCE Table 4. Final rt cunts (cm-2) at different sil depths Marquette C Frtier SiCL Suris LS Sil depth (cm) z 1989-15 15- -45 45-6 199-15 15- -45 45-6 2. 1.9 1..7 1..4.1.2 2. L] ( 7)* ( 7) 1..8 (.3) (.3) 1.1.1 (.4) (.3) 1.2.7 ( 4) (.3) 1..7 r.4 1..9.7.5.2.3 1.9 2. (.4) ( 6) r.1 1.2 ( 5) ( 6) 1.1 1.1 ( 4) (.s).6.5 (.3) ( 3) -i- 1.5.1.2.2.6.6 (.3) ( 2).4.4 (. 2) ( 2).4.3 (.2) (. r).2.2 (.1) ( 1) z : cnventinal tillage, : zer trllage. I N data were btained n the Suris LS Cf in 19g9. xnumbers in parentheses are standard deviatin f rt cunts. Standard deviatin data fr 1989 are nt available. fr the higher regressin cefficients fr the Suris sandy lam sil than fr the Frtier silty clay lam sil. The sam-e principle wuld have applied t-sil iayers where prsity was_reduced by lading prcess such as the actin ftillage implements ( at 15 cm depth at the Frtier site; r slumping (deep hrizns f sil). penetratin bv rts wuld have depended primariiy n the existence f cntrnuus macrpres and cracks. Rt Penetratin At the Marquette site, rt prfiles were similar in bth years (Table 4). In general, rt cunts were slightly lwer in 19g9 than in 199, pssibly due t sme rt de;mpsitin durins the perid between sampling and cunting in the first r.urnl Hwever, in bth years, the treatment had higher rt cunts than, especially in the deeper samples. The 1w values fr the treatment at depths greater than cm may have been related t the high pr at 1 cm depth in this treaiment (Fig. 1). In additin, even rhush n dlfferences were registered in PSD (Table 2). shrinkage cracks were bserved (nt measured) t ccur mre frequently under, pssibly the result f mre extreme variatins in sil misture cntent. Thus, rts in the system apparently penetrated the sil alng these cracks reaching deeper depths than tn. At the Frtier site rt cunts were higher in 199 than in 1989 (Table 4), prbably as a cnseque-nce frt deteriratin during strage f the 1989 samples, as discussed abve. In bth years there were n significant differences between tillage treatmenrs (P <.5,;. tn bth tillage treatments there was n evident relatinship between pr and rt penetratin. The higher surface cmpactin under did nt translate int decreased rt penetratin. This suggested the existence f a cntinuus pre system r the presence f abundant bichannels and cracks thiugh which rbts may have grwn, despite high mechanical impedances in this treatment. Equally plausible explanatins fr the lack f relatinship between PR and rt penetratin are: (l) as pinted ut by Bland (1989), the cre-break methd prvides nly a "lw precisin estimate f rt length density", and thus may nt have been capable fdetecting differences, and(2) the large variance assciated with the rt measurement made it difficult t detect differences. If ne accepts the lack f differences in rt cunts between tillage treatments as real, then the results f the Frtier silty clay lam have sme significance. The tillage treatments have been established fr 1 yr and therefre shuld have reached an equilibrium cnditin. There were real differences in the penetratin resistance in the tp 15 cm, with the zer tillage treatment having a resistance as much as.5 MPa higher than cnventinai tillage (Fig. 2). Lack fdifferences init cunts suggests that, while the resistance t cne penetratin increased, an equally favrable envirnment fr rt grwth was maintained. This was likeiy due t develpment f a sil structure mre favrable t rt srwth. At the Suris site, the final rt cunt f wheat in 1989 was much higher than that f the barley crp in 199. Cnsidering the fact that sme rts may have deterirated in sample strage in 1989, it is likely that the real difference between years may have been even greater. Barley has been shwn t be mre sensitive t mechanical stress than wheat (Gss 1977), and this may have been the main reasn fr the bserved utcme. There was n significant effect f tillage treatment n hnal rt density. This was expected in view fthe lack ftillage effects n the varius sil physical prperties evaluated (Tables I and2). In general, rt cunt at this site was measurably lwer than at the ther tw sites, prbably because f the higher mechanical impedance bserved in this sil. Relatins Between Cne Penetratin Resistance and Final Rt Cunt Because mechanical impedance in all three sils varied widely bth temprally and spatially, it was difficult t assess hw plants in the field integrated such variatins in their respnse. The final rt cunt des nt prvide an indicatin f the timing f rt grwth in respnse t shrt-term variatins in mechanical impedance and aeratin status. In additin, averaging PR and rt cunt values btained fr a given treatment culd mask imprtant spatial variatins characteristic f these variables. The lack f a clear relatinship between rt cunt and PR in treatment averages may well have been due t the use f an inapprpriate scale f cmparisn. Since the lcatin f all samples taken in 199 was recrded, the relatinship culd be studied at a mre reduced scale. Fr every depth increment and site sampled in the field, the rati between rt cunt at a given depth and that at the depth immediately abve (relative rt density) was pltted as a functin f average PR between bth depths (Fig. 5). (As expected, a number f the values f relative rt density were greater than 1.. They amunted t 12 ut f78, 16 ut f285 and ut f 143 fr the Marquette, Frtier and Suris sites, respectively.) This resulted in a randm distributin fpints with a well-defined bundary t the left, which became evident when ver 5 pints frm all three sites were pled in the same plt (Fig.5d). The shape f this bundary
MARTINO AND SHAYKEWICH _ SOIL BESISIATVCE TO ROOT PENETRATION 199 a) l\ tni LJlv. vv : Ir + u " ; tls1 c) v"-vw ''I h v v _ wvw-' VEv vgv j jvtv,h*d t ;q9w 'w e v resembled very clsely the curve btained by Taylr et al. (1966) fr the relatinship between PR and the prprtin f cttn rts passing thrugh cres f a wide range f sils. The relative rt density seemed t be a sigmidal functin f PR, with the maximum effect ccurring between.8 and 1.1 MPa (Fig 5d). The critical sil strength, defined as the PR which cmpletely suppressed rt grwth, was slightly less than 2 MPa. This was in clse agreement with a number f studies using recnstructed sils (Taylr and Gardner 1963; Taylr etal. 1966; Camp and Lund 1968; Cckrft et al. 1969). Other authrs (Grimes et al.19'15; Gerard et al. 1982; Ehlers et al. 1983; Yapa et al. 1988; Vepraskas and Wagger 1989) have reprted higher values. Hwever, these latter studies were perfrmed under field cnditins, and the higher critical strength was usually attributed t the presence fbipres thrugh which rts penetrated, despite high sil matrix strength. The pints abve the bundary in Fig. 5d represent situatins in which rts grew in spite f mechanical stress. There are several ways, all f them based n tempral and/r spatial variability in the sil physical prperties, in which this culd have happened: (a) grwth thrugh bichannels and cracks which, because f their size r trtusity, were nt detected by cne penetrmeter measurements; (b) lateral grwth frm adjacent sil regins f lw strength; and (c) tempral decreases in sil strength, such as immediately after a rainfall. r' ffi. v -v Yv v v F\gJ7 v v "hh.t##a"' WF- "%"*"I Il" r{l \AI v{ V* w vw Penetrctin Reslstnce (MPc) v vv v-flvv -w lz 'v' -v v Jw v vvfl Fig. 5. Rati f rt cunts at a given depth ver that at the depth immediately abve, as related t penetrmeter resistance between bth depths in 199. (a) Marquette, (b) Frtier, (c) Suris, (d) all sites. vv An attempt was made t assess whether tillage systems r sil depths differed in the degree f dispersin frm the bundary, but n clear pattern was fund, prbably because: (a) the resulting number f pints in each class was t small t detect any effect; and (b) it was shwn that the effects f tillage systems n the sil physical prperties were relatively unimprtant, and likely less imprtant than the spatial variability within the treatments. Als, the data culd nt be related t PSD measurements, because the latter were nt made at the same sites at which PR and rt cunts were determined. The ability f rts t penetrate sil against a mechanical stress was bserved in all three sils (Fig. 5), and analysis f the dispersin f pints did nt reveal any differences between sil types. Despite high PR values and lw abslute rt cunt at Suris, relative rt densities at similar PR levels were nt less than in the ther tw sils. This suggested that the few rts which culd vercme high mechanical impedance in this sil, develped independently fpr levels, very likely thrugh ld rt channels. ACKNOWLEDGMENTS Financial supprt fr this study was prvided by the Canadian Internatinal Develpment Agency. Abdalla, A. M., Herriaratchi, D. R. P. and Reece' A. R. 1969. The mechanics f rt grwth in granular media. J. Agric' Eng. Res. 14: 236-248..v v rd
2OO CANADIAN JOURNAL OF SOIL SCIEIVCE Andersn, G., Pidgen, J. D., Spencer, H. B. and parks, R. 198. A new hand-held recrding penetrmeter fr sil studies. J. Sil Sci. 3l:27-296. Ayers, P. D. and Perumpral, J. V. f982. Misture and density effect n cne index. Trans. Am. Sc. Agric. Eng. 25: 1169-11'72. Ball, B. C. and O'Sullivan, M. F. 1982. Sil strength and crp emergence in direct-drilled and plughed cereal seedbeds in seven field experiments. J. Sil Sci. 33: 69-622. Barley, K. P., Farrell, D. A. and Greacen, E. L. 1965. The influence f sil strength n the penetratin f a lam by plant rts. Aust. J. Sil Res. 3: 69-79. Barley, K. P. and Grecean, E. L, 1967. Mechanical resistance as a sii factr influencing the grwth f rts and undergrund shts. Adv. Agrn. 19: 1-43. Bland, W. L. 1989. Estimating rt length densiry by the cre break methd. Sil Sci. Sc. Am. J. 53: 1595-1597. Biihm, W. 1979. Methds f studying rr systems. Eclgical studies. Vl. 33. Springer Verlag, Berlin, Germany. 188 pp. Camp, C. R. and Lund., Z. F. 1968. Effecr f mechanical impedance n cttn rt grwth. Trans. Am. Sc. Agric. Eng. 11:188-19. Chan, K. Y. and Mead, J. A. 1988. Surface physical prderries fa sandy lam sil under differenr rillage praitiies. Rust. l. Sit Res.26: 549-559. Cckrft, B., Barley, K. P. and Greacen, E. L. 1969. The penetratin fclays by fine prbes and rt tips. Aust. J. Sii Res. 7:333-348. Dexter, A. R. 1987. Cmpressin f sil arund rts. plant Sil 97: 1-6. Ehlers, W., Kiipke, U. Hesse, F. and Biihm, W. 1983. Penetratin resistance and rt grwth f ats in tilled and untilled less si1. Sil Tillage Res. 3: 26I-2i5. Ellis, F. B. and Barnes, B. T. 198. Grwth and develpment f rt systems f winter cereals grwn after different tillage methds including direct drilling. Plant Sil 55: 283-295. Francis, G. S., Camern, K. C. and Swift, R. S. 1987. Sil physical cnditins after 6 years fdirect drilling n cnventinal cultivatin n a silt lam sil in New Zealand. Aust. J. Sil Res. 25: 51'7-529. Francis, G. S., Camern, K. C. and Kemp, R, A. 1988. A cmparisn f sil prsity and slute leaching after six years fdirect drilling r cnventinal cultivatin. Aust. J. Sil Res. 26: 63'.1-649. Gantzer, C. J. and Blake, G. R. 1978. phvsical characteristics f LeSueur clay lam sil fliwing n-till and cnventinal tillage. Agrn. J. 7: 853-857. Gerard, C. J., Sextn, P. and Shaw, G. 1982. physical factrs influencing sil strength and rt grwth. Agrn. J. 74:875-879. Gill, W. R. and Blt, G. H. 1955. Pfeffer's studies f the rt grwth pressures exerted by plants. Agrn. J. 47: 166-168. Gmez, K. A. and Gmez, A. A. 1984. Statistical prcedures fr agricultural research. Jhn Wiley, Trnt, ON. 68 pp. Gss, M. J.1977. Effects f mechanical impedance n rt grwth in barley (Hrdeum vulgare L.). L Effects n the elngatin and branching f seminal rt axes. J. Exp. Bt. 28: 96-111. Grimes, D. W., Miller, R. J. and Wiley, P. L. 1975. Cttn and crn rt develpment in tw field sils f different strength characteristics. Agrn. J. 67: 519-523. Letey, J. 1985. Relatinship between sil prperties and crp prductin. Adv. Sil Scr. l:273-294. Misra, R. K., Dexter, A. R. and Alstn, A. M. 1986. Maximum axial and radial grwth pressures f plant rts. Plant Sil 95: 3t5-326. O'Sullivan, M. F., Dickinsn, J. W. and Campbell, D. J. 1987. Interpretatin and presentatin f cne resistance data in tillage and traffic studies. J. Sil Sci. 38: 137-148. Perfect, E., Grenevelt, P. H., Kay, B. D. and Grant, C. D. 199. Spatial variability f sil penetrmeter measurements at the messcpic scale. Sil Tillage Res. 16:257-271. Stlzy, L. H. and Barley, K. P. 1968. Mechanical resistance encuntered by rts entering cmpact sils. Sil Sci. 15: 297-1. Taylr, H. M. and Gardner, H. R. 1963. Penetratin f cttn seedling taprts as influenced by bulk density, misture cntent, and strength f sil. Sil Sci. 96: 153-156. Taylr, H. M., Rbersn, G. M. and Parker, J. J., Jr. 1966. Sil strength-rt penetratin relatins fr medium-t-carsetextured sil materials. Sil Sci. 12: 18-22. Trangmar, B. B., Yst, R. S. and Uehara, G. 1985. Applicatin f gestatistics t spatial studies f sil prperties. Adv. Agrn. 38: 45-94. Vepraskas, M. J. and Wagger, M. G. 1989. Cne index values diagnstic fwhere subsiling can increase crn rt grwth. Sil Sci. Sc. Am. J. 53: 1499-155. Warrick, A. W., Myers, D. E. and Nielsen, D. R. f986. Gestatistical methds applied t sil science. Pages 53-82 in A. Klute, ed. Methds f sil analysis. Part I, 2nd ed. Physical and mineralgical methds. Agrnmy N. 9. American Sciety f Agrnmy, Madisn, WI. Wiersum, L. K. 1957. The relatinship f the size and structural rigidity fpres t their penetratin by rts. Plant Sil 9: 75-85. Yapa, L. G. G., Frittn, D. D. and Willatt, S. T. 1988. Effect f sil strength n rt grwth under different water cnditins. Plant Sil 19: 9-19. Zwarich, M. A. and Shaykewich, C. F. 1969. An evaluatin f several methds f measurins bulk densitv f sils. Can. J. Sil Sci. 49: 241-245.