Soil bulk density and penetration resistance under different tillage and crop management systems, and their relationship with barley root growth

Transcription

1 Soil ulk density nd penetrtion resistnce under different tillge nd crop mngement systems, nd their reltionship with rley root growth $EVWUDFW Few studies hve een reported on the effect of fllow on physicl properties of soil nd the root growth of the following crop. It is known tht soil strength increses in the first few yers fter introduction. To detect whether this increse could ffect the root growth of rley nd whether fllow cn hve eneficil effect on the physicl ehviour of soil, ulk density nd penetrtion resistnce were mesured t different times in tillge experiment on two soils of contrsting depths. Soil A ws Fluventic Xerochrept of 12 cm depth nd Soil B ws Lithic Xeric Torriorthent of 3 cm depth. In Soil A three tillge systems were compred: susoil tillge, minimum tillge nd no-tillge. In Soil B only two were compred: minimum tillge nd no-tillge. Three field situtions were compred in oth soils: continuous crop, fllow, nd crop fter fllow. Grvimetric wter content, grvel content nd root length density were lso determined. Anlysis of covrince ws used to nlyse ulk density nd penetrtion resistnce, using s covriles grvimetric wter content nd grvel content for ulk density, nd grvimetric wter content nd ulk density for penetrtion resistnce. Bulk density rnged from.69 to 1.66 Mg m -3 in Soil A, nd from.67 to 1.46 Mg m -3 in Soil B. In Soil A, ulk density ws lower in the fllow nd crop fter fllow plots (1.26 Mg m -3 ) thn in the continuous crop plots (1.32 Mg m -3 ). In this soil, notillge showed the lrgest ulk densities (men of 1.34 Mg m -3 ), followed y minimum tillge (men of 1.27 Mg m -3 ), nd finlly susoil tillge (men of 1.22 Mg m -3 ), ccording to tillge intensity. In Soil B no differences were found etween field situtions or tillge systems. Lrger penetrtion resistnce (sometimes.5 to 1. MP) ws found in no-tillge thn in susoil tillge nd minimum tillge in oth soils soon fter tillge opertions. 59% of the penetrtion resistnce redings rnged from 1.3 to 3.7 MP, nd these vlues re reported to produce 5% to 1% reduction in root growth. However, root length density profiles sometimes showed greter vlues for no-tillge thn for the other tillge systems, reveling good soil condition for root growth under no-tillge. Therefore, there is n increse in soil strength under no-tillge in the first yers fter its introduction tht does not gretly ffect root growth in well-structured soils. Fllow reduces soil strength due to the effect of tillge nd nturl loosening fctors. This effect extends to the following crop..h\zrugv soil strength, ulk density, penetrtion resistnce, cone index, penetrometer, root growth, fllow, no-tillge. J. Lmpurlnés nd C. Cntero-Mrtínez. In preprtion. 57

2 ,QWURGXFWLRQ Interest in no-tillge is growing in the rinfed cerel cropping res of Spin due to the effective reduction in time nd costs which this tillge system llows. However, the concept "do not distur the soil" clshes strongly with most frmers, who for yers hve distured the soil to otin soft medium for etter crop growth. For the frmer, undistured soil seems to e hrder nd more resistnt to root penetrtion thn tilled soil. In fct, high soil strength hs een proved to reduce nd even to stop root growth. The most common vriles used to ssess soil strength in tillge studies re ulk density nd penetrometer resistnce. They re interrelted nd the use of only one of these vriles my led to misleding results (Cmpell nd Henshll, 1991). Bulk density is inversely relted to totl porosity (Crter nd Bll, 1993), which gives us n ide of the porous spce left in the soil for ir nd wter movement. The optiml ulk density for plnt growth is different for ech soil. In generl, less-thn-optiml ulk density (high porosity) leds to poor wter reltions, nd high ulk density (low porosity) reduces ertion nd increses penetrtion resistnce, limiting root growth (Cssel, 1982). Bulk density is relted to nturl soil chrcteristics such s texture, orgnic mtter, soil structure (Cssel, 1982; Chen HWDO, 1998) nd grvel content (Frnzen HWDO, 1994), nd vries over the yer due to the ction of severl processes: freezing nd thwing (Blevins HW DO, 1983; Unger, 1991), settling y desicction nd kinetic energy of rinfll (Cssel, 1982), nd loosening y root ction nd niml ctivity. Crop opertions, especilly tillge, my lso lter ulk soil density. One of the gols of tillge is to reduce ulk density (incresing soil porosity). This effect of tillge on ulk density is temporry, nd fter tillge the soil rpidly settles, recovering its former ulk density (Hernnz nd Girón, 1988; Cmpell nd Henshll, 1991; Frnzen HWDO, 1994; Frnzlueers HW DO, 1995). In the first yers of no-tillge, ulk density of the soil my increse due to the repeted psses of the trctor nd the lck of the loosening ction of tillge. Numerous experiments performed to compre no-tillge with other conservtion or more conventionl tillge systems hve given different results. In most of them, ulk density ws greter in no-tillge in the first 5 to 1 cm of soil (Ehlers, HW DO., 1983; Pelegrin HW DO, 1988; Rdclifee HWDO, 1988; Hmmel, 1989; Hill, 199; Cmpell nd Henshll, 1991; Grnt nd Lfond, 1993; Rhoton HW DO, 1993; Frnzen HW DO, 1994; Hurd HW DO, 1994; Frnzlueers HWDO, 1995; Unger nd Jones, 1998; Terügge nd Düring, 1999; Wnder nd Bollero, 1999). In others, no differences in ulk density were found etween tillge systems (McCll nd Army 1961; Cssel, 1982; Blevins HWDO, 1983; Burch HWDO, 1986; Blevins nd Frye, 1993; Tod HW DO, 1998; Arshd HW DO, 1999; Logsdon HW DO, 1999; Ferrers HW DO, 2; Logsdon nd Cmrdell, 2). In third group, ulk density even decresed under no-tillge (Morn HW DO, 1988; Pikul nd Ase, 1995; Edwrds, 1996; Crovetto, 1998), 58

3 especilly when n increse in orgnic mtter ws oserved in the first lyer of the soil (Edwrds, 1996; Crovetto, 1998). Owing to the progressive increse in ulk density fter tillge, the difference etween tillge nd no-tillge ecomes smller s the time since tillge increses. In some soils, porosity under no-tillge decreses in the first few yers until the soil recovers its nturl structure (Kinsell, 1995). The most importnt fctors ffecting penetrtion resistnce or the cone index of the soil re soil wter content nd ulk density (Cssel, 1982; Hmlin, 1985; Brdford, 1986; Klepper, 199; Cmpell nd O Sullivn, 1991; Unger nd Jones, 1998). Texture, orgnic mtter, prticle surfce roughness (Cssel, 1982, Cmpell nd O Sullivn, 1991) nd structure (Brdford, 1986; Cmpell nd O Sullivn, 1991) my lso produce different penetrtion resistnce in different soils or in different lyers of the sme soil. Penetrtion resistnce increses with depth due to the increse in shft friction (Brdford, 1986; Cmpell nd O Sullivn, 1991; Frnzen HW DO, 1994), nd vlues from the different depths re correlted with ech other (Stelluti HWDO, 1988; Cmpell nd O Sullivn, 1991). Ysin HWDO (1993) found cuic reltionship etween cone index nd depth. In severl studies compring tilled nd non-tilled soils, greter penetrtion resistnce ws found under no-tillge, especilly in the upper 1 cm (Ehlers HW DO, 1983; Rdclifee, 1988; Hmmel, 1989; Hill, 199; Pelegrin HWDO, 199; Ageng nd Mree, 1991; Grnt nd Lfond, 1993; López HW DO, 1996; Wnder nd Bollero, 1999; Ferrers HWDO, 2). Frnzen HWDO (1994) oserved significntly smller cone index vlues under no-tillge down to 1 cm soil depth due to mulching. As for ulk density, differences etween no-tillge nd more conventionl soil-disturing tillge methods re gret soon fter tillge opertions, ut fll quickly during the growing seson nd my dispper t the end (Pelegrin HW DO, 199; Frnzen HWDO, 1994; López HWDO, 1996). The tillge system ffects not only penetrtion resistnce ut lso its relted vriles: soil wter content nd ulk density. For this reson some reserchers hve tried to seprte the direct effect of tillge on cone index from its indirect effect through the effect on wter content nd ulk density in different wys in order to llow etter comprisons. Cmpell nd O Sullivn (1991), proposed mesuring t field cpcity nd simultneously mesuring ulk density. Busscher HWDO (1997) djusted different functions to correct cone index vlues from wter content. Others used nlysis of covrince to reduce the effect of wter content nd ulk density in the cone index comprisons (Ysin HW DO, 1993; Frnzen HW DO, 1994). After correction, the dependence of cone index on these vriles is reduced (Busscher HWDO, 1997). The most importnt physicl fctors ffecting root growth re porosity, mechnicl impednce, wter content nd soil structure (Klepper, 199; Gregory, 1994). In generl, root tips re unle to penetrte pores nrrower thn their dimeter (Tylor, 1983; Hmlin, 1985; Cmpell nd Henshll, 1991). They cn exert verticl pressure rnging from.7 to 2.5 MP, depending on crop species (Gregory, 1994). Bulk density vlues tht limit root growth 59

4 re dependnt on soil wter content (Pin HWDO, 1998) nd rnge etween 1.46 nd 1.9 Mg m-3 (Cmpell nd Henshll, 1991). Mechnicl impednce increses s ulk density increses nd wter content decreses (Ehlers HWDO, 1983). Penetrtion resistnce mesured with the penetrometer is usully 2 to 8 times greter thn tht ctully undergone y the root tip (Bengough, 1991; Atwell, 1993; Gregory, 1994), owing to the different wy in which roots nd proes penetrte the soil. However, it is well correlted with the soil strength perceived y roots in soils with reltive homogeneous mtrix (Atwell, 1993). Root growth decreses s penetrtion resistnce increses (Tylor, 1983; Atwell, 1993; Gregory, 1994), showing liner (Ehlers, 1983), inverse (Bengough, 1991; Atwell, 1993) or exponentil (Hmlin, 1985) reltionship. Penetrometer vlues greter thn 2 MP re generlly reported to produce significnt root growth reduction (Atwell, 1993). However, in well-structured soils or those in which iochnnels re preserved (s in non-tilled soils), roots continue to extend t greter penetrometer redings ecuse they cn grow in the interggregte spces (Ehlers, 1983; Tylor, 1983; Klepper, 199; Cmpell nd Henshll, 1991). Fllow hs een proved to ffect wter nd nitrogen lnces in the soil (French, 1978; McDonld nd Fischer, 1987). It lso seems resonle to hypothesise tht fllow cn hve n effect on soil strength ecuse greter humidity generlly encountered under fllow thn under cultivted soils my modify the restructuring process of the soil nd its iologicl ctivity. Better knowledge of this effect cn help to elucidte the est wy to perform fllow in the set-side fields forced y the Europen Union Agriculturl Policy. The ojective of this study ws to follow the evolution of ulk density nd penetrtion resistnce in the firsts few yers fter no-tillge ws estlished on fllow nd continuous crops of rley, nd to determine whether it hd limiting or eneficil effects on root growth. 6

5 DWHULDOVDQGPHWKRGV The experimentl fields of this study were locted in El Cnós, in the semirid re of the north-est Ero Vlley, Spin (men nnul precipittion of 44 mm), on two soils of contrsting depth tht re representtive of the soils in the re. The deep soil (Soil A) ws fine-lomy, mixed, mesic Fluventic Xerochrept (Villr, 1989) of 12 cm depth. The shllow soil (Soil B) ws lomy, mixed, clcreous, mesic, shllow Lithic Xeric Torriorthent of 3 cm depth. The two soils showed high grvel content, minly t the surfce ( 15). Some selected soil properties re shown in Tle 1. Tle 1 Selected properties for the lyers of Soil A (Villr, 1989) nd Soil B. Depth (cm) Orgnic Mtter (%) Equivlent CO 3 C (%) Texture USDA (%) Snd Silt Cly 6RLO$ RLO% The experiment ws designed s rndomised complete lock with four replictions. The plots (1 y 6 m in re) were rrnged in three contiguous strips. In the centrl strip, rley (+RUGHXP YXOJDUH L.) ws cropped every yer. Lterl strips were lterntively under fllow or cropped with rley ech yer. With this rrngement we investigted three field situtions: Continuous Crop (CC), Crop After Fllow (CAF) nd Fllow (F). Three tillge systems were compred in Soil A (susoil tillge, minimum tillge nd no-tillge), nd two in Soil B (minimum tillge nd no-tillge). Susoil Tillge () consisted of susoiler tilling t 4 cm depth in August, nd field cultivtor t 15 cm depth in Octoer (nd cultivtor in spring in the fllow plots if weeds were present). Minimum Tillge () consisted of field cultivtor working to depth of 15 cm efore sowing (nd in My in the fllow plots if weeds were present). No-Tillge () consisted of mintining the soil free of weeds y totl hericide sprying (2 l of 36% glyphoste [N-(phosphonomethyl)glycine] h -1 ) in Octoer, nd in spring if weeds were present. Rinfll nd temperture were monitored t wether sttion situted 25 m from the experimentl field. Bulk Density (BD) ws determined y tking two undistured soil cores from ech plot, from to 7 nd from 7 to 14 cm depth. We took the cores y hmmering into the ground stinless steel cutter edge cylinders 5 mm high nd 6 mm in dimeter ( cm 3 inner volume). The cores were stored nd trnsported in hermetic cns to determine the Grvimetric Wter Content (GWC). The cores were dried, weighed, nd wshed through 2 mm sieve to

6 determine the Grvel Content (GC). Bulk density of the fine soil (< 2 mm) ws clculted s (Mc-Mg)/(Vc-Vg)*1, where Mc nd Vc re the dry mss nd the volume of the soil core, nd Mg nd Vg re the mss nd the volume of the grvel. This is nother form of Russos s eqution (cited y Frnzen HW DO, 1994). Grvel content ws clculted s Mg/Vc to llow comprison with Frnzen s (1994) results. Grvimetric wter content ws otined from the fresh nd dry weights of the cores. Grvimetric rther thn volumetric wter content ws used ecuse volumetric wter content is ffected y ulk density tht t the sme time vries during penetrtion mesurements (Cmpell nd O Sullivn, 1991). To mesure Penetrtion Resistnce (PR) we used hnd-held penetrogrph (Stiok penetrogrph, EIJKELKAMP ) tht drws grph of the resistnce to penetrtion vs. depth to depth of 8 cm. The conicl point ws 1 cm 2 in re nd the point ngle ws 6º. The mesurement rnge ws to 5 MP. At ech mesurement time we otined two grphs per plot with totl of 8 replictions per tretment. Wter content nd root length density profiles were otined y tking soil cores etween rows with Edelmn or Riverside ugers (EIJKELKAMP ) t importnt developmentl stges of the rley: tillering, stem elongtion, nthesis, mturity nd hrvest. Additionl smples for wter content were tken t sowing nd during winter. In ech plot of Soil A, soil cores were tken from -25, 25-5, 5-75 nd 75-1 cm depth. In Soil B, the cores smpled the profile from to 1 nd 1 to 3 cm depth. The dtes on which tillge opertions nd ulk density nd penetrometer resistnce determintions were mde re shown in Tle 2. More detils out crop opertions nd root length density determintion cn e found in Lmpurlnés HWDO (2 nd 2). Sttisticl nlyses were ccomplished using SAS softwre, grouping the plots y their condition: continuous crop, crop fter fllow or fllow. The dt were nlysed s repeted mesures over time nd spce (Steel nd Torrie, 198; Gómez nd Gómez, 1984). Due to unequl cell size, this nlysis ws done s split-split plot (Littell HWDO, 1991) with tillge s min plot nd smpling dte nd depth s successive su-plots. For ulk density nlysis, we used grvimetric wter content nd grvel content of the smple s covriles. For penetrometer resistnce dt, we used grvimetric wter content nd ulk density of the nerest smpling time s covriles. Lest squre mens (corrected y the covriles) were used, nd differences for min effects nd interctions were tested with the PDIFF option of the LSMEA sttement (Littell HWDO, 1991). 62

7 Tle 2 Dtes of tillge opertions, Penetrtion Resistnce (PR) or Bulk Density (BD) mesurements, nd dys nd rinfll from the lst tillge opertion in Soil A nd Soil B. Dte Tillge opertion Vrile smpled From lst tillge Dys Rinfll (mm) 6RLO$ Susoiling of plots Cultivtor in nd plots Sowing PR (except fllow plots) Resowing BD Cultivtor in nd (fllow plots) PR (fllow plots) (34) (23) BD (fllow plots) (36) (23) Susoiling of plots BD PR Cultivtor in nd plots Sowing BD PR BD Cultivtor in nd fllow plots PR BD 235 (3) 41 (19) Susoiling of plots Cultivtor in nd plots Sowing Compctor roller in nd plots PR Resowing continuous crop plots PR BD RLO% Cultivtor in plots Sowing Resowing PR PR BD Cultivtor in fllow plots PR (fllow plots) (34) (23) BD (fllow plots) (36) (23) PR 335(181) 263(194) Cultivtor in plots Sowing BD PR BD Cultivtor in (fllow plots) PR BD 235 (3) 41 (19) Cultivtor in plots Cultivtor in plots Sowing Compctor roller PR PR BD , Susoil Tillge;, Minimum Tillge;, No-Tillge. Vlues in prenthesis re for fllow plots nd were computed from the spring tillge. 63

8 5HVXOWV 5DLQIDOO Dily rinfll nd tillge opertions re shown in Fig 1. The driest yer ws , with little winter nd spring rinfll. The wettest yers were nd , with high winter rinfll. In , spring rinfll ws lso high. In , rinfll etween the first nd second tillge opertions (142 mm) ws higher thn in (74 mm) or in (52 mm). On the other hnd, rinfll etween the second nd third tillge opertions ws higher in (294 mm) nd (275 mm) thn in (69 mm). Precipittion etween the third nd first tillge ws lso higher in (293 mm) thn in (122 mm). 5DLQIDOOPP T1 7 6 T1 5 4 T2 T3 T1 T2 T3 T2 3 2 T3 1 oct-94 fe-95 jun-95 oct-95 fe-96 jun-96 oct-96 fe-97 jun-97 Fig. 1. Dily rinfll nd tillge opertions during the experiment. (T1: Susoiler in susoil tillge plots; T2: Cultivtor in susoil nd minimum tillge plots; T3: Cultivtion in susoil nd minimum tillge fllow plots). %XON'HQVLW\%' 2.1. Soil A Preliminry results (Lmpurlnés nd Cntero-Mrtínez, 1996), showed significnt positive reltionship etween Grvimetric Wter Content (GWC) nd Bulk Density (BD). Working in soil with grvel lyer, Frnzen HWDO (1994) lso found negtive reltionship etween BD nd Grvel Content (GC). Therefore, the first step to nlyse our BD dt ws to investigte its reltionship with GWC nd GC. The generl regression of BD vs. GWC nd GC (Tle 3) ws significnt for oth vriles, with coefficients of.83 for GWC nd.135 for GC. In spite of this, the test of homogeneity of slopes (Littell HW DO, 1991) showed tht this reltionship ws different for ech strip: Continuous Crop (CC), Crop After Fllow (CAF) nd Fllow (F). For this reson, we performed seprte nlysis of covrince for ech strip, including s covriles GWC, 64

9 GC or oth, ccording to their significnce in the regression nlysis. Therefore, we use s covriles GWC nd GC for F, GWC for CC, nd neither for CAF. Tle 3 Coefficients of the generl nd y strip regressions of Bulk Density (BD) vs. Grvimetric Wter Content (GWC) nd Grvel Content (GC) for Soil A nd Soil B. Intercept GWC GC R 2 C.V. 6RLO$ Generl 1.15 ***.83 *** *.5 *** 13.1 Continuous crop (CC) 1.16 ***.9 * Crop fter fllow (CAF) 1.17 *** Fllow (F) 1.6 ***.148 *** ***.15 *** RLO% Generl 1.5 ***.652 *** ***.14 *** 12.3 Continuous crop (CC) 1.6 *** ***.15 ** 13.7 Crop fter fllow (CAF) 1.2 *** * Fllow (F).89 ***.152 *** ***.31 *** 11.6 C.V. Coefficient of Vrition. Significnce P<.1; * P<.5; ** P<.1; *** P<.1. As is shown in Tle 4, no trnsformtion ws necessry to meet the ssumptions of the ANOVA model. Mesured BDs rnged etween.69 nd 1.66 Mg m -3. Men BD ws similr for CAF nd F strips (1.26 nd 1.27 Mg m -3 ), nd greter for CC strip (1.32 Mg m -3 ). As expected, the most significnt fctors ffecting BD were DATE nd DEPTH (P<). The smllest BDs were found during the fll nd winter months (out 1.12 Mg m -3 ), nd the gretest in spring nd summer (1.36 Mg m -3 ). BD ws smller in the to 7 cm soil lyer (1.22 Mg m -3 ) thn in the 7 to 14 cm lyer (1.34 Mg m -3 ). The effect of tillge on BD ws significnt in the three strips (P<.3 for CC, P<.9 for CAF, nd P<.2 for F). BD ws greter, in generl, for (men of 1.34 Mg m -3 ), medium for (1.27 Mg m -3 ), nd smller for (1.22 Mg m -3 ), ccording to tillge intensity. The BD trends in Fig. 2 show tht over the three strips BD ws greter under in the to 7 cm lyer, nd smller under in the 7 to 14 cm lyer, though differences were more significnt in F, the only strip with significnt TILLxDEPTH interction (P<, Tle 4). The gretest difference etween tillge systems ws found in F in June 1996 from to 7 cm depth, 3 dys nd 19 mm fter the lst tillge opertion. 65

10 Tle 4 Bulk Density (BD, Mg m -3 ) nlysis of covrince nd LSMEA seprtion for the different field conditions. Soil A. Source of Vrition Continuous crop (CC) Crop fter fllow (CAF) Fllow (F) GWC GC TILL DATE TILLxDATE DEPTH TILLxDEPTH DATExDEPTH TILLxDATExDEPTH Model Pr>F MSE D.F. R-squre C.V. Trnsformtion TILL DATE Mr/95 My/95 Sep/95 Nov/95 Fe/96 Jun/96 Mr/97 DEPTH Unnecessry c.78 d Unnecessry Unnecessry 1.19 c cd 1.26 cd 1.16 d 1.16 d c GWC Grvimetric Wter Content (%). GC Grvel Content (Mg m -3 ). TILL Tillge system: Susoil Tillge (), Minimum Tillge (), No-Tillge (). DATE Dte on which mesurements were mde. DEPTH Depth of soil (cm). MSE Men Squre Error. D.F. Degrees of Freedom. Non-significnt t the.1 proility level. C.V. Coefficient of Vrition. 66

11 # "!, + * /. - ) ( # "!, + * /. - ) ( &KDSWHU,,, &RQWLQXRXVFURS&& &URSDIWHUIDOORZ&$) )DOORZ) oct-94 fe-95 jun-95 oct-95 fe-96 jun-96 oct-96 fe-97 jun oct-94 fe-95 jun-95 oct-95 fe-96 jun-96 oct-96 fe-97 jun oct-94 fe-95 jun-95 oct-95 fe-96 jun-96 oct-96 fe-97 jun-97 &' $ % oct-94 fe-95 jun-95 oct-95 fe-96 jun-96 oct-96 fe-97 jun oct-94 fe-95 jun-95 oct-95 fe-96 jun-96 oct-96 fe-97 jun oct-94 fe-95 jun-95 oct-95 fe-96 jun-96 oct-96 fe-97 jun-97 &' $ % Fig. 2. Bulk Density (BD) trends for three tillge systems: Susoil Tillge (), Minimum Tillge () nd No-Tillge (). Soil A. 67

12 2.2. Soil B The regression nlysis of BD vs. GWC nd GC found significnt regression coefficients (P<) for oth vriles (Tle 3), though the slopes were significntly different for ech strip, especilly for GWC. Accordingly, GC nd GWC were used s covriles in the nlysis of covrince of CC nd CAF strips, wheres only GC ws used in the F strip. The results of the nlysis of covrince (Tle 5) show tht neither TILL nor TILLxDEPTH interction were significnt in ny strip. BD rnged from.67 to 1.46 Mg m -3. Men BD for oth tillge systems ws 1.1 Mg m -3. Clerly, DEPTH hd significnt effect (P<), with men BD of 1.4 Mg m -3 from to 7 cm depth nd 1.17 Mg m -3 from 7 to 14 cm. For CC (P<.3) nd F (P<.2) strips, BD incresed with time (from 1.1 Mg m -3 in Mrch 95 to 1.2 in Mrch 97 for CC, nd from 1.11 to 1.25 Mg m -3 for F on the sme dtes), wheres for CAF it decresed (from 1.21 to 1.14 Mg m -3 ), though this decrese ws not sttisticlly significnt. Tle 5 Bulk Density (BD, Mg m -3 ) nlysis of covrince nd LSMEA seprtion for the different field conditions. Soil B. Source of Vrition Continuous crop (CC) Crop fter fllow (CAF) Fllow (F) GWC GC TILL DATE TILLxDATE DEPTH TILLxDEPTH DATExDEPTH TILLxDATExDEPTH Model Pr>F MSE D.F. R-squre C.V. Trnsformtion TILL DATE Mr/95 My/95 Sep/95 Fe/96 Jun/96 Mr/97 DEPTH Unnecessry c -.97 c 1.13 c GWC Grvimetric Wter Content (%). GC Grvel Content (Mg m -3 ). TILL Tillge system: Minimum Tillge (), No- Tillge (). DATE Dte on which mesurements were mde. DEPTH Depth of soil (cm) Unnecessry Unnecessry MSE Men Squre Error. D.F. Degrees of Freedom. Non-significnt t the.1 proility level. C.V. Coefficient of Vrition.

13 3HQHWUDWLRQ5HVLVWDQFH Soil A To nlyse PR we considered GWC nd BD s covriles. Both showed strongly significnt regression coefficients (-.92 for GWC nd 5.15 for BD, Tle 6). As coefficients were different in ech strip, seprte covrince nlysis ws performed for y strips. As is shown in Tle 7, though TILL fctor ws not significnt, TILLxDEPTH interction ws very significnt in the three strips (P<). TILLxDATExDEPTH interction ws lso significnt in the CAF (P<.2) nd F (P<.7) strips. Differences etween tillge systems were more evident in the first 2 cm depth, principlly on the following dtes (Fig. 3): My 1995 in the F strip, 34 dys fter cultivtor tillge (23 mm of ccumulted rinfll); Septemer 1995, 3 dys fter tillge (72 mm); nd Mrch 1997, 176 dys fter lst tillge (274 mm). On these dtes, hd PR tht ws.5 to 1 MP greter thn nd in the first 1 cm depth, especilly in the F strip. Also showed PR tht ws out 1 MP lower thn nd from 1 to 25 cm depth in Septemer 1995, especilly on the CAF nd F strips. The gretest differences etween CC nd CAF strips were found in the first 2 cm of soil. In this lyer, men PR ws greter for CC thn for CAF (.43 MP on,.27 on, nd.36 on ). Tle 6 Coefficients of the generl nd y strip regressions of Penetrtion Resistnce (PR) vs. Grvimetric Wter Content (GWC) nd Bulk Density (BD) for Soil A nd Soil B. Intercept GWC BD R 2 C.V. 6RLO$ Generl *** -.92 *** 5.15 ***.31 *** 4.8 Continuous crop (CC) -3.1 *** -.87 *** 5. ***.28 *** 39.4 Crop fter fllow (CAF) *** *** 5.18 ***.4 *** 42. Fllow (F) *** -.72 *** 5.9 ***.26 *** 4.8 6RLO% Generl 1.94 *** -.99 *** 1.89 ***.12 *** 5.8 Continuous crop (CC) 2.45 *** -.67 *** 1.12 ***.6 *** 48.4 Crop fter fllow (CAF) 1.16 *** -.17 *** 2.51 ***.23 *** 5.2 Fllow (F) 1.53 *** *** 2.34 ***.15 *** 51.9 C.V. Coefficient of Vrition. Significnce *** P<.1. 69

14 Tle 7 Penetrtion Resistnce (PR) nlysis of covrince for the different field conditions. Soil A. Source of Vrition Continuous crop (CC) Crop fter fllow (CAF) Fllow (F) GWC BD TILL DATE TILLxDATE DEPTH TILLxDEPTH DATExDEPTH TILLxDATExDEPTH Model Pr>F MSE D.F. R-squre C.V. Trnsformtion Unnecessry GWC Grvimetric Wter Content (%). GC Grvel Content (Mg m -3 ). TILL Tillge system. DATE Dte on which mesurements were mde. DEPTH Depth of soil (cm). MSE Men Squre Error. D.F. Degrees of Freedom. Non-significnt t the.1 proility level. C.V. Coefficient of Vrition Unnecessry Unnecessry 7

15 s rqp ~ } { z y x w v >= s rqp ~ } { z y x w v i h g f e d c K J I H G F E D >= s rqp ~ } { z y x w v K J I H G F E D C &KDSWHU,,, &RQWLQXRXVFURS&& &URSDIWHUIDOORZ&$) )DOORZ) on l m j k u p t YZ X V RW U T R S Q R <; : on m ƒ ` [ _ ]^ B = A u p t ˆn m j k P; O : N9 L M u p t B = A Fig. 3. Penetrtion resistnce (PR) profiles in the three strips t different times for the three tillge systems: Susoil Tillge (), Minimum Tillge () nd No-Tillge (). Soil A. 71

16 3.2. Soil B As in Soil A, GWC nd BD were used s covriles ecuse their regression coefficients were significntly different from zero (P<): -.99 for GWC nd 1.89 for BD (Tle 6), nd seprte covrince nlysis ws performed y strips. TILL, s min fctor, ws only significnt (P<.12) in the F strip (Tle 8). On the other hnd, TILLxDEPTH interction ws significnt in the three strips (P<.9 for CC, P< for CAF nd F strips). Fig. 4 shows PR profiles for Ferury 1995, Ferury 1996 nd Mrch 1997, 112, 119 nd 14 dys fter the lst tillge opertion respectively. In CC, PR incresed with yer from out 3 MP in 1994 to nerly 4 MP in The differences etween nd lso incresed from zero to 1 MP in the to 1 cm lyer for the sme period of time. On the CAF strip, PR incresed with yer nd the differences etween nd lso incresed from.5 MP in Ferury 1996 to 1 MP in Mrch In the F strip, oth men PR nd differences in PR etween tillge systems decresed with yer. The lrgest differences in PR etween nd, up to 2 MP, were found in the first 1 to 15 cm of soil in the F strip in My 1995 (34 dys fter tillge), nd June 1996 (3 dys fter tillge) (Fig. 5). In the first 2 cm of soil, PR ws greter in the CC thn in the CAF strip, with difference of 55 MP for nd 32 MP for. Tle 8 Penetrtion Resistnce (PR) nlysis of covrince for the different field conditions. Soil B. Source of Vrition Continuous crop Crop fter fllow Fllow GWC GC TILL DATE TILLxDATE DEPTH TILLxDEPTH DATExDEPTH TILLxDATExDEPTH Model Pr>F MSE D.F. R-squre C.V. Trnsformtion Unnecessry Unnecessry GWC Grvimetric Wter Content (%). GC Grvel Content (Mg m -3 ). TILL Tillge system. DATE Dte on which mesurements were mde. DEPTH Depth of soil (cm). MSE Men Squre Error. D.F. Degrees of Freedom. Non-significnt t the.1 proility level. C.V. Coefficient of Vrition Unnecessry 72

17 ª Œ Š µ ³ ² ± ª Œ Š š µ ³ ² ± Ÿ ž ª Œ Š š µ ³ ² ± Ÿ ž &KDSWHU,,, &RQWLQXRXVFURS&& &URSDIWHUIDOORZ&$) )DOORZ) 5 Ž « Ž Ž œ œ « « Fig. 4. Penetrtion Resistnce (PR) chnges in time for the three strips nd the two tillge systems: Minimum Tillge () nd No-Tillge (). Soil B. 73

18 º¹¼» ½ ¾ ÀÂÁÄà ÅÆÀÂÇ È Ò ÓÕÔ ÖØ» ½ Ù ÀÚÁÄà ÅÆÀÂÇ È Ñ Î Í ÏÐ Ì É ÊË Ñ Î Í ÏÐ Ì É ÊË Fig. 5. Two Penetrtion Resistnce (PR) profiles in the fllow strip showing the lrgest PR differences etween the tillge systems: Minimum Tillge () nd No-Tillge (). Soil B. 'LVFXVVLRQ )DOORZHIIHFWRQVRLOVWUHQJWK During fllow, tillge ws performed to control weeds in nd plots. This tillge lso hd loosening ction tht reduced BD nd PR in these plots. This effect extended to the following crop, s is shown y the lower BD nd PR found on the CAF strip thn on the CC strip. In plots lower soil strength ws lso found on the CAF strip thn on the CC strip. In these plots only nturl soil-loosening fctors, such s drying nd wetting cycles or fun ctivity, could reduce soil strength ecuse weeds were removed chemiclly. Strength reduction due to fllow in plots ws greter thn in plots of Soil A nd smller thn in plots of Soil B. This indictes tht in some situtions nturl fctors induced y fllow my e s effective s tillge in reducing soil strength. 7LOODJHHIIHFWVRQVRLOVWUHQJWK Bulk density on ws greter thn on tilled plots in the first 7 cm of Soil A, s hs lso een reported y numer of uthors (Pelegrin HW DO, 1988; Rdclifee HW DO, 1988; Hmmel, 1989; Hill, 199; Grnt nd Lfond, 1993; Rhoton HWDO, 1993; Frnzen HWDO, 1994; Hurd HW DO, 1994; Frnzlueers HW DO, 1995; Unger nd Jones, 1998; Terügge nd Düring, 1999; Wnder nd Bollero, 1999), nd incresed from 1.29 Mg m -3 in Mrch 1995 to 1.44 Mg m -3 in Mrch This effect ws especilly cler on fllow plots (Fig. 2). There could e two resons for these results. First, we mesure BD in the first five yers fter the chnge from conventionl tillge to no-tillge. According to Kinsell (1995), the soil ws in the trnsition or repir period in which it uilds humus, regins its structurl stility nd restores the pore spce. During this period there is first n increse in BD until mximum, nd then decrese due to the restructuring process, until n equilirium level is reched when the structure is fully restored. The second reson ws the low quntity of residues left on the soil (strw ws pcked nd removed fter hrvest), which delyed the increse in orgnic mtter nd the restructuring process of the soil. 74

19 Susoiling ws effective in reducing BD in depth (7 to 14 cm). Lrger differences would proly hve een found if BD hd een mesured t greter depths. In Soil B, with greter grvel content thn Soil A, no differences were found in BD etween tillge systems due to the structurl effect of grvel (Frnzen HW DO, 1994), which protected the soil ginst compction in oth nd plots. When significnt differences were found etween tillge systems in PR, plots under showed lrger PR in the first 1 to 2 cm of soil thn tilled plots, s hs lso een oserved y numer of uthors (Rdclifee, 1988; Hmmel, 1989; Hill, 199; Pelegrin HWDO, 199; Ageng nd Mree, 1991; Grnt nd Lfond, 1993; Frnzen et l, 1994; López HWDO, 1996; Wnder nd Bollero, 1999; Ferrers HW DO, 2). In Soil A, differences found in the first 2 cm depth were ccording to tillge intensity: lower PR for, medium for, nd higher for. In Mrch 1997, fr from tillge opertions, gret differences were found etween nd or on CC in Soil A (Fig. 3), nd etween nd on CC nd CAF in Soil B (Fig 4). This seems to indicte, s in the cse of BD, tht the soil under is in the trnsition period, when the soil strength increses. 6RLOVWUHQJWKDQGURRWJURZWK The mximum BDs recorded in Soil A were etween the criticl (1.67 Mg m -3 ) nd the non-limiting (1.46 Mg m -3 ) BD vlues for root growth stted y Pierce HW DO 1983 (cited y Godwin, 199). In Soil B, BD ws elow the non-limiting vlue. On the other hnd, 59% of the PR recorded in Soil A nd Soil B rnged from 1.3 to 3.7 MP. This PR vlues re reported to produce 5-1% reduction in elongtion rte for rley (Hds, 1997). On the other hnd, root length density profiles (Lmpurlnés HW DO, 2, Lmpurlnés HW DO, 2) do not denote wrong conditions for root growth in these soils ecuse the gretest root length densities were found on, the tillge tretment tht lso showed the gretest soil strength. In Soil A, under CC, the root profile in Ferury 1996 (Fig. 6-A) showed.7 cm cm -3 smller root length density (LV) for thn for or in the -3 cm depth lyer. BD (Fig. 2, continuous crop) ws lso significntly lrger for this tretment (.14 Mg m -3 ). In Septemer 1995, t the eginning of the root growth, PR ws more thn 1 MP higher for thn for or in the first 1 cm of soil (Fig. 3, continuous crop). High soil strength, s mesured y BD nd PR, could produce this result. On the other hnd, in the CAF strip in Ferury 1996 showed less root length density (Fig. 6-B), lthough BD in Ferury 1996 nd PR in 1995 were smller for this system thn for the other. Lower wter reltions in these conditions my hve reduced root growth (Cssel, 1982). In Soil B, showed consistently higher root length density thn in in the -1 cm lyer of the CAF strip (Lmpurlnés HWDO, 2, Fig. 9). PR ws lso consistently higher under t this depth (Fig. 4) nd my e responsile for the oserved root growth differences. 75

20 &KDSWHU,,, A) ÛÝÜ Þ Þ ßÚàáÜ âõãåäçæ æ¼è¼é æ êåë ìîíºâõï Ü âñðñí¼ðõòôóøàúí¼õ ö øçùäú û üîû üñý þ ÿ B) ÛÝÜ Þ Þ ßÚàØÜ âñãåäºæ¼æ¼è¼é æ êåë ìîàúí¼õ ï ßÚà ºÞ Þ íîö øâùîú û üäû üñý þ ÿ LSD LSD.5 Fig. 6. Two Root Length Density (LV) profiles in the Continuous Crop (CC) nd Crop After Fllow (CAF) strips under three tillge systems: Susoil Tillge (), Minimum Tillge () nd No-Tillge (). Soil A. To quntify the reltionship etween soil strength nd root growth, multiple regression nlysis ws performed on ech soil (Tle 9), with the growth rte of the root length density s the response vrile nd GWC, BD nd PR s predictors. The results indicte positive nd very significnt reltionship etween the growth rte nd the GWC, negtive ut not significnt reltionship with BD, nd significnt reltionship with PR which ws negtive for Soil A nd positive for Soil B. The reltionship with GWC ws positive ecuse most PR dt were from the pre-nthesis period. The fct tht the reltionship with BD ws not significnt corroortes the ide tht BDs in these soils were elow the criticl rnge for root growth. Though significnt, oth regression coefficients found for PR were very low, indicting smll effect of PR on root growth. The positive regression coefficient found in Soil B my e due to the high grvel content in this soil, especilly in the first lyer where root length density is lso greter. Low regression coefficients my lso e cused y the high PR encountered in oth soils. Tle 9 Regression coefficients of the growth rte of the root length density vs. Grvimetric Wter Content (GWC), Bulk Density (BD) nd Penetrtion Resistnce (PR) for Soil A nd Soil B. Intercept GWV BD PR R 2 C.V. Soil A.2.6 ** *.12 *** 142 Soil B *** *.1 *** 212 C.V. Coefficient of Vrition. Significnce * P<.5; ** P<.1; *** P<.1. Severl fctors could contriute to these high PR redings. Firstly, the soil hs high grvel content. The grvel interfered with the penetrometer mesurements, incresing the vlues (Hmlin, 1985) nd the vrince of the PR redings (Cmpell nd O Sullivn, 1991). Secondly, these soils hve modertely high cly content (Tle 1) tht increses with depth nd leds to the formtion of strong columnr ggregtes (Villr, 1989) which increse the PR redings (Atwell, 1993). Finlly, the orgnic mtter ws reltively high for these semirid 76

21 soils (2-3% in the top lyer of the soil, Tle 1), which is lso reported to increse PR (Cmpell nd O Sullivn, 1991). The stress required to drive proe into compcted soil is four to eight times tht required for the roots to penetrte the soil (Bengough, 1991; Atwell, 1993), ecuse roots grow long the oundries etween the peds, therey voiding the resistnce to penetrtion of the ulk soil (Cmell nd Henskll, 1991; Atwell, 1993), s ws oserved y Villr (1989) in Soil A. The reduction in soil strength in the CAF strip due to fllow resulted in higher growth rtes of the root length density thn in the CC strip (Tle 1). This effect ws oserved in oth Soil A nd Soil B, demonstrting the fvourle effect tht fllow cn hve on root growth in the first stges of the crop. Tle 1 Growth rte of the root length density (cm cm -3 dy -1 ) in the pre-nthesis stges for three tillge systems (Susoil Tillge (), Minimum Tillge (), No Tillge ()) nd two field situtions (Continuous Crop (CC) nd Crop After Fllow (CAF)). Soil A nd Soil B. Soil A Soil B Field sitution Field sitution Tillge CC CAF HDQ CC CAF HDQ HDQ &RQFOXVLRQV Fllow hs een effective in reducing soil strength for the following crop. This effect is s importnt in tilled s in non-tilled fllows, indicting tht nturl loosening fctors my e s effective s tillge in reducing soil strength. After the introduction of no-tillge there is n increse in soil strength compred with tilled soils. In our well-structured soils, this increse in strength does not limit root growth ecuse roots cn grow etween the ggregtes. In grvely soils, the increse in strength is smller due to the structurl supporting effect of grvel. Differences in root growth due to the cropping (fllow or continuous crop) or tillge system were smll ecuse the soil strength do not rised limiting levels for root growth. 77

22 5HIHUHQFHV Ageng, G.A., Mree, P.C.J., Effect of tillge on some soil properties, plnt development nd yield of spring whet (7ULWLFXPDHVWLYXP L.) in stony soil. Soil Till. Res. 21, Arshd, M.A., Frnzlueers, A.J., Azooz, R.H., Components of surfce soil structure under conventionl nd no-tillge in northwestern Cnd. Soil Till. Res. 53, Atwell, B.J., Response of roots to mechnicl impednce. Environ. Exp. Bot. 33, Bengough, A.G., The penetrometer in reltion to mechnicl resistnce. In: Smith, K.A. nd Mullins, C.E. (Eds.), Soil nlysis. Physicl methods. Mrcel Dekker, New York, USA, pp Blke, G.R., Hrtge, K.H., Bulk density. In: Klute, A. (Ed.), Methods of soil nlysis, Prt 1. Physicl nd minerlogicl methods-agronomy monogrph Nº 9 (2 nd edition), ASA-SSSA, Mdison, USA, pp Blevins, R.L., Frye, W.W., Conservtion tillge: n ecologicl pproch to soil mngement. Adv. Agron. 51, Blevins, R.L., Smith, M.S., Thoms, G.W., Frye, W.W., Influence of conservtion tillge on soil properties. J. Soil nd Wter Cons. My-June, Brdford, J.M., Penetrility. In: Klute, A. (Ed.), Methods of soil nlysis, Prt 1. Physicl nd minerlogicl methods-agronomy monogrph Nº 9 (2 nd edition), ASA-SSSA, Mdison, USA, pp Burch, G.J., Mson, I.B., Fischer, R.A., Moore, I.D., Tillge effects on soils: physicl nd hydrulic responses to direct drilling t Lockrt, N.S.W. Aust. J. Soil Res. 24, Busscher, W.J., Buer, P.J., Cmp, C.R., Sojk, R.E., Correction of cone index for soil wter content differences in costl plin soil. Soil Till. Res. 43, Cmpell, D.J., Henshll, J.K., Bulk density. In: Smith, K.A. nd Mullins, C.E. (Eds.), Soil nlysis. Physicl methods. Mrcel Dekker, New York, USA, pp Cmpell, D.J., O'Sullivn, M.F., The cone penetrometer in reltion to trfficility, compction, nd tillge. In: Smith, K.A. nd Mullins, C.E. (Eds.), Soil nlysis. Physicl methods. Mrcel Dekker, New York, USA, pp Cssel, D.K., Tillge effects on soil ulk density nd mechnicl impednce. In: Unger, P.W. nd Vn Doren, D.M. (Eds.), Predicting tillge effects on soil physicl properties nd processes. ASA-SSSA, Mdison, USA, pp Crter, M.R., Bll, B.C., Soil porosity. In: Crter, M.R. (Ed.), Soil smpling methods of soil nlysis. Lewis Pulishers, pp Chen, Y., Tessier, S., Rouffignt, J., Soil ulk density estimtion for tillge systems nd soil textures. Trns. ASAE. 41, Crovetto, C.C., No-till development in Chequén frm nd its influence on some physicl, chemicl nd iologicl prmeters. J. Soil nd Wter Cons. 53,

23 Edwrds, W.M., Soil structure: processes nd mngement. In: Ll, R. nd Pierce, F.J. (Eds.), Soil mngement for sustinility. Soil nd Wter Conservtion Society. Ankeny, USA, pp Ehlers, W., Köpke, U., Hesse, F., Böhm, W., Penetrtion resistnce nd root growth of ots in tilled nd untilled loess soil. Soil Till. Res. 3, Ferrers, L.A., Cost, J.L., Grci, F.O., Pecorri, C., 2. Effect of no-tillge on some soil physicl properties of structurl degrded Petroclcic Pleudoll of the southern "Pmp" of Argentin. Soil Till. Res. 54, Frnzen, H., Ll, R., Ehlers, W., Tillge nd mulching effects on physicl properties of tropicl Alfisol. Soil Till. Res. 28, Frnzlueers, A.J., Hons, F.M., Zuerer, D.A., Tillge nd crop effects on sesonl dynmics of soil CO 2 evolution, wter content, temperture, nd ulk density. Applied Soil Ecology. 2, French, R.J., The effect of fllowing on the yield of whet. I. The effect on soil wter storge nd nitrte supply. Aust. J. Agric. Res. 29, Godwin, R.J Agriculturl engineering in development: tillge for crop production in res of low rinfll. FAO, Rome, Itly, 124 pp. Gómez, K.A., Gómez, A.A., Sttisticl Procedures for Agriculturl Reserch. John Wiley & Sons, New York, USA, 68 pp. Grnt, C.A., Lfond, G.P., The effects of tillge systems nd crop sequences on soil ulk density nd penetrtion resistnce on cly soil in southern Ssktchewn. Cn. J. Soil Sci. 73, Gregory, P.J., Root growth nd ctivity. In: Boote, K.J., Bennett, J.M., Sinclir, T.R. nd Pulsen, G.M. (Eds.), Physiology nd determintion of crop yield. ASA-CSSA-SSSA, Mdison, USA, pp Hds, A., Soil tilth-the desired soil structurl stte otined through proper soil frgmenttion nd reorienttion processes. Soil Till. Res. 43, 7-4. Hmlin, A.P., The influence or soil structure on wter movement, crop root growth, nd wter uptke. Adv. Agron. 38, Hmmel, J.E., Long-term tillge nd crop rottion effects on ulk density nd soil impednce in northern Idho. Soil Sci. Soc. Am. J. 53, Hernnz, J.L., Girón, V.S., Experiments on the growing of cerels with different tillge systems in centrl Spin. Proceedings of the 11 th Interntionl Conference of the Interntionl Soil Tillge Reserch Orgniztion th July, Edinurgh, Scotlnd, Volume 2, pp Hill, R.L., 199. Long-term conventionl nd no-tillge effects on selected soil physicl properties. Soil Sci. Soc. Am. J. 54, Hurd, R.K., Hrgrove, W.L., Lowrnce, R.R., Willims, R.G., Mullinix, B.G., Physicl properties of clyey costl plin soil s ffected y tillge. J. Soil nd Wter Cons. 49,

24 Kinsell, J., The effects of vrious tillge systems on soil compction. In: Soil nd Wter Conservtion Society (Ed.), Frming for etter environment. Ankeny, pp Klepper, B., 199. Root growth nd wter uptke. In: Stewrt, B.A. nd Nielsen, D.R. (Eds.), Irrigtion of griculturl crops. ASA-CSSA-SSSA, Mdison, USA, pp Lmpurlnés, J., Cntero-Mrtínez, C., Evolución de l densidd prente de un suelo cultivdo jo distintos sistems de loreo en condiciones semiárids del vlle del Ero. In: Grcí, L., Vrel, A. nd González, P. (Eds.), Agricultur de conservción: rentilidd y medio miente. 2-4 Octoer, Córdo, Spin, pp Lmpurlnés, J., Angás, P., Cntero-Mrtínez, C., 2. Root growth, soil wter content nd yield of rley under different tillge systems on two soils in semirid conditions. Field Crops Res. (Accepted on 26 th Septemer 2). Lmpurlnés, J., Angás, P., Cntero-Mrtínez, C., 2. Tillge effect on wter storge efficiency during fllow, nd soil wter content, root growth nd yield of the following rley crop on two different soils in semirid conditions. Soil Till. Res. (Sumitted). Littell, R.C., Freund, R.F., Spector, P.C., SAS System for Liner Models. SAS Institute Inc., Cry, USA, 329 pp. López, M.V., Arrúe, J.L., Sánchez-Girón, V., A comprison etween sesonl chnges in soil wter storge nd penetrtion resistnce under conventionl nd conservtion tillge systems in Argón. Soil Till. Res. 37, Logsdon, S.D., Cmrdell, C.A., 2. Temporl chnges in smll depth-incrementl soil ulk density. Soil Sci. Soc. Am. J. 64, Logsdon, S.D., Kspr, T.C., Cmrdell, C.A., Depth-incrementl soil properties under no-till or chisel mngement. Soil Sci. Soc. Am. J. 63, McCll, T.M., Army, T.J., Stule mulch frming. Adv. Agron. 13, McDonld, G.K., Fischer, R.A., The role of soil mngement in the mintennce of crop production in semi-rid environments. In: Acevedo, E., Fereres, E., Giménez nd C., Srivstv, J.P. (Eds.), Improvement nd mngement of winter cerels under temperture, drought nd slinity stresses. Proceedings of the ICARDA-INIA Symposium, Octoer 1987, Córdo. MAPA-INIA, Mdrid, Spin, pp Morn, C.J., Koppi, A.J., Murphy, B.W., McBrtney, A.B., Comprison of the mcropore structure of sndy lom surfce soil horizon sujected to two tillge tretments. Soil Use nd Mngement. 4, Pin, J., Lipiec, J., Wlodek, S., Biskupski, A., Kus, A., Criticl soil ulk density nd strength for pe seedling root growth s relted to other soil fctors. Soil Till. Res. 46, Pelegrin, F., Moreno, F., Mrtin-Arnd, J., Cmps, M., The influence of tillge methods on soil-wter conservtion in SW Spin. In: Tillge nd trffic in crop production, Proceedings of the 11 th Interntionl Conference of the Interntionl Soil Tillge Reserch Orgniztion th July, Edinurgh, Scotlnd, volume 2, pp

25 Pelegrin, F., Moreno, F., Mrtin-Arnd, J., Cmps, M., 199. The influence of tillge methods on soil physicl properties nd wter lnce for typicl crop rottion in SW Spin. Soil Till. Res. 16, Pikul, J.L., Ase, J.K., Infiltrtion nd soil properties s ffected y nnul cropping in the northern Gret Plins. Agron. J. 87, Rdclifee, D.E., Tollner, E.W., Hrgrove, W.L., Clrk, R.L., Goli, M.H., Effect of tillge prctices on infiltrtion nd soil strength of Typic Hpludult soil fter ten yers. Soil Sci. Soc. Am. J. 52, Rhoton, F.E., Bruce, R.R., Buehring, N.W., Elkins, G.B., Lngdle, C.W., Tyler, D.D., Chemicl nd physicl chrcteristics of four soil types under conventionl nd no-tillge systems. Soil Till. Res. 28, Steel, R.G.D., Torrie, J.H., 198. Principles nd Procedures of Sttistics. A Biometricl Approch. McGrw-Hill, New York, USA, 633 pp. Stelluti, M., Miorn, M., De Giorgio, D., Multivrite pproch to evlute the penetrometer resistnce in different tillge systems. Soil Till. Res. 46, Tod, M.A., Micucci, F.G., Cosentino, D.J., Lvdo, R.S., Comprison of compction induced y conventionl n zero tillge in two soils of the Rollin Pmp of Argentin. Soil Till. Res. 49, Tylor, H.M., Mnging root systems for efficient wter use: nd overview. In: Tylor, H.M., Jordn, W.R. nd Sinclir, T.R. (Eds.), Limittions to efficient wter use in crop production. ASA-CSSA-SSSA, Mdison, USA, pp Terügge, F., Düring, R.A., Reducing tillge intensity - review of results from longterm study in Germny. Soil Till. Res. 53, Unger, P.W., Overwinter chnges in physicl properties of no-tillge soil. Soil Sci. Soc. Am. J. 55, Unger, P.W., Jones, O.R., Long-term tillge nd cropping systems ffect ulk density nd penetrtion resistnce of soil cropped to drylnd whet nd grin sorghum. Soil Till. Res. 45, Villr, J.M., Evpotrnspirción y productividd del gu en ced (+RUGHXPYXOJDUH L.) y triticle (; 7ULWLFRVHFDOH Wittmrt) en condiciones de secno en l Segrr (Lleid). UPC, ETSEAL, Lleid, Spin, 168 pp. Wnder, M.M., Bollero, G.A., Soil qulity ssessment of tillge impcts in Illinois. Soil Sci. Soc. Am. J. 63, Ysin, M., Grisso, R.D., Bshford, L.L., Jones A.J., Mielke, L.N., Normlizing cone resistnce vlues y covrince nlysis. Trns. ASAE. 36,