soil compaction and root distribution for okra as affected by tillage and vehicle parameters

Soil & Tillage Research 74 (2003) 25–35

Soil compaction and root distribution for okra as affectedby tillage and vehicle parameters

Nidal H. Abu-Hamdeh∗Jordan University of Science and Technology, P.O. Box 422, Irbid 21110, Jordan

Received 3 April 2002; received in revised form 11 March 2003; accepted 26 March 2003

Abstract

Many soil physical properties and crop yield are affected by compaction and tillage systems. The effect of three differentfactors, i.e. tillage treatments (no-tillage, chisel plowing, and moldboard plowing), axle load (6 and 16 t/axle vehicle), andtire inflation pressure (120 and 350 kPa inflation pressures) on okra (Abelmoschus esculentus) root density and soil physicalproperties (bulk density and cone penetration resistance) was studied. This study was initiated in April 2000 on a loam soil(42% sand, 36% silt, 22% clay) at an experimental farm in Irbid, a district in the northern part of Jordan. Response to the abovefactors was measured down to 48 cm. Axle load and tire inflation pressure caused an increase in bulk density (BD) and conepenetration (CI) resistance from the soil surface down to 48 cm. The loaded vehicle with 350 kPa tire inflation pressure onno-tillage plots had the greatest effect soil physical properties measured while the unloaded vehicle with 120 kPa tire inflationpressure on chisel-plowed plots had the least effect. Plants in the no-tillage and moldboard-plowed treatments had a higherconcentration of roots near the base of the plant compared to the plants in the chisel-plowed treatment.© 2003 Elsevier B.V. All rights reserved.

Keywords: Tillage treatments; Axle load; Inflation pressure; Soil physical properties; Root density

1. Introduction

Soil compaction as defined by many researchersrefers to the packing effect of applied forces on thesoil. This packing effect decreases the volume occu-pied by pores and increases the density and strengthof the soil mass. Bulk density and penetrometer resis-tance are indices of soil compaction.Carpenter et al.(1985)in discussing the effect of wheel loads on sub-soil stresses stated “although soil compaction affectsmany important soil physical properties, perhaps themost detrimental effect is the drastic reduction in hy-draulic conductivity, which ultimately results in soil

∗ Tel.: +962-2-7095111x22330; fax:+962-2-7095018.E-mail address: [email protected] (N.H. Abu-Hamdeh).

erosion and reduced crop yields due to reduced infil-tration, increased run-off and poor drainage”.Baileyet al. (1988)reported that excessive compaction maycause such undesirable effects as restriction of rootgrowth and increased run-off. These detrimental ef-fects can increase soil erosion.

Soil compaction under tractors is of special concernbecause the weight of these machines has increaseddramatically between 1970 and 1990. Soil responseto pressures and deformations imposed by wheels andtracks, and by soil-engaging tools is the main mecha-nisms of soil compaction. Pressures exerted on the soilsurface depend on the wheel or track size and designcharacteristics and soil properties.Taylor et al. (1986)measured and compared subsurface soil pressures be-neath dual tires and single tires, with similar loads.

0167-1987/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0167-1987(03)00122-3

26 N.H. Abu-Hamdeh / Soil & Tillage Research 74 (2003) 25–35

They found that dual tires significantly reduced sub-soil pressures when compared to a single tire with thesame axle load. Pressures exerted on the soil surfacedepend on the static load characteristics of the wheelor track and soil properties. The product of the verticalcomponent of surface stress with the soil surface areaon which it acts is equal to the total weight carriedon any wheel or track. The degree of soil compactionis dependent upon the axle load and contact pressurebetween the soil and tire (Hakansson et al., 1988). Intheir field experiments, vehicles with high axle loadshave reduced crop yield and caused detrimental en-vironmental effects.Abu-Hamdeh et al. (2000)foundhigher soil density values with increasing axle load inthe tillage and subsoil zones. Theory suggests that thetotal axle load is a better indicator factor for deep soilcompaction than the surface contact pressure.Soehne(1958) studied the effects of loads and distributionof loads on resulting pressures in the soil profile andconcluded, “the pressure in the upper soil layer is de-termined by the specific pressure at the surface, andthe pressure in the deeper soil layer is determined bythe amount of the load”.Abu-Hamdeh and Al-Widyan(2000) found that soil bulk density was maximum atdepths ranging from 20 to 30 cm underneath a loadedtruck tire on clay soil.

One of the most frequently used measures of com-paction is soil bulk density. Bulk density is foundby determining the weight of dry soil that occupiesa known volume.Ngunijiri and Siemens (1993)ob-served an increase in bulk density of the topsoil from0.96 to 1.31 g/cm3 as a result of soil compaction bywheel track.Gameda et al. (1987)reported that com-mutative high axle loading contributed to an annualincrease in soil bulk density.Soane et al. (1976)sug-gested that the zone of maximum density moves closerto the surface with repeated passes.

Numerous researchers have used cone penetrome-ters to measure soil strength. Cone penetrometers, de-vices used to sense the penetration resistance of soils,have been in use for many years, and have varied appli-cations in many fields because of their easy, rapid andeconomical operation (Perumpral, 1987). Hartge et al.(1985)used a cone penetrometer to relate compactionfrom deep ploughing to increases in cone penetrationresistance.Ayers and Bowen (1987)used a cone pen-etrometer along with moisture content to predict bulkdensity.

Tillage treatments are expected to affect soil re-sponse and crop yield (Comia et al., 1994; Larney andBullock, 1994; Shafiq et al., 1994; Stewart and Vyn,1994; Tessier et al., 1997). Erbach et al. (1992)eval-uated the effect of the following tillage treatments:no-tillage, chisel plow, moldboard plow, and para plowsystems on three soils (poorly drained, medium, andfine textured) in Iowa. Results showed that all tillagetools reduced bulk density and cone penetration resis-tance to the depth of tillage. However, after planting,only the soil tilled with the para plow remained lessdense than before tillage.Voorhees (1983)measuredsoil physical properties in the topsoil following nor-mal farming operations using a tractor weighing 7.3 t.He found that fall moldboard plowing decreased bulkdensity on the tilled layer to essentially the same levelas in nontrafficked soil.Chen and Tessier (1997)foundthat the effect of wheel traffic associated with the plow-ing operation on soil density could be extended to thedepth of 30 cm.

Optimum crop yields are dependent upon optimumroot growth, and when soil is in good condition, rootsystems are large, deep, and expansive (Trouse, 1977).Roots are capable of reaching depths of 180 cm inless than 1 month, and spreading laterally more than100 cm. Dominant roots of most plants are able toelongate rapidly for many days and develop branchroots that supply the plants with the nutrients andmoisture they demand. Bulk densities associated withdecrease root growth or crop yield have been measuredfor several crops. Decreased root penetration by cottonwas associated with an increase in soil bulk density to1.65 g cm−3 (Taylor and Gardner, 1963). Reeves et al.(1984)found that spring wheat in Australia grown insoil with a bulk density of 1.52 g cm−3 in the 0–20 cmdepth had less root growth than that grown in soilwith a bulk density of 1.32 g cm−3. Laboski et al.(1998) conducted a field experiment to determine ifsoil strength and/or available water could be the fac-tors limiting corn rooting depth on an irrigated finesand soil. They found that a compacted soil layer con-fined roots almost entirely to the top 0.60 m of soilbecause it had high soil strength and bulk density andthe compacted layer, in turn, retained more water forcrop use.Boone et al. (1986)defined the lower criticalmechanical limit (LCML) as the soil strength whereroot growth was reduced to 50% of unimpeded growth.The upper critical mechanical limit (UCML) was the

N.H. Abu-Hamdeh / Soil & Tillage Research 74 (2003) 25–35 27

soil strength where root growth ceased or was com-pletely a function of mechanical resistance. They de-termined that the UCML for corn was 3.0 and 1.5 MPafor the LCML. These numerical limits were developedfor homogeneous soils, where soil texture or organicmatter show no significant variation with depth.

Although the relationship between soil compaction,vehicle load and ground contact stress has been quan-tified in general, many features of this relationshipare dependent on the characteristics of the particularsoil involved and on specific parameters of the tiresor tracks employed. Furthermore, the relation betweensoil compaction and yield is not straightforward; it in-volves the interaction of soil, air, and water as it af-fects various stages of plant development. Optimumcrop yields are dependent upon root growth which ishighly affected by soil compaction. This paper con-siders the important effects of compaction from highaxle loads and tire inflation pressures in a geographi-cal area not well covered for this subject in the past.Soil compaction is a relatively new problem in Jor-dan and many countries around the world. Much workis still needed to evaluate the compaction effects fordifferent field conditions and operating parameters toprovide management strategies to minimize the detri-mental effects of compaction. The objectives of thisstudy were to (1) evaluate the changes in soil physicalproperties including compaction to a depth of 50 cmas affected by vehicle mass and tire size and tillagefactors, and (2) investigate the effect of tillage meth-ods on root density for okra. This work is importantto the production of okra because of its economic im-portance in the region.

2. Materials and methods

2.1. Experimental design

This research was initiated in April 2000 on a loamsoil (42% sand, 36% silt, 22% clay) at an experi-mental farm in Irbid, a district in the northern partof Jordan. The soil is classified as fine, mixed, mesicTypic Haplustox under the USDA Soil Taxonomyclassification system. Three parameters were studied:tire inflation pressure (120 and 350 kPa), axle load (6and 16 t/axle) and tillage treatment (no-tillage, chiselplowing with disk harrow, moldboard plowing with

Table 1The combinations of treatments used in this study

Treatment Axle load(t/axle)

Tillagesystem

Inflation pressure(kPa)

6C120 6 Chisel 1206M120 6 Moldboard 1206N120 6 No till 1206C350 6 Chisel 3506M350 6 Moldboard 3506N350 6 No till 35016C120 16 Chisel 12016M120 16 Moldboard 12016N120 16 No till 12016C350 16 Chisel 35016M350 16 Moldboard 35016N350 16 No till 350U – – –

rotor tiller). A 2 × 2 × 3 factorial experiment wasthen conducted with additional untrafficked row (U)treatment. The combinations of treatments used inthis study are shown inTable 1. The experiment wasarranged in two blocks. Each block consisted of 26plots representing two replications of the 12 combina-tions of tire inflation pressures, axle loads, and tillagesystems treatments and the untrafficked treatment.Plots were 5 m wide and 25 m long. The field had notbeen cultivated or cropped for at least 2 years beforestarting the experiment.

2.2. Field operations

Soil compaction was applied in April 2000 witha grain cart at two loading levels (6 and 16 t/axle).Tires (0.68 m× 0.65 m), were bias-ply with recom-mended pressure of 230 kPa at 8 t load and 130 kPaat 3 t load. The 16 t/axle treatment was obtained byloading the cart with grains until a load of 16 t isexerted by the axle. The first level of compaction wasimposed to pre-specified plots by the loaded cart withtires inflated to 350 kPa. Then, while keeping the cartloaded, the tires inflation pressure was reduced to120 kPa and the second level of compaction was ap-plied. The third compaction level was then imposedby the empty cart with tires inflated to 120 kPa. Fi-nally, the tires of the unloaded cart were inflated to350 kPa while keeping the cart empty to apply thefourth compaction level. Control plots (untraffickedtreatments) were not compacted by any level of axle


load. All compactive loads were applied such that theentire area of each plot was covered completely withwheel tracks. Soil water content (from the surface to48 cm depth) at the time of compaction ranged from10.2 to 18.3 kg kg−1. The three tillage treatments(no-tillage, chisel plowing, and moldboard plowing)were then applied to the respective plots. The workingdepth of the tillage implements was approximately25 cm. Next, okra was hand-planted at the begin-ning of May at 55,000 seeds ha−1 without fertilizeror insecticide at a 40 cm, in-row, spacing with 50 cmbetween-row spacing. The plots were hand-plantedbecause plots were small and to ensure accurateplacement of seeds. Conventional farming practiceswere then performed. All tillage and field operationswere performed by an 80 kW two-wheel drive KUB-OTA M8030 tractor weighing 4 t (front tires were14.9R30 with inflation pressure set to the recom-mended level of 190 kPa and rear tires were 18.4R46bias-ply set to recommended inflation pressureof 105 kPa).

2.3. Soil physical properties

Soil physical properties were measured across thecompacted plots in August 2000 (the time of harvest).The measured soil physical properties were bulk den-sity and cone penetration resistance. Soil bulk densitywas measured on cores obtained by a manually oper-ated tool to obtain cores 5 cm in diameter and 6 cmin length (Blake and Hartge, 1986). Cores were ob-tained at four locations in each treatment and at eightdepths to a depth of 48 cm (0–6, 6–12, 12–18, 18–24,24–30, 30–36, 36–42, and 42–48 cm). Extra soil sam-ples were taken to measure soil water content at thespecified depths using the oven drying technique. Wetbulk density of soil sample was obtained by weighingthe known volume of the core filled with soil and thensubtracting the weight of the core itself. Since wetbulk density is a dynamic function, wet bulk densitywas converted to dry bulk density using the followingequation (Hillel, 1982):

ρd = ρw

1 + w(1)

whereρd is the dry bulk density (g/cm3), ρw the wetbulk density (g/cm3) andw the gravimetric water con-tent (g/g).

Twenty cone penetrometer measurements weretaken in each plot at randomly selected locations us-ing a standard cone penetrometer (ASAE StandardS313.2). The penetrometer had a standard 30◦ conewith a base diameter of 1.28 cm. Data for each mea-sured property were compiled and individual valueswere averaged for each 12 cm depth increment to adepth of 48 cm. Statistical analysis was performedon the experimental data using the statistical analysissoftware,MINITAB (1994). Statistically significantdifferences of the 90% level were considered.Tables 2and 3 show the bulk density and cone penetrationdata, respectively.

2.4. Root density and distribution

During the growing season root density and dis-tribution were monitored and evaluated. Root densitywas assessed for okra using the procedure presentedby Ngunijiri and Siemens (1993). A hole was dug toexpose the plant roots to a depth of 20 cm and a widthof 20 cm centered across an okra-row. A 2 cm× 2 cmgrid was placed on the exposed surface and a root in-dex recorded for each square of the grid. Root indicesranged from a low of 0 for squares with no roots toa high of 8 for squares with numerous roots (Fig. 1).The indices were used to estimate root density usingthe relationship presented byNgunijiri and Siemens(1993)and shown inFig. 1.

3. Results and discussion

3.1. Root density

Thirty samples were used to estimate root densityfor okra in each tillage system. A statistical analysiswas performed to test the null hypothesis that “dif-ferent samples” have no effect on the results obtainedfor each tillage system. Means were separated by theleast significant differences (LSD) procedure at 0.05probability level to compare means among samplesfor each tillage system. The analysis indicated thatthere were no significant differences among samplesfor each tillage system. Thus, all the results were aver-aged over the 30 samples for each tillage system.Fig. 2shows the average root density for okra under the threetillage systems. Mapping the root density showed that

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Table 2Dry bulk density (g/cm3) by depth for all treatmentsa

Depth(cm)

U Low axle load High axle load

Low inflation pressure High inflation pressure Low inflation pressure High inflation pressure

Chisel(6C120)

Moldboard(6M120)

No till(6N120)

Chisel(6C350)

Moldboard(6M350)

No till(6N350)

Chisel(16C120)

Moldboard(16M120)

No till(16N120)

Chisel(16C350)

Moldboard(16M350)

No till(16N350)

0–12 1.15 a 1.17 a 1.16 a 1.32 cd 1.18 a 1.21 ab 1.38 de 1.25 bc 1.29 c 1.37 d 1.29 c 1.35 d 1.44 e12–24 1.15 a 1.21 b 1.27 bc 1.34 d 1.21 b 1.23 b 1.35 de 1.29 c 1.32 d 1.40 e 1.30 cd 1.33 d 1.43 e24–36 1.15 a 1.21 b 1.27 c 1.19 b 1.24 c 1.25 c 1.27 c 1.29 c 1.34 d 1.27 c 1.32 d 1.37 d 1.42 e36–48 1.19 a 1.21 a 1.21 a 1.21 a 1.21 a 1.30 b 1.26 b 1.31 b 1.33 bc 1.30 b 1.38 c 1.39 c 1.39 c

a Means in rows, within a depth range, followed by the same letter were not significantly different at alpha= 0.1 using Tukey’s Studentized range test.

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Table 3Cone penetration resistance (MPa) by depth for all treatmentsa

Depth(cm)

U Low axle load High axle load

Low inflation pressure High inflation pressure Low inflation pressure High inflation pressure

Chisel(6C120)

Moldboard(6M120)

No till(6N120)

Chisel(6C350)

Moldboard(6M350)

No till(6N350)

Chisel(16C120)

Moldboard(16M120)

No till(16N120)

Chisel(16C350)

Moldboard(16M350)

No till(16N350)

0–12 0.53 a 0.63 b 0.82 d 0.83 d 0.76 c 0.809 d 0.97 e 0.91 e 0.95 e 0.99 fe 0.85 d 0.74 c 1.09 f12–24 1.10 a 1.11 b 1.29 cd 1.25 c 1.13 b 1.22 c 1.23 c 1.21 c 1.26 c 1.34 de 1.12 b 1.22 c 1.40 e24–36 1.32 a 1.48 b 1.50 b 1.51 b 1.50 b 1.45 b 1.68 d 1.58 bc 1.65 d 1.59 c 1.77 e 1.70 d 1.82 e36–48 1.49 a 1.60 a 1.81 c 1.66 ab 1.60 a 1.58 a 1.88 d 1.69 b 1.82 c 1.75 bc 1.73 b 1.80 c 1.86 cd

a Means in rows, within a depth range, followed by the same letter were not significantly different at alpha= 0.1 using Tukey’s Studentized range test.


Root density (g cm-3) = ( 0.029 + 0.047 N + 0.011 N2 ) / Volume

N = 0 N = 2 N = 4 N = 8

1 root with diameter = 0.5-1mm + small roots

OR several roots diameter < 0.5 mm

1 root with diameter > 0.5 mm

no roots 2 roots with diameter > 0.5 mm and small roots

Fig. 1. Root indices,N, as influenced by root count and size.

the no-tillage treatment restricted root distribution.Plants in no-tillage and moldboard-plowed treatmentshad higher concentration of roots near the base ofplants compared to the plants in the chisel-plowedtreatment (Fig. 2). From the same figures it can beseen that roots had greater distribution between rowsin the chisel treatment than in the no-tillage and

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

-10 -5 0 5 10 Distance From Row (cm)

chisel moldboard no-till

Roo

t D

ensi

ty (

g/cm

3 )

Fig. 2. Root density relative to crop row in chisel-plowed, moldboard-plowed, and no-till plots.

moldboard-plowed treatments. Chisel plowing sig-nificantly affected root density as resulted from theanalysis of variance (ANOVA) procedure which wasused to evaluate the significance of each tillage sys-tem on root density (P = 0.099, F = 2.58). It seemsthat the chisel plow provided loose soil that wasconducive to root penetration in the tilled layer (top


Fig. 3. Average soil dry density under different treatments (treatment symbols are described inTable 1).

25 cm) than the moldboard plow. Widely distributedroots would seem to be desirable for maximumproductivity.

3.2. Bulk density

Dry bulk density was significantly increased toa depth of 48 cm for most treatments (Fig. 3 andTable 2). No-tillage at high load and high inflationpressure (16N350) treatment caused the maximumpercentage increase of dry bulk density at the 0–12,12–24, and 24–36 cm depth. The average percentageincrease in dry density at the 0–24 cm depth showedthat no-tillage at high load and high inflation pres-sure (16N350) treatment had the highest effect whilethe chisel-plowed at low load and low inflation pres-sure (6C120) treatment had the least effect. At the24–36 cm depth, the no-tillage at low load and lowinflation pressure (6N120) treatment had the leasteffect. The no-tillage at high load and high inflationpressure (16N350) and the moldboard-plowed at highload and high inflation pressure (16M350) treatmentshad the highest effect at the 36–48 cm depth, as it wasthe lowest with chisel-plowed at low load and highinflation pressure (6C350) treatments. The averagevalues at the 24–48 cm depth show that the no-tillage

at high load and high inflation pressure (16N350)treatment had the greatest percentage increase of drybulk density while the no-tillage at low load and lowinflation pressure (6N120) treatment had the low-est percentage increase of dry bulk density. In otherwords, increasing tire inflation pressure and axle loadincreased soil dry bulk density. Available informationclearly demonstrates that the axle load and tire infla-tion pressure are crucial factor affecting the depth ofsubsoil compaction.Gameda et al. (1987)observedhigher bulk densities under the 20 t/axle treatmentthan those under the 10 t/axle treatment and in controlplots. An increase in axle wheel loads and pressureresulted in greater soil compaction due to increasedshear and vertical soil stresses.Danfors (1990)ob-served that compaction of the 30–50 cm layer slightlydecreased when the tire inflation pressure in wheelsloaded by 6 t was reduced from 150 to 100 kPa.

3.3. Cone index

Cone index values (MPa) in traffic areas at alldepths were significantly different (P ≤ 0.1) fromvalues in untrafficked areas except for some treat-ments at the 36–48 cm depth (Fig. 4 and Table 3).Cone index values were significantly greater under


Fig. 4. Average cone penetration resistance under different treatments.

the no-tillage at high load and high inflation pressure(16N350) than under other treatments except at the36–48 cm depth where it was not significantly differ-ent from the no-tillage at low load and high inflationpressure (6N350). As shown inFig. 4 and Table 3,the no-tillage at high load and high inflation pressure(16N350) had the greatest percentage increase in coneindex, followed by the no-till at high load and lowinflation pressure (16N120). The chisel-plowed atlow load and low inflation pressure (6C120) had thelowest percentage increase in cone index. The resultsagain indicate that compaction occurring from the tiregoes deeper with increasing axle load and tire inflationpressure.Chaplin et al. (1986)found that chisel plow-ing effectively removed dense layers developed fromsoil compaction and the highest mean penetrationresistance was for no-tillage at 24 cm below the soilsurface.

It is worth noting that bulk density at the 24–36 cmdepth in the no-tillage treatments was lower than inshallower soil. These results reflect a more compactsoil layer at shallower depths in the no-tillage treat-ments than at deeper depths. Soil bulk density in-creases with increasing the axle load and tire inflationpressure. An increase in axle wheel loads and pres-

sures have greater effects on soil compaction in thetillage zone than in deeper depths due to increasedin both shear and vertical soil stresses. Since tillageobliterates the effect of the axle load and tire infla-tion pressure on soil strength in the tilled layer, bulkdensity in the topsoil was lower than in the subsoil inchisel and moldboard treatments while it was higherin the topsoil in the no-tillage treatments than in thesubsoil. This phenomenon was absent in cone indexmeasurements. Cone penetration resistance is highlydependent on soil water content at the time of mea-surements. In this study, the soil water content pro-files were fairly uniform with depth and were closer tothe field capacity at the time of cone index measure-ments, which might have resulted in more consistentmeasurements among treatments.

4. Conclusions

The effects of tillage treatments, axle load, and tireinflation pressure on soil physical properties and rootdensity were investigated. The experimental results re-vealed that the intensity of subsoil compaction occur-ring from a vehicle tire goes deeper with increasing


axle load and tire inflation pressure. The results alsoshowed that the type of tillage system affected rootdensity. To this endeavor, the study showed that:

(1) Increasing tire inflation pressure and axle load in-creased soil dry bulk density and cone penetrationresistance.

(2) Tillage treatments restricted root distribution.Okra in no-tillage and moldboard-plowed treat-ments had higher concentration of roots near thebase of plants compared to roots of okra in thechisel-plowed treatment.

Results of this experiment may suggest the impor-tance of restricting high axle loads to permanent trafficlanes, especially in no-tillage farming systems. The re-sults also showed the importance of using correct tireinflation pressure in reducing soil compaction damage.Thus, it appears that future designs based upon lim-ited ground contact pressures are essential. This willrequire limitations on vehicle wheel loads and the useof more tires and axles on heavy equipment.

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