Wheel traf®c impact on soil conditions as in¯uenced
by tillage system in Central Anatolia
H. GuÈcluÈ Yavuzcan
*Faculty of Agriculture, Department of Agricultural Machinery, Ankara University, 06130 Aydinlikevler, Ankara, Turkey
Received 12 April 1999; accepted 14 October 1999
Abstract
The increased limiting effects of soil compaction on Central Anatolian soils in the recent years demonstrate the need for a detailed analysis of tillage system impacts. This study was undertaken to ascertain the effects of seven different tillage systems and subsequent wheel traf®c on the physical and mechanical properties of typical Central Anatolian medium textured clay loam soil (Cambisol), south of Ankara, Turkey. Both tillage and ®eld traf®c in¯uenced soil bulk density, porosity, air voids and strength signi®cantly except the insigni®cant effect of traf®c on moisture content. Traf®c affected the soil properties mostly down to 20 cm. However, no excessive compaction was detected in 0±20 cm soil depth. The increases of bulk density following wheel traf®c varied between 10±20% at 0±5 cm and 6±12% at 10±15 cm depth. In additions, traf®c increased the penetration resistance by 30±74% at 0±10 cm and 7±33% at 10±20 cm. Less wheel traf®c-induced effects were found on chisel tilled plots, compared to ploughed plots. Soil stress during wheel passage was highly correlated with soil strength. Also, both tillage and traf®c-induced differences were observed in mean soil aggregate sizes, especially for mouldboard ploughed plots. The obtained data imply that chiselcultivator-tooth harrow combination provides more desirable soil conditions for resisting further soil compaction.#2000 Elsevier Science B.V. All rights reserved.
Keywords:Soil compaction; Tillage; Traf®c; Bulk density; Soil strength
1. Introduction
Soil compaction has been identi®ed as one of the leading causes of soil degradation threatening future productivity of Central Anatolian farm lands in the
recent years (Anonymous, 1991, 1994, 1997). This problem became evident in relatively level areas, particularly after the wide introduction of tractors and agricultural machinery. Many Central Anatolian soils are low in organic matter and are subjected to low annual rainfall amounts (Akalan, 1983; Anonymous, 1997). Most importantly, the greatest portion of the annual rainfall occurs in the early or mid spring, when most ®eld operations are performed, providing a soil condition that is very conductive to compaction.
The cropland in the region accounts for around 6 million ha, most (80%) of which is devoted to grain
*Present address: Institut fuÈr Landtechnik, Technische
Universi-taÈt MuÈnchen, Am Staudengarten 2, 85350 Freising-Weihenstephan, Germany. Tel.: 49-8161-713447/49-8161-498838; fax: 49-8161-713895.
E-mail addresses: yavuzcan@tec.agrar.tu-muenchen.de, yavuzcan@agri.ankara.edu.tr (H.G. Yavuzcan)
production (Anonymous, 1994, 1997). The production system of annual crops includes seedbed preparation for each crop cycle. Conventional local tillage system involves soil loosening by mouldboard ploughing and then crumbling the soil for the seedbed through disc harrowing.
Many farmers in the Central Anatolia perform tillage operations without knowledge or regard for effects on soil properties and crop responses. The irregularity through freezing and thawing as well as wetting and drying create dif®culties in quantifying the traf®c-induced consequences of soil compaction. It is a fairly common practice for farmers to periodically subsoil cropland to alleviate perceived compaction caused by vehicular wheel traf®c. Waste of energy and soil degradation by erosion and compaction are recognised problems caused by inadequate tillage management. Although natural processes such as freezing and thawing, wetting and drying, shrinking and swelling and fracturing and aggregation caused by plant root growth and organic matter tend to alleviate soil compaction (Bicki and Siemens, 1991), these may be effective only to shallow soil depths.
Studies evaluating the effects of wheel traf®c on soil properties in Central Anatolia have been limited. Research has generally focused on conventional tillage systems and rotary tilling. Comparisons of conventional and reduced (rotary tilling) till indicated that relative increase of penetration resistance after wheel traf®c was greater in conventionally tilled plots at 0±15 cm (OÈ zguÈven and Aydinbelge, 1990; Carman, 1996). Other research conducted in the region reported that a classical conventional tillage (ploughdisctooth harrow) provides better condi-tions for resisting further compaction (Taser and Metinoglu, 1997). However, the interactive roles that tillage system Ð especially conservation tillage sys-tems and traf®c on soil compaction have not been clari®ed.
An important step towards understanding and con-trolling compaction is the ability to predict compac-tion. However, the soil conditions that in¯uence compaction and that are changed by compaction are dif®cult to de®ne and to quantify. After analysing the impacts of four different tillage systems (chisel, mouldboard plough, no-till and ridge-till) and subse-quent ®eld traf®c, Bauder et al. (1985) found that
tillage system had no effect on porosity and gravi-metric water content while both tillage and traf®c had signi®cant effects on bulk density and penetration resistance down to a depth of 22 cm on a Nicollet± Webster soil in south-central Minnesota. Gruber and TebruÈgge (1990) reported that the highest ground pressure and rut depth were obtained in conventional tillage systems and susceptibility of subsoil compac-tion is much lower for reduced tillage systems due to the even distribution of wheel loads.
A soil that has developed suf®cient load-carrying capacity can resist further changes in bulk density or total porosity (Anonymous, 1992). In such a soil, wheel traf®c may change the pore distribution slightly by elastic deformation. Ngunjiri and Siemens (1995) investigated the effects of different wheel traf®c pat-terns during ploughing and reported that wheel traf®c was found to increase soil bulk density and cone index to a depth of 30 cm with the most important impacts in the 0±15 cm depth on a silt loam soil of Illinois. A study to determine soil cone index variability under no-till, conventional and reduced tillage systems in Watkinsville, GA (Manor et al., 1991) indicated that much of the variability in cone index under ®eld conditions is caused by tillage and traf®c. Another study reported that ploughing loosens soil more than chiseling, however, natural processes and tillage for seedbed preparation caused the soil after planting to be re-compacted to about the same density as before (Erbach et al., 1992).
2. Materials and methods
2.1. Site, soil and experimental design
Experiments were carried out at the research farm of Ankara University, 40 km south of Ankara, Turkey where the average rainfall was 410 mm in 1996 and 426 mm in 1997. The site altitude is 1050 m and the soil is an imperfectly drained, medium tex-tured clay loam soil of the Central Anatolian series (Cambisol) with 210, 580 and 210 g kg±1of clay, silt and sand, respectively. Soil pH averaged 7.8 and organic matter 11.3 g kg±1. The experiment was arranged in three blocks each consisting of seven tillage treatments. The plots were 25 m wide and 75 m long. Tillage practices were performed on wheat (Triticum aestivum L.) stubble. Climatological data were obtained from the farm station. Monthly rainfall and precipitation were compared to the long term averages.
2.2. Field equipment and procedures
A 65 kW, two wheel drive tractor weighed to 32.4 kN was used for both tillage and traf®c treat-ments. The rear axle was ®tted with 13.6/12±36 bias-ply single tires, in¯ated to 160 kPa while the front axle was ®tted with 7.50±16 bias-ply single tires, in¯ated to 240 kPa. This type of tractor and wheels are most commonly used in Central Anatolia.
A total of seven tillage systems were performed in this study as mentioned below:
1. No-till (control) (S0);
2. Chiseldisc harrow (chiselling to a depth of 28 cm followed by two passes of a disc harrow to a depth of 13 cm) (S1);
3. Chiselcultivator-tooth harrow combination (chi-selling to a depth of 28 cm followed by two passes of a cultivator-tooth harrow combination to a depth of 12 cm) (S2);
4. Mouldboard ploughcultivator-tooth harrow com-bination (ploughing to a depth of 22 cm followed by two passes of a cultivator-tooth harrow combination to a depth of 12 cm) (S3);
5. Mouldboard ploughdisc harrow (ploughing to a depth of 22 cm followed by two passes of a disc harrow to a depth of 10 cm) (S4);
6. Rotary tiller Ð horizontal axis-rotary harrow combination to a depth of 13 cm (S5);
7. Rotary tiller Ð vertical axis-rotary harrow com-bination to a depth of 13 cm (S6).
At the beginning of the study, in the early spring of 1996, the entire experimental area was subsoiled down to a depth of40 cm with a three leg subsoiler (legs spaced at 0.5 m). The randomly selected plots were then subjected to the above-mentioned tillage systems, and received the same subsequent ®eld operations (drilling, fertilising, spraying) for production of Gerek 79 wheat variety. In the late September 1996, the wheat was harvested and plots were fallowed until the early spring 1997 when spring tillage was seeded. As soon as the tillage treatments had been com-pleted traf®c treatment was performed twice, with 1.5 m/s forward speed and 22 kN rear axle load before planting. It was accomplished by making repeated passes with the tractor so that alternate plant rows would be located in a wheel track (Bicki and Siemens, 1991). Nine passes were made at approximately equal distances along each plot. Thus, approximately 25% of the plots were traf®cked. The second passes were made exactly over the established traf®c lanes. At the time of traf®c, moisture content of the plots were checked and no signi®cant difference was found compared to the values obtained after tillage.
2.3. Soil measurements
Measurements were taken in the spring of 1997. Soil bulk density, moisture content, penetration resis-tance and shear strength were measured both after tillage and after wheel traf®c treatments. All measure-ments related with soil physical properties and strength were performed with regard to row position rather than randomly within each plot in order to reduce sampling error. Accordingly, equally distanced six transects perpendicular to the rows in each plot were determined and measurement points were selected within these transects (Manor et al., 1991). Orientation and position of transects were always the same. After traf®c, the measurements were taken from the centre of tracks.
and 20±25 cm. Three set of samples were collected on each plot transect following tillage and traf®c. Soil moisture content was determined gravimetrically from bulk density samples. Total porosity was also calcu-lated from bulk density assuming a particle speci®c gravity of 2.65. Percentage air voids was calculated from the ratio of volume of air to the total volume of soil. All of these parameters were calculated after oven drying of cores at 1058C.
The changes in soil strength resulting from tillage and traf®c treatments were evaluated by the measure-ments of both shear vane and cone penetrometer. A vane borer with a 16 mm32 mm bladed vane and measurement range of 0±260 kPa was used for shear strength measurements. Vane shear strength was mea-sured to a depth of 30 cm at 5 cm increments using three replications on each plot transect. Penetration resistance was measured with a hand operated record-ing type cone penetrometer which had a 308steel cone of 1 cm2base area. Penetration values were recorded at each 1 cm depth interval down to 30 cm at three positions within each plot transect. The data were reduced by averaging in 0.10 m increments.
Vertical stress in soil under wheel load (after wheel passage) was measured with a developed pressure gage (Gruber and TebruÈgge, 1990). Vertical stress measurements were performed at 10 and 20 cm depths and at least six replications per plot. In each plot, rut depth of the rear tire was measured after traf®c treat-ments with a pro®le meter. The pro®le meter consists of vertical metal rods sliding through a 100 cm long iron bar at a regular spacing of 2.5 cm. The bar was
placed across the wheel tracks to measure the shape of the depression (Carman, 1996).
Five kg soil samples were taken from randomly selected three transects of each plot at 0±15 cm both after tillage and traf®c. After air drying, samples were sieved with a rotary sieve to determine size distribu-tion of aggregates (Voorhees et al., 1985). The mean weight diameter was calculated as the sum of the product of aggregate diameters in each size and the fractional weight of aggregate in that size (Adam and Erbach, 1992).
The effects of various tillage system and wheel traf®c on the above soil properties were evaluated by analysis of variance with tillage as the main effect, wheel traf®c as the ®rst split plot and depth as the second split plot. Comparisons of mean values were accomplished using least signi®cant differences (LSD) ata0.05.
3. Results and discussion
Summary of ANOVA representing the effects of tillage, wheel traf®c and depth on some physical and mechanical properties are shown in Table 1. Tillage and traf®c had signi®cant effects (p<0.01) on soil bulk density, total porosity, percentage air voids, vane shear strength and penetration resistance. As expected, no-till plots and wheel traf®c were characterised by greater mean values of bulk density and soil strength. Despite the signi®cant impact of tillage, ®eld traf-®c did not result in appreciably different moisture
Table 1
Summary of ANOVA, indicating the effects of tillage, wheel traf®c (WT) and soil depth on physical and mechanical properties of soil
Effect Bulk
contents among the plots (p>0.05). Tillage had also considerable effects on soil stress and rut depth during subsequent ®eld traf®c (p<0.01).
3.1. Bulk density
Excluding the no-tilled plots, the greatest bulk densities after tillage were observed in S1 plots within the depth of 15 cm (Table 2). In all plots, the lowest bulk densities were obtained at 0±5 cm depth ranging between 16 and 24% less than that of the no-tilled plots. This reduction varied between 11 and 19% at 10±15 cm. Cultivator-tooth harrow combination in secondary tillage reduced the bulk density more than disc harrow. Rotary tiller mixed up the surface resi-dues mostly in the upper soil layer, causing the great-est bulk density reduction compared to the other treatments. Wheel traf®c signi®cantly increased soil bulk density over the 0±15 cm depth. The greatest traf®c-induced increases were observed in the S5 and S3 plots, respectively, for 0±5 cm and 10±15 cm soil depths. The increases in the bulk density varied from 10 to 20% at the 0±5 cm and from 6 to 12% at the 10± 15 cm soil depth. These results support the opinion that the impact of traf®c on compaction is greater under loose soil conditions (Anonymous, 1992). Apparently, conventional tillage systems (S3, S4) are more susceptible to compaction during secondary tillage and ®eld traf®c, than chisel systems. This observation is supported by Bauder et al. (1985) and Erbach et al. (1992).
However in the present study, the critical bulk density levels (1.3±1.5 Mg mÿ3) reported by Godwin (1990) for similar kinds of soil type were not exceeded. This suggests that the soil compaction levels in this study were not excessive.
3.2. Moisture content and air voids
The mean comparisons of moisture content exhib-ited signi®cance for different tillage systems (Fig. 1). This observation does not comply with the results of Bauder et al. (1985) and Hill and Meza-Montalvo (1990) stating that tillage did not have signi®cant effect on gravimetric water content. However, it is in general agreement with Raper et al. (1993). Tillage reduced moisture content by 3±6% at 0±5 cm and by 2±3% at 10±15 cm, compared to untilled soil. The soil water content under rotary till at 0±5 cm and conven-tional till at 10±15 cm was considerably less than that found in the other tillage treatments. Wheel traf®c did not induce changes in soil water.
The largest percent of air voids after tillage was found in S5 and S3 plots, respectively, at the 0±5 and 10±15 cm soil depth, and in chisel tilled (S1, S2) plots at 20±25 cm soil depth (Table 3). Apparently, the air voids were in¯uenced by ®eld traf®c down to 20 cm in all the tillage treatments. However, reduction in air voids was not so excessive to approach a critical rate of 10% proposed by Godwin (1990). Traf®c-induced decrease rate in air voids were found to be lower in chisel tilled plots.
Table 2
Mean comparisons of dry bulk density as affected by tillage and traf®c within the 0±25 cm pro®le of a Central Anatolian clay loam soil
Depth (cm) Dry bulk density (Mg mÿ3)a,b
S0 S1 S2 S3 S4 S5 S6
After tillage
0±5 0.98 a 0.85 b 0.82 c 0.80 de 0.82 cd 0.79 e 0.80 e
10±15 1.10 a 0.99 b 0.95 cd 0.92 e 0.94 de 0.96 cd 0.97 c
20±25 1.16 a 1.08 b 1.08 b 1.18 a 1.18 a 1.17 a 1.17 a
After wheel traffic
0±5 1.01 a 0.93 b 0.92 b 0.92 b 0.93 b 0.92 b 0.92 b
10±15 1.11 a 1.05 b 1.02 c 1.02 c 1.03 bc 1.04 bc 1.05 b
20±25 1.17 a 1.09 b 1.08 b 1.18 a 1.18 a 1.17 a 1.17 a
aNumbers in a row within a depth range followed by the same letter are not signi®cantly different at 0.05 con®dence level
(LSD0.050.022 Mg mÿ3).
bS0: no-till, S1: chiseldisc, S2: chiselcultivator-tooth harrow, S3: ploughcultivator-tooth harrow, S4: ploughdisc, S5: rotary tiller
Since it is determined that traf®c had no signi®cant effect on moisture content, the decrease in total por-osity (data not shown) after wheel traf®c might thor-oughly attributable to the reduction in air voids.
3.3. Soil strength
Vane shear strength and cone penetrometer mea-surements exhibited similar trends due to tillage and traf®c (Tables 4 and 5). The lowest soil strength after tillage were obtained in S6 at 0±10 cm, in S4 at 10± 20 cm and in chisel systems at 20±30 cm depth ranges. On the other hand, excluding no-till, the lowest tillage-induced reductions were observed in chisel systems and S7, respectively, for 0±10 and 10±20 cm depths. Compared to untilled soil S3, S4 and S5 plots
indi-cated a signi®cantly higher strength below the tillage depth at 20±30 cm. The impact of wheel traf®c on mechanical impedance of differently tilled plots was evident particularly down to 20 cm depth. Wheel traf®c increased the penetration resistance by 30± 74% at 0±10 cm and 7±33% at 10±20 cm depth. Chisel tilled plots (S1, S2) were affected quite less at any given soil depth and differed the most from the other treatments. Erbach et al. (1992) observed that the relative in penetration resistance due to wheel traf®c was60% less in chisel plots, compared to ploughed plots. In general, the highest traf®c-induced incre-ments were detected in S4 plots. Irrespective of the tillage system used, traf®c patterns' impact on soil mechanical impedance are mostly con®ned to 0± 20 cm.
Fig. 1. Percent moisture content as affected by tillage system. Error bars indicate the range of the mean comparisons for LSD0.050.5% (S0:
no-till, S1: chiseldisc, S2: chiselcultivator-tooth harrow, S3: ploughcultivator-tooth harrow, S4: ploughdisc, S5: horizontal rotary tiller, S6: vertical rotary tiller).
Table 3
Mean comparisons of percentage air voids as affected by tillage and traf®c within the 0±25 cm pro®le of a Central Anatolian clay loam soil
Depth (cm) Air voids (%)a,b
S0 S1 S2 S3 S4 S5 S6
After tillage
0±5 43.1 e 54.1 d 54.6 d 57.6 bc 57.2 c 58.9 a 58.3 ab
10±15 31.5 f 44.0 e 41.9 d 45.7 a 44.3 b 43.2 c 42.0 d
20±25 26.7 b 32.6 a 33.1 a 26.6 b 26.8 b 26.6 b 25.9 b
After wheel traffic
0±5 41.3 d 49.3 c 49.3 c 51.0 b 51.6 ab 52.4 a 51.5 ab
10±15 30.5 e 36.5 d 36.7 c 39.6 a 38.9 ab 38.1 bc 38.3 bc
20±25 26.3 b 32.4 a 32.3 a 26.4 b 26.4 b 26.1 b 26.2 b
aNumbers in a row within a depth range followed by the same letter are not signi®cantly different at 0.05 con®dence level
(LSD0.050.98%).
bS0: no-till, S1: chiseldisc, S2: chiselcultivator-tooth harrow, S3: ploughcultivator-tooth harrow, S4: ploughdisc, S5: rotary tiller
Raper et al. (1993) suggested that it seems appro-priate to take 2 MPa as the criterion for prevention of optimum root growth. In the present study, this limit was exceeded at 20±30 cm depth in conventionally tilled (S3, S4), reduced tilled (S5, S6) and no-till (S0) plots. This suggests that the above mentioned systems need periodic deep tillage.
3.4. Vertical soil stress
Comparisons of vertical soil stress appeared during traf®c process at 10 and 20 cm depth are shown in
Fig. 2. The highest stress remained in S3, S4 and S6 plots at 10 cm, and in S3 and S4 at 20 cm depth. In comparison, the lowest vertical stress were in S1 and S6 plots, respectively, at 10 cm and 20 cm. Analysis for the dependency of soil bulk density, shear strength, and penetration resistance on vertical stress approached a linear relationship as shown in Table 6. These linear ®ts illustrate that the dependency of shear strength and penetration resistance on soil stress was statistically more signi®cant and reliable than bulk density. Hill and Meza-Montalvo (1990) reported that soil strength is a more sensitive indicator of wheel traf®c effects than bulk density.
Table 4
Mean comparisons of vane shear strength as affected by tillage and traf®c within the 0±30 cm pro®le of a Central Anatolian clay loam soil
Depth (cm) Vane shear strength (kPa)a,b
S0 S1 S2 S3 S4 S5 S6
After tillage
0±10 41.33 a 12.00 b 10.67 b 9.00 b 7.67 b 6.67 b 7.50 b
10±20 124.708 a 68.33 d 60.00 e 49.88 f 54.00 ef 95.67 c 102.67 b
20±30 168.75 b 143.17 c 140.83 c 175.17 a 177.17 a 174.83 ab 178.17 a
After wheel traffic
0±10 47.83 a 22.67 b 22.17 bc 22.17 bc 18.83 bc 15.67 c 16.50 bc
10±20 128.60 a 85.67 c 76.50 d 73.50 d 73.17 d 107.67 b 110.33 b
20±30 169.25 b 146.50 c 147.50 c 178.50 a 179.17 a 179.50 a 183.33 a
aNumbers in a row within a depth range followed by the same letter are not signi®cantly different at 0.05 con®dence level
(LSD0.056.687 kPa).
bS0: no-till, S1: chiseldisc, S2: chiselcultivator-tooth harrow, S3: ploughcultivator-tooth harrow, S4: ploughdisc, S5: rotary tiller
(horizontal), S6: rotary tiller (vertical).
Table 5
Mean comparisons of cone penetration resistance as affected by tillage and traf®c within the 0±30 cm pro®le of a Central Anatolian clay loam soil
Depth (cm) Penetration resistance (kPa)a,b
S0 S1 S2 S3 S4 S5 S6
After tillage
0±10 1037 a 505 b 403 c 348 cd 265 d 352 cd 382 c
10±20 1675 a 1023 d 827 ef 725 f 890 e 1320 c 1443 b
20±30 2035 c 1765 d 1715 d 2158 ab 2185 a 2120 ab 2090 bc
After wheel traffic
0±10 1125 a 655 b 555 c 543 cd 460 d 567 bc 582 bc
10±20 1737 a 1275 a 1102 de 1022 e 1180 d 1465 b 1548 b
20±30 2047 b 1792 c 1798 c 2195 a 2197 a 2180 a 2115 ab
aNumbers in a row within a depth range followed by the same letter are not signi®cantly different at 0.05 con®dence level
(LSD0.0592 kPa).
bS0: no-till, S1: chiseldisc, S2: chiselcultivator-tooth harrow, S3: ploughcultivator-tooth harrow, S4: ploughdisc, S5: rotary tiller
3.5. Rut depth
Rut depth during traf®c treatment also exhibited appreciable difference among differently tilled plots (Fig. 3). The average highest rut depths were measured in S3 and S4 plots, whereas smallest was in S1 plots. Rut depth was found to be closely related with the soil conditions after tillage and became greater when the strength and bulk density were relatively low over a soil depth of20 cm. Regression analysis between rut depth and vertical stress exhibited the following empirical equation:
R ÿ2:3093:118S100:602S20
R0:977; p<0:01 (1)
Fig. 2. Average vertical soil stress during traf®c treatment as affected by tillage system. Error bars indicate the range of the mean comparisons for LSD0.051.6 kPa (S0: no-till, S1: chiseldisc, S2: chiselcultivator-tooth harrow, S3: ploughcultivator-tooth harrow, S4: ploughdisc,
S5: horizontal rotary tiller, S6: vertical rotary tiller).
Table 6
Linear regression for the variation of vane shear strength, penetration resistance and bulk density due to soil stress at 10 and 20 cm depth
Parameter Regression equationa R Significance level
Shear strength K2.7671.031KT0.373S(10 cm) 0.963 **
K21.9510.839KT0.498S(20 cm) 0.968 **
Penetration resistance Pÿ48.2001.049PT10.726S(10 cm) 0.967 **
P396.3500.786PT4.764S(20 cm) 0.970 **
Dry bulk density H1.052ÿ0.114HT±910ÿ6S(10 cm) 0.708 NSb
H0.692±0.341 HT±14.510ÿ6S(20 cm) 0.789 *
*Signi®cant at the 0.05 probability level (p<0.05). **Signi®cant at the 0.01 probability level (p<0.01).
aK: shear strength after traf®c (kPa), KT: shear strength after tillage (kPa),P: penetration resistance after traf®c (kPa), PT: penetration
resistance after tillage (kPa),H: dry bulk density after traf®c (Mg mÿ3), HT: dry bulk density after tillage (Mg mÿ3),S: vertical soil stress
(kPa).
bNon-signi®cant at the 0.05 probability level (p>0.05).
Fig. 3. Average tire sinkage depth (rut depth) during traf®c treatment as affected by tillage system. Error bars indicate the range of the mean comparisons for LSD0.054.3 mm (S0: no-till,
whereRis the rut depth (mm),S10the average vertical stress at 10 cm (kPa), and S20 the average vertical stress at 20 cm (kPa).
3.6. Aggregate mean weight diameter
Both tillage and wheel traf®c-induced differences were also observed in soil aggregate mean weight diameter (Fig. 4). After tillage, the largest and smallest diameters were found in S2 and S5, respectively. These trends were also similar for traf®c patterns. Of all the plots, differences between tillage and traf®c were greatest in the S3 and S4 plots. Conversely, there were no signi®cant wheel traf®c-induced differences in aggregate size of reduced tilled (S5, S6) plots. This does not agree with the soil strength and bulk density increases in these plots after wheel traf®c. Owing to this fact, a reliable relationship between the compres-sion and particle size distribution could not be obtained. Adam and Erbach (1992) reported that use of cultivator instead of disc harrow results in smaller aggregate size. This is supported in the present study.
4. Conclusions
Although, vehicular traf®c affected the soil proper-ties, the type of tillage used had a much greater effect on overall soil physical and mechanical properties in Central Anatolian conditions. Traf®c affected the soil properties mostly down to 20 cm. However, in all plots, no excessive compaction was detected at the
0±20 cm soil depth. Few of soil properties exceeded the critical levels at the 20±30 cm soil depth. No-till plots exhibited the greatest soil strength and bulk density regardless of depth. In these plots there were few signi®cant changes in bulk density, soil strength and air voids attributable to wheel traf®c.
The relative increase in soil strength and bulk density was less for chisel plots following wheel traf®c. Thus, the use of chisel systems was found to be more reasonable to alleviate the effects of subse-quent wheel traf®c. From the standpoint of soil con-dition and without considering the further crop response, it can be stated that system S2 (chi-selcultivator-tooth harrow) provides better and more desirable soil conditions for resisting subsequent ®eld traf®c. This system contained the lowest relative increases in bulk density and soil strength after ®eld traf®c. Mechanical measures of soil con®rmed the detrimental effect of traf®c particularly after conven-tional tillage. Wheel traf®c-induced changes espe-cially in soil strength were detected to be larger and more prevalent for conventionally tilled soils. Thus, it seems that conventional systems do not ®t in well with controlling further soil compaction in the region.
A trend of excessive soil strength was observed in the 20±30 cm depth of the conventionally tilled (S3, S4) and reduced tilled (S5, S6) plots. Thus, increased emphasis on these systems may result in increased root restricting pans in advance unless supported by deep tillage practices periodically.
Wheat is the most important crop in the agricultural production of Central Anatolia. For wheat growing, it Fig. 4. Mean weight diameter of soil aggregates as affected by tillage and subsequent ®eld traf®c. Error bars indicate the range of the mean comparisons for LSD0.051.1 mm (S1: chiseldisc, S2: chiselcultivator-tooth harrow, S3: ploughcultivator-tooth harrow, S4:
is often not possible to minimise some of the adverse effects of wheel traf®c since spacings are generally much narrower than width of implement or tractor tires. Thus, a considerable portion of the seedbed and early growth zone is subjected to compactive force of wheel either before or after seeding. In this study, a contribution towards the understanding of tillage and subsequent random vehicular traf®c-induced changes on soil conditions was established in terms of taking necessary measures in Central Anatolian conditions. However, the crop response still remains in question.
Acknowledgements
I owe very special thanks to Prof. M. Arif Erol for his continuous help during this study.
References
Adam, K.M., Erbach, D.C., 1992. Secondary tillage tool effect on soil aggregation. Trans. ASAE 35, 1771±1776.
Akalan, I., 1983. Toprak Bilgisi, Vol. 873. Ankara University Publishers, Ankara, 360 pp.
Anonymous, 1991. Agricultural Structure and Production in 1989. Turkish Statistics Institute, Ankara.
Anonymous, 1992. Conservation Tillage Systems and Manage-ment. Midwest Plan Service, Iowa State University, Ames, 260 pp.
Anonymous, 1994. Agricultural Structure and Production in 1991. Turkish Statistics Institute, Ankara.
Anonymous, 1997. Agricultural Structure and Production in 1994. Turkish Statistics Institute, Ankara.
Bauder, J.W., Randall, G.W., Schuler, R.T., 1985. Effects of tillage with controlled wheel traf®c on properties and root growth of corn. J. Soil Water Conser. 49, 382±385.
Bicki, T.J., Siemens, J.C., 1991. Crop response to wheel traf®c soil compaction. Trans. ASAE 34, 909±913.
Carman, K., 1996. Prediction of penetration resistance, sinkage and bulk density in soil±tire interaction. In: Proceedings of the Sixth International Congress on Agricultural Mechanisation and Energy, September 2±6, 1996, Ankara, Turkey, pp. 417± 423.
Erbach, D.C., Benjymin, J.G., Cruse, R.M., Elamin, M.A., Mukhtar, S., Choi, C.H., 1992. Soil and corn response to tillage with paraplow. Trans. ASAE 35, 1347±1354.
Godwin, R.J., 1990. Agricultural engineering in development: tillage for crop production in areas of low rainfall. FAO Agricultural Services Bulletin, Vol. 83, Rome, 132 pp. Gruber, W., TebruÈgge, F., 1990. In¯uence of different tillage
systems on ®eld traf®cability and soil compaction. ASAE Paper No. 90-1090, St. Joseph, 14 pp.
Hill, R.L., Meza-Montalvo, M., 1990. Long term wheel traf®c effects on soil physical properties under different tillage systems. Soil Sci. Soc. Am. J. 54, 865±870.
Manor, G., Clark, R.L., Radcliffe, D.E., Langdale, G.W., 1991. Soil cone index variability under ®xed traf®c tillage systems. Trans. ASAE 34, 1952±1956.
Ngunjiri, G.M.N., Siemens, J.C., 1995. Wheel traf®c effects on corn growth. Trans. ASAE 38, 691±699.
OÈ zguÈven, F., Aydinbelge, M., 1990. Ikinci uÈruÈn icin tohum yatagi hazirliginda kullanilan toprak isleme aletlerinin toprak sikisik-ligina etkisi uÈzerine bir arastirma. In: Proceedings of the Fourth International Congress on Agricultural Mechanisation, Sep-tember 4±7, 1990, Adana, Turkey, pp. 166±173.
Raper, R.L., Reeves, D.W., Burt, E.C., Torbert, H.A., 1993. Conservation tillage and traf®c effects on soil condition. Trans. ASAE 37, 763±768.
Taser, OÈ .F., Metinoglu, F., 1997. Farkli tohum yatagi hazirlama yoÈntemlerinin toprak sikismasi ve toprak nem duÈzeyine etkileri uÈzerine bir arastirma. In: Proceedings of the 17th National Congress on Agricultural Engineering, 17±19 September, 1997, Tokat, Turkey, pp. 298±310.
