Soil Compaction Induced by Small Tractor Traffic in Northeast China

Published online February 27, 2006

Soil Compaction Induced by Small Tractor Traffic in Northeast China Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.

X. Y. Zhang,* R. M. Cruse, Y. Y. Sui, and Z. Jhao tionally owned collective farms were dismantled and divided into small blocks and rented to interested farmers. One farm family may cultivate from 0.2 to 2 ha, so large tractors were and are not necessary. The development of economical, low-powered tractors (ST) accelerated due to their convenience to farmers. The number of ST and MT increased from 2 to 13.2 million and from 0.79 to 0.83 million, respectively, during the last two decades (CDA, 1950–2001). During the same period, the area mechanically tilled increased by 36% (CDA, 1950–2001). A representative town, Qianjin (Hailun City, Heilongjiang Province), demonstrates the current trend of mechanical tillage in the black soil region in Northeast China, where the total arable cropland is 13 600 ha. Three different tractor types were used for power in Qianjin in 2001: 26 track MTs, 5 tire MTs, and 798 STs (CDA, 1950– 2001). About 20% of the fields were cultivated by track MT, 10% by tire MT, and 70% by ST. Due to the lower power (,15 kW) and the narrow implement width, ST requires 6 to 10 passes in the field during a growing season, while MTrequires 5 to 7 passes during a growing season. Frequent passes and narrow implements result in a high percentage of the surface being wheel tracked at least once, with many areas receiving multiple passes. Studies suggest that soil stress and rut depth increase with increasing load at constant inflation pressure, and with increasing inflation pressure at constant load (Way et al., 1998; Raper and Erbach, 1990); and topsoil compaction increases with tire inflation pressure (Gassman et al., 1989; Eldin et al., 1993). These observations occurred with tractors more powerful and of greater mass than those commonly used in China. Subsoil (below the Ap horizon) compaction increases as total axle load increases and is less dependent on ground pressure than is surface compaction (Botta et al., 2002; Jones et al., 1988; Jones, 1999). The mass of ST and MT are about 870 and 3075 kg, respectively, and ST typically has a lower ground pressure than MT—400 vs. 800 kPa, respectively. Although the ST mass is low and inflation pressure is lower than that for the MT, the tracking frequency discussed previously may lead to elevated compaction potential using the ST tillage system. Tracking frequency impacts on compaction with relatively small tractors has not been addressed in the literature. Based on ground pressure, a FIAT FA150 tractor with a mass of 13 000 kg and rubber-track ground pressures of about 50 kPa (Marsili et al., 1998) has less compaction potential in general than the smaller ST (870 kg) with its normal tire inflation pressure of about 400 kPa because of higher ground contact pressure of the lighter ST. Thus, tractor size may not be a good indicator of compaction potential.

ABSTRACT With the introduction of the small four-wheel tractors (ST), agricultural mechanization has developed rapidly in China. In this study, we investigated the impact of these relatively small tractors on soil compaction of a silty clay loam (fine-silty, mixed, superactive, mesic Typic Argiboroll) in northeast China using soil penetration resistance (PR) and bulk density as soil compaction indicators. The relationship between soil water content and PR, the comparison of soil compaction induced by ST and the medium power tractor (MT), the effect of tractor mass on compaction, the effect of number of tractor passes and tillage on PR, and the effect of compaction on crop yield were studied. Compared with MT-powered tillage, the ST-powered tillage system created a more compacted plow layer due to increased passes required with this system. Small four-wheeled tractors had a statistically significant higher PR than MT in the topsoil and subsoil. The PR had a significant negative correlation (r 5 2 0.640, P , 0.01) with soil water content at time of PR measurement. After trafficking in a wheat (Triticum aestivum L.) field, the highest PR values were found in the 5- to 14-cm depth interval. Crop yield decreased with increasing numbers of tractor passes. There were different yield-loss responses to compaction for spring wheat and corn (Zea mays L.). With the development of mechanical tillage in China, the problem of soil compaction may be significantly increased, causing potential yield reductions.

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of mechanized farming has impacted all industrialized nations of the world. As tractors replace horses, tillage efficiency improves. With improved tillage efficiency and increased tractor power and size, the severity of soil compaction has also increased (Keller et al., 2002). Excessive compaction causes undesirable effects that can lead to reduction in crop yields and soil quality; these effects include increases in water runoff and reduction in root growth and plant development rates (Unger and Kaspar, 1994; Schafer et al., 1992). Soil compaction has negative effects on crop production and the environment, and presents problems in all climatic regions (Soane and van Ouwerkerk, 1994). A region with growing agricultural industrialization is China. With this growth comes increase use of tractor power. There are two main tractor types widely used in China: STand MT. The ST has ,15 kW (20 hp) of power, and the MT has between 37 and 60 kW (50 and 80 hp) of power. The manufacture of tractors has increased markedly, especially since the development of the ST in 1980 (CDA, 1950–2001), largely because the former naEVELOPMENT

X.Y. Zhang, Y.Y. Sui, and Z. Jhao, Northeast Institute of Geography and Agricultural Ecology, the Chinese Academy of Sciences, 138 Haping Road, Harbin 150040, P.R. China; R.M. Cruse, 3212 Agronomy Hall, Iowa State Univ., Ames, IA 50011-1010. Received 12 Apr. 2005. *Corresponding author ([email protected]). Published in Soil Sci. Soc. Am. J. 70:613–619 (2006). Soil & Water Management & Conservation doi:10.2136/sssaj2005.0121 ª Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA

Abbreviations: MT, medium power tractor; PR, penetration resistance; ST, small four-wheel tractor.

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Reproduced from Soil Science Society of America Journal. Published by Soil Science Society of America. All copyrights reserved.

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SOIL SCI. SOC. AM. J., VOL. 70, MARCH–APRIL 2006

With the development of mechanical tillage and extensive use of ST, soil compaction is becoming a potential problem in China and one that is not well documented (Zhang, 2001). Farmland in China is concentrated north of the Yellow River. Thus, this region located in Northeast China (Heilongjiang and Jilin Provinces) was chosen for this compaction study. The objectives were to: (i) determine the degree of soil compaction resulting from management with ST and MT systems, (ii) identify the impact of numbers of tractor passes vs. tractor mass on soil compaction, and (iii) relate observed soil compaction to crop yield. The hypothesis tested include: (1) use of ST vs. MT would not affect soil bulk density or PR on fields managed with these tractors for 6 yr; (2) increasing ST weight would not affect soil PR or soil bulk density resulting from a single tractor pass; (3) the number of tractor passes would not affect soil PR; (4) moldboard plowing would not change the pattern of soil PR with depth; (5) the number of tractor passes in the furrow of ridged corn rows would not affect corn grain yield; and (6) the number of tractor passes applied to a field would not affect spring wheat yield. MATERIALS AND METHODS Study Region The black soil is mainly found in Heilongjiang and Jilin Provinces in Northeast China and covers more than 10 million hectares. The soil is noted for its high organic matter content of 0.02 to 0.06 kg kg21 dry soil (Wanyun et al., 1992). Before cultivation, soil organic matter contents ranged from 0.10 to 0.12 kg kg21 dry soil (Wanyun et al., 1992). Today over 90% of the farmland is cultivated by tractors. The main crops planted are soybeans (Glycine max L.), corn (Zea mays L.), and spring wheat (Triticum aestivum L.). Soybean and cornfields are typically ridged and spring wheat is seeded on a flat soil surface. The study site is located in the North Temperate Zone and continental monsoon area. This region is cold and arid in the winter, and hot and rainy in the summer, with heavy rainfall and high temperatures in the same season. The annual average air temperature is 1.58C, ranging from typical mean maximum temperature of 328C in the summer to a typical mean minimum temperature of 2378C in the winter. The annual mean rainfall is about 500 mm. The average air temperature and rainfall during the growing season (1 May to 30 Sept.) is 18.18C and 423 mm, respectively.

The Experimental Site The study was performed at Hailun Agricultural Ecology Experimental Station (HAEES), Chinese Academy of Science, located in Hailun City (478269 N and 1268389 E), in northeast China. The soil is a fine-silty, mixed, superactive, mesic Typic Argiboroll (Lee Burras, personal communication, 2005) referred to as “black soil” in China. Physical and chemical properties of this soil (0–0.2 m) are: 10% sand (1– 0.05 mm), 55% silt (0.05–0.001 mm), 35% clay (,0.001 mm); 0.056 kg kg21 organic matter; 0.0026 kg kg21 total N, pH of 6.80; and wilting-point gravimetric water content of 11.2% (Meng et al., 1996). The topsoil and subsoil texture of this loess-derived soil is similar (silty clay loam). There is also no significant spatial variability of soil texture in the surface 0.6 m of the 20-ha field in which these trials were conducted (Meng and Zhang, 1996).

Tracking and Measurement Methods Tractors Used Field tests were performed during 2000, 2001, and 2002. Six trials were completed and are described below. The DFH75 track tractor 55.9 kW (75 hp) and Fujin 11.2 kW (15 hp) ST are the main agricultural tractors in this region and were the tractors used for these trials. The DFH75 (MT) has a mass of 3075 kg and the Fujin (ST) has a mass of 900 kg. Tires on the ST were inflated to 400 kPa. The rear wheel diameter is 96 cm and the front wheel diameter is 64 cm. Rear wheel tire width is 22 cm and the front wheel width is 12 cm. Weight distribution for both tractors was 60% on the rear axle and 40% on the front axle. Ground pressure for the MT was about 175 kPa. Penetration Measurements In selected trials, PR and bulk density were measured. Penetration resistance was measured with a TE-3 penetrometer made in Nanjing. The penetrometer had a drive mechanism powered by hand crank, allowing insertion at a constant rate. The cone had a 108 angle with a cone base cross-sectional area of 1 cm2. The penetrometer was pressed into the soil at a constant rate of 0.02 m s21. The measurement range of the penetrometer was 0.2 to 8.0 MPa with a precision of 0.04 MPa. Bulk Density Undisturbed soil cores were collected by driving with a rubber hammer 5-cm diam. metal cylinders into the soil to the depth specified for each trial. Bulk densities were calculated based on the volumes, calculated from the length and diameter of the section, and dry weights of the soil samples (IJAFT, 1979).

Field Trials Trial 1: Relationship between Soil Water Content and Penetration Resistance Fifteen randomly located open-ended 25-cm diam. metal cylinders were inserted to a depth of 3 cm in weed-free plot areas. Five different amounts of water—0, 1.25, 2.5, 3.75, or 5 kg—were gently applied to the soil surface within the cylinder. Three replications were arranged in a completely randomized design. Forty-eight hours after water application, all water had infiltrated. At this time, gravimetric soil water contents at 5, 15, and 25 cm below the soil surface and the corresponding PR to a depth of 20 cm were measured. Five undisturbed topsoil (0–10 cm) samples were collected for determining the coefficient of linear extensibility (COLE) (Grossman and Reinsch, 2002). The sample heights were measured by a vernier caliper under air-dry, wilting-point, fieldcapacity, and saturated conditions. Sample bulk densities were calculated based on the volumes calculated from the length and diameter of the section, and dry weights of the soil samples (IJAFT, 1979). COLE was calculated based on the bolt-length method (Buol et al., 1990). Trial 2: Soil Compaction Comparison between ST and MT The effects of ST- and MT-powered tillage systems on soil bulk density, soil water content, and PR in the top 20 cm were evaluated in 2000 after 6 yr of continuous tillage management treatments. Soil bulk density and PR were used as indicators of soil compaction. Two 1-ha fields (134 m long, 75 m wide) separated by 50 m were selected. One field had received continuous ST and the other MT for the six prior years. Measurements were made in 2000. Both fields contained a crop rotation

ZHANG ET AL.: SOIL COMPACTION INDUCED BY SMALL TRACTOR TRAFFIC


of spring wheat-corn-soybean for 6 yr before this study. At the time of this study, soybeans were planted in this field. The track coverage rate, TCR, was calculated for both ST and MT based on the method proposed by Medvedev et al. (1993):

TCR 5 (N 3 w/W) 100%

[1]

where N is the number of tractor passes during the growing season (generally one tillage operation counts as one pass), w is the width of the two rear tires, and W is the width of tillage operation. Typical values for w and W are 0.44 and 1.44 m, respectively, for ST, and 0.80 and 3.20, respectively, for MT. Typical values for N are 6 to 10 for ST and 5 to 7 for MT. Row crop traffic with the MT system was limited to the between-row areas, while for ST it was not. When spring wheat was present, traffic control did not exist for either system. Five locations, one in each of the four corners and one in the center, were selected in each field as measurement sites. Three replicated measurements of soil bulk density, soil water content, and PR to a depth of 20 cm in both the row and between-row areas were made on 19 May and 19 July in each of the measurement sites. Least significant differences (LSD) were used to evaluate treatment effects at a common depth and date for PR, soil water content, and bulk density. Significance level used for this and all subsequent studies was the 0.05 level of probability. Trial 3: Impact of Load A ST with different tractor weights (900—tractor with no weights, 1050, 1200, 1350, 1500, 1650, and 1800 kg) was used in a spring-plowed field in 2001 to evaluate the effect of tractor mass on soil bulk density and PR. The weight distribution between front and rear axles was 40 and 60% of the total load, respectively, when measured statically. Tire pressures were not altered for the different weight additions. A tractor of a given weight made one pass with a forward velocity of 6.5 km h21. Plowing was conducted 10 d before imposing the tracking treatments. Each plot was 15 m long and 2 m wide. Treatments were replicated three times in a randomized complete block design. After the tractor pass, bulk density and PR were measured in the middle of a wheel track to a depth of 20 cm at three randomly selected locations. An analysis of variance (ANOVA) was conducted for each depth and if significant treatment effects were detected, LSD values were determined. Trial 4: Impact of the Number of Tractor Passes on Penetration Resistance Different numbers of tractor passes (0, 1, 3, 5, and 7) were imposed on the same track following soybean planting in 2002. An equivalent comparison took place in fall on a fall-plowed spring wheat field with 0, 1, 2, 3, 5, 7, and 9 passes. Treatments were replicated three times in a completely randomized design. Each plot was 20 m in length and 5.6 m wide. Three points in each plot were chosen for measurement. Penetration resistance and gravimetric soil water content were measured immediately after tracking. Gravimetric soil water content at the time of tracking and measurement was 0.23 and 0.19 kg kg21 for soybean and spring wheat soils, respectively. An ANOVA was conducted for each depth and if significant treatment effects were detected, LSD values were determined. Trial 5: Plowing Impact on Penetration Resistance To determine the effect of plowing on PR, measurements were conducted in three random locations in 2002 on a field of harvested spring wheat to a depth of 20 cm both before and after plowing. The top 20 cm was plowed with a DFH75 track

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tractor 55.9 kW (75 hp) pulling a 4-bottom moldboard plow (1L-435). The preplowing measurements were considered the control and were compared with those after plowing. The data was analyzed using an ANOVA as a completely randomized design with a separate analysis conducted for each depth. Trial 6: Effect of Soil Compaction on Crop Response A ridged seedbed prepared with the MT system was planted to corn (Var. Haiyu 8) on 1 May 2001 and 2002, and received 127.5 kg N ha21 applied as (NH4)2HPO4. Emerged plant population was 54 600 plants ha21. All furrow areas were trafficked by the ST 0, 1, 3, 5, or 7 times on 5 May 2001 and 2002. In the furrow position, the average 15-cm gravimetric water content at time of trafficking was 0.21 g g21. Eight furrows were compacted in a plot 20 m long and 6.7 m wide. Treatments were randomized and replicated three times in a completely randomized design. No secondary tillage was conducted. Weeds were controlled manually. Sixty corn plants were hand-harvested from each plot at maturity for grain yield. Crop yield samples were selected randomly from three locations in each plot, threshed manually, and converted to yield at a water content of 0.14 kg kg21. A comparable trial addressing the effect of soil compaction on spring wheat (Var Kehan 19) yield was also conducted. After seeding wheat at 250 kg ha21, the field was trafficked by ST with 0, 1, 3, 5, 7, or 9 passes. When trafficking the average 15-cm depth gravimetric water content was 0.24 kg kg21. Each plot was 10 m long and 2 m wide. The entire soil surface of each plot was tracked. Treatments were randomized and replicated three times. The seedling emergence rate was measured in randomly selected 1-m2 areas before the tillering stage. Number of wheat spikes per plant and grain yield was measured manually. These were collected in three random 1-m2 locations in each plot. Data were analyzed with an ANOVA testing for significance of tractor passes on corn yield, wheat population, wheat spike numbers, and wheat yield. With significant treatment effects, means were separated with LSD procedures.

RESULTS AND DISCUSSION Trial 1: Relationship between Soil Water Content and Penetration Resistance Soil PR readings were affected by soil water content as expected. Similar to that observed by Ekwue and Stone (1995) and Kamaruzaman (1991), PR varied linearly with gravimetric water content from wilting point to field capacity and was inversely related to soil water content (Fig. 1). This likely occurred because soil strength is known to be reduced as soil moisture increases (Yong and Warkentin, 1966). Additionally, bulk density of this soil is reduced by wetting as evidenced by the COLE measurements. From oven-dry to saturated soil conditions, the bulk density decreased from 1.37 to 1.32 Mg m23, its volume expanded 3.2%, which is another factor influencing soil strength or PR reduction (Yong and Warkentin, 1966).

Trial 2: Soil Compaction Comparison between Small Four-Wheeled Tractor and Medium-Powered Tractor The track coverage rate averaged 193% for ST and 127% for MT, indicating that soil managed with ST


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Fig. 1. The relation between soil gravimetric water content and penetration resistance (PR) in the 0- to 20-cm depth.

received more wheel tracking, although loads were lower than those of the MT treatments. Bulk density, soil water content, and PR were affected by tractor treatments. When comparing bulk density on a given date, position, and depth, ST differences were not statistically separable by the LSD (Table 1). Bulk density differences tended to be higher in the later sampling date (Table 1). The potential importance of this is explained later. From 19 May to 19 July, the bulk density increased with time at the 0- to 5- and 5- to 10-cm depths for all treatments except ST-furrow (0–5 cm). Bulk density increased significantly with time in the 10- to 20-cm layer only for MT-ridge. On 19 May, the 0- to 10-cm layer bulk density tended to be higher in the ST-furrow than in the MTfurrow position, although differences were not significant at the 0.05 level. Bulk density differences between tractor types within either ridge or furrow were nonexistent at the 10- to 20-cm depth. The ANOVA detected that no depth effects existed on 19 July. Penetration resistance was consistently higher in the ST- than in the MT-ridge position to the 24-cm depth (Fig. 2). In the furrow, PR was similar for both tractor types in the top 14 cm (Fig. 2), after which the ST values tended to be smaller. Penetration resistance increased for both tillage systems to a depth of 8 cm in the furrow (Fig. 2). Gravimetric soil water contents were higher with ST than with MT management for both positions and at most depths (Fig. 3). Figure 1 indicates PR increased rapidly as soil water content decreased. The higher water content observed for ST in the top 7.5 cm between ST-F and MT-F (Fig. 3) should result in much higher MT than ST PR if all other factors were equal

Fig. 2. Comparison of penetration resistance (PR) in the ridges (R) and furrows (F) in two fields that were continuously managed for 6 yr using small four-wheel tractor (ST) or medium-power tractor (MT). Open symbols at a given depth within ridge or furrow locations indicate significant PR differences existed.

(based on the regression relationship in Fig. 1), which did not occur. This suggests the soil physical condition created with ST had characteristics associated with elevated compaction compared with MT, characteristics which resulted in comparable PR even though ST soil was considerably wetter. Additionally note that observed ST bulk density values were higher than for comparable MT values, and although statistically they were not separable (Table 1), this cannot be ignored as a potential factor affecting PR. The lower soil water content in the ridge position of both treatments is likely due to the ridged surface configuration enhancing evaporation rate and the lower bulk density favoring drainage (Wang, 1996). Canarache et al. (1984) observed similar water content/bulk density results reporting bulk density and gravimetric water content in compacted plots increased 20 to 25% and 2 to 3%, respectively, compared with noncompacted control plots.

Trial 3: Impact of Load Both PR and bulk density increased with load (Fig. 4 and 5). Penetration resistance was highest in the 5- to 10-cm depth, and differed significantly among the highest, lowest, and intermediate loads. Treatments caused similar bulk density changes in the 0- to 5-, 5- to 10-, 10- to 15-, and 15- to 20-cm depth intervals. Based on PR, there was a noticeable influence of load on soil compaction in the plow layer. When the spring-plowed wheat field was compacted (load varied from 900 to 1800 kg), bulk density at

Table 1. Soil bulk density (Mg m23) in the furrow (F) and ridge (R) for ST- and MT-powered tillage systems. 19 May Depths 0–5 cm 5–10 cm 10–20 cm

19 July

ST-F

MT-F

ST-R

MT-R

ST-F

MT-F

ST-R

MT-R

1.23abA† 1.34aB 1.32abB

1.15cA 1.21bB 1.32abC

1.11dA 1.24cB 1.28cdB

1.06eA 1.11eB 1.26dC

1.22a 1.31a 1.32ab

1.27a 1.32a 1.35a

1.25a 1.28d 1.30bc

1.14cd 1.28d 1.28cd

† Row means with the same small letter are not significantly different at the 0.05 level, and column means with the same capital letter on 19 May are not significantly different at the 0.05 level.



Fig. 3. Gravimetric water content in the furrow (F) and ridge (R) in two fields that were continuously managed for 6 yr using ST or MT. Differences were not significant between treatments containing darkened symbols at a given depth.

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Fig. 5. Bulk density under a single-pass ST track with tractor having no traffic (0) or a mass of 900, 1050, 1200, 1350, 1500, 1650, or 1800 kg. Horizontal bars give the LSD for each depth.

all depths of the plow layer increased. Compaction, as determined by bulk density changes, was more difficult to detect than those related to PR changes. Carpenter et al. (1985) evaluated the theoretical effect of wheel loads of high-power tractors on subsoil stresses using Froelich’s equations as developed by Soehne. Carpenter et al. (1985) indicated that maximum allowable subsoil stress for most soils results from individual wheel loads of approximately 3000 kg. The different loads created by the tractors and tillage operations in this study were sufficient to affect bulk density and PR at depths below 10 cm. The increase in the value of these variables with increasing tractor load suggests the critical stress to cause maximum compaction below the surface 10 cm was not reached with loads used in this study.

Trial 4: Number of Tractor Passes Impact on Penetration Resistance

Fig. 4. Penetration resistance (PR) under a single-pass ST track with the tractor having a mass of 900, 1050, 1200, 1350, 1500, 1650, or 1800 kg. Horizontal bars give the LSD for each depth.

Wheel-track PR increased with the initial ST passes (Fig. 6 and 7) for both the fall-plowed spring wheat field (flat configuration) and the ridged soybean field. One pass significantly increased PR, and the tractor’s first three passes produced a greater PR increase than did the remaining passes in both fields. Kamaruzaman (1991) observed similar results with a heavier, unloaded tree harvester. Greater PR differences were seen in the 4- to 12-cm depth interval than in other depths, which follows a similar pattern observed by Pagliai et al. (2003) with a larger wheeled tractor. While trends for PR change with tracking passes were similar between fields, PR values were lower for the soybean field, which had a higher gravimetric water content (0.23 kg kg21) than


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Fig. 6. Impact of ST passes on wheel-track penetration resistance (PR) in a fall-plowed spring wheat field at the 0- to 20-cm depth.

did the spring wheat field (0.19 kg ka21), respectively, at the time of measurement.

Trial 5: Plowing Impact on Penetration Resistance Penetration resistance in the plow layer following spring wheat and before tillage was about 2 MPa and was more variable with depth than after plowing (Fig. 8). After plowing, the plow layer PR was quite homogeneous and had a PR of about 0.3 MPa, only 20% of the untilled value, which is supported by Murphy et al. (1993) results on loamy sand indicating PR was reduced 45% by plowing. Plowing can alleviate soil compaction in the surface layer. However, the tradeoff frequently

Fig. 8. Penetration resistance (PR) in the plow layer (0–20 cm) (a) before and (b) immediately after plowing.

is elevated PR at the base of the plow layer (Weisskopf et al., 2000; Keller et al., 2002), a zone of typically high PR that is difficult to mechanically alleviate and a condition deleterious to crop production. In this study, PR at the 20-cm depth before and after plowing was different (Fig. 8). One must keep in mind that for this study, data were collected from a ridge system and normally the ST system does not contain ridges. During the plowing operation, a tractor wheel may travel in the base of the plow layer, elevating soil PR at this depth as identified by other studies identified above.

Trial 6: Effect of Soil Compaction on Crop Response Soil compaction significantly affected crop yields. Corn yields were reduced with increasing ST passes in the furrow; the first pass reduced corn yield by 8.6%. Corn yield subsequently decreased with additional tractor passes through the maximum of seven passes used in this study. Canarache et al. (1984) reported corn grain Table 2. Effect of soil compaction by different passes on corn yield, spring wheat plant population, spike number, and yield. Number of passes

Fig. 7. The effect of ST passes in the furrow of a ridge-tilled soybean field on penetration resistance (PR) in the 0- to 20-cm depth. Horizontal bars give the LSD for each depth.

0 1 3 5 7 9

Corn yield

Wheat population

Wheat spike(S) number

Wheat yield

Mg ha21 7.43 a† 6.79 b 6.55 c 6.36 cd 6.25 d —

Plant m22 496 a 510 b 519 b 418 c 385 d 388 d

S m22 450 a 394 b 368 c 344 d 298 e 269 f

Mg ha21 2.5 a 2.5 ab 2.4 bc 2.3 c 2.0 d 2.0 d

† Column means with the same letter are not significantly different at the 0.05 level.



yield linearly decreased 13 kg ha21 for each 1 kg m23 increase in bulk density in Romania. Spring wheat responded differently than corn to soil compaction. The tractor’s first three passes did not reduce the yield of spring wheat, and actually increased the seedling populations (Table 2). With more than three passes, yield and plant populations decreased. Spring wheat spike numbers were also reduced with increasing tractor passes (Table 2).

CONCLUSIONS Small, four-wheel tractors are the principal tractor power in the black soil region of Northeast China, but because of its lower tillage efficiency relative to MT, it produces significant soil compaction in the topsoil (20 cm). Compared with MT, the ST managed system created a significantly higher PR. With increasing axle loads and number of passes, both bulk density and PR increased. Soil compaction induced by tractor traffic reduced crop yield in the black soil farmland. Even though the ST has smaller mass and lower ground pressure than MT, the increased frequency of passes associated with ST tillage management results in soil conditions potentially more detrimental than those associated with MT. ACKNOWLEDGMENTS This research was sponsored by the Chinese Academy of Science, the People’s Republic of China, under grant no. KZCX1-ST-19-3-01.

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